learning ipv4 routes with ripv2 -...

APPENDIX G

Learning IPv4 Routes with RIPv2

This chapter covers the following exam topics:

3.0 Routing Technologies

3.2 Interpret the components of routing table

3.2.a Prefix

3.2.b Network mask

3.2.c Next hop

3.2.d Routing protocol code

3.2.e Administrative distance

3.2.f Metric

3.2.g Gateway of last resort

3.3 Describe how a routing table is populated by different routing information sources

3.3.a Admin distance

3.5 Compare and contrast static routing and dynamic routing

3.7 Configure, verify, and troubleshoot RIPv2 for IPv4 (excluding authentication, filtering,

manual summarization, redistribution)

This appendix is from Chapter 19 of CCENT/CCNA ICND1 100-105 Official Cert Guide.

Although the ICND2 exam objectives do not explicitly reference the content covered here,

the skills and concepts covered are important to have a solid understanding of as a future

IT professional. All references to other content in this appendix refer to CCENT/CCNA ICND1 100-105 Official Cert Guide and not this book.

Routers route IP packets. However, they cannot route packets without meaningful routes,

routes for all destinations in the internetwork. And the most common way for routers to

learn routes to remote subnets is to use a dynamic routing protocol.

Routing Information Protocol (RIP) Version 2 (RIPv2) is the only IP routing protocol dis-

cussed in depth in this book. The big idea is simple: An engineer enables RIPv2 on each

router. RIPv2 then takes the connected routes it knows because of interface IP address con-

figuration and then advertises them by sending messages to neighboring routers. Over time,

as each router learns more routes, they advertise about those routes as well. By the end of

the process, all routers know about all subnets, including details about redundant routes to

reach each subnet. Each router can then put the best route for each subnet into its routing

table, completing the goal of learning good routes to forward traffic to all subnets.

G

Appendix G: Learning IPv4 Routes with RIPv2 3

This chapter, one of the longer chapters in this book, takes RIPv2 and the topic of rout-

ing protocols from initial concept, into configuration and verifications, and ends with

troubleshooting.

Foundation Topics

RIP and Routing Protocol ConceptsMany IP routing protocols exist, in part due to the long history of IP. However, if you com-

pare all the IP routing protocols, they all have some core features in common. Each routing

protocol causes routers (and Layer 3 switches) to

1. Learn routing information about IP subnets from other neighboring routers

2. Advertise routing information about IP subnets to other neighboring routers

3. If a router learns of more than one route to reach one subnet, choose the best route

based on that routing protocol’s concept of a metric

4. React to changes when the network topology changes—for example, when a link

fails, and converge to use a new choice of best route for each destination subnet

All the routing protocols do these same four functions, but the protocols differ in the

details of how they accomplish these tasks. The rest of this chapter works through the

details of how Routing Information Protocol Version 2 (RIPv2) accomplishes these tasks,

with a few comments about other protocols sprinkled throughout .

History of Interior Gateway Protocols Historically speaking, RIP Version 1 (RIPv1) was the first popularly used IP routing proto-

col, with the Cisco proprietary Interior Gateway Routing Protocol (IGRP) being introduced

a little later, as shown as the first wave in Figure G-1.

20102000

EIGRP

RIP-2

1990

OSPFv2IGRP

1980

RIP-1

1st Wave IPv4:Distance Vector

2nd Wave:IPv4

3rd Wave:IPv6

EIGRPv6

RIPngOSPFv3

OSPFv3AddressFamilies

4th Wave:IPv4 + IPv6

Figure G-1 Timeline for IP IGPs

By the early 1990s, business and technical factors pushed the IPv4 world toward a second

wave of better routing protocols. That second wave includes RIP Version 2 (RIPv2), OSPF

Version 2 (OSPFv2), and Enhanced Interior Gateway Routing Protocol (EIGRP), all proto-

cols found in use today for IPv4.

The first and second wave of routing protocols worked with IPv4 but not IPv6. IPv6

emerged in the mid-1990s as a long-term solution to IPv4 growth issues in the Internet.

These new IPv6 routing protocols included EIGRP for IPv6 (sometimes called EIGRPv6),

OSPF Version 3 (OSPFv3), and RIP next generation (RIPng). (Yes, RIPng was named after

the Star Trek series.)

4 CCNA Routing and Switching ICND2 200-105 Official Cert Guide

The fourth wave shown in Figure G-1 is there mainly to overcome a bit of history with

OSPF and is listed here just to be fully correct. When OSPFv3 was created, it supported

advertising IPv6 routes only. So, for many years, OSPFv2 implied IPv4-only, and OSPFv3

implied IPv6-only. Around 2010, OSPFv3 was improved to advertise both IPv4 and IPv6

routes using a feature called address families .

Comparing IGPsToday, you would most likely see the second- and third-wave routing protocols in most

networks. In fact, Cisco considers IGRP to be so old that it does not even include IGRP in

its more recent IOS versions. EIGRP and OSPFv2 are easily the most popular IPv4 routing

protocols, with RIPv2 used much less. However, RIPv2 is listed in the ICND1 exam topics,

and it has one huge advantage versus EIGRP and OSPFv2: RIPv2 is easier to learn.

What is an IGP in the first place? All the routing protocols mentioned so far in this chap-

ter happen to be categorized as interior gateway protocols (IGP) rather than as an exterior

gateway protocols (EGP). These two terms use the word gateway instead of router because

routers were called gateways in the earliest days of IP routing. The designers of some rout-

ing protocols intended the routing protocol for use inside one company or organization

(IGP), with other routing protocols intended for use between companies and between

Internet service providers (ISP) in the Internet (EGPs).

This chapter falls back to using the term IGP when talking about all the routing protocols

mentioned in this chapter.

Any time an engineer thinks about what routing protocol to use, he can make some basic

comparisons between the routing protocols. The following list describes four of the major

points to consider when comparing these routing protocols:

■ The underlying routing protocol algorithm: Specifically, whether the routing protocol

uses logic referenced as distance vector (DV) or link state (LS).

■ The usefulness of the metric: The routing protocol chooses which route is best based on

its metric; so the better the metric, the better the choices made by that routing protocol.

■ The speed of convergence: How long does it take all the routers to learn about a change

in the network and update their IPv4 routing tables? That concept, called convergence time, varies depending on the routing protocol.

■ Whether the protocol is a public standard or a vendor-proprietary function: RIP and

OSPF happen to be standards, defined by RFCs. EIGRP happens to be defined by Cisco,

and until 2013, was kept private.

RIP’s hop count metric treats each router as a hop, so the hop count is the number of other

routers between a router and some remote subnet. RIP’s hop-count metric means that RIP

picks the route with the smallest number of links and routers. However, that shortest route

may have the slowest links. In fact, Figure G-2 shows just such a case on the left, with RIP

choosing the one-hop route from Router B to subnet 10.1.1.0, even though it crosses the

slower 100-Mbps link instead of the two-hop route over two 1-Gbps links.

A routing protocol whose metric was based (at least in part) on link bandwidth might be

a better choice in the topology shown in Figure G-2. For example, EIGRP does base its

metric in part on link bandwidth. EIGRP, on the right side of the figure, chooses the route

that happens to have more links through the network (and more hops), but both links have a

faster bandwidth of 1 Gbps on each link .

G


Subnet 10.1.1.0

G0/2

G0/1

RIP

Bandwidth 100 Mbps

Bandwidth 1 Gbps

Bandwidth 1 Gbps

EIGRPSubnet 10.1.1.0

G0/2

G0/1

Bandwidth 100 Mbps

Bandwidth 1 Gbps

Bandwidth 1 Gbps

A B

C

A B

C

Figure G-2 EIGRP Choosing the Longer but Better Route to Subnet 10.1.1.0

Distance Vector BasicsEach IGP can be categorized based on its internal logic, either distance vector (used by RIP)

or link state. The next few pages explain more about how a DV protocol actually exchanges

routing information, using RIPv2 as an example. The ICND2 book’s chapters about OSPF

describe link state logic, and the ICND2 book’s chapters on EIGRP get into more detail

about some advanced distance vector features.

The Concept of a Distance and a Vector

The term distance vector describes what a router knows about each route. When a router

learns about a route to a subnet, the routers learn three important facts related to each

route: the destination subnet, the distance (that is, the routing protocol metric), and the vec-

tor (that is, the link and next-hop router to use as part of that route).

Figure G-3 begins to develop that concept showing RIP updates in the small boxes on the

left. That is, Router R1 receives RIP updates from three neighboring routers. Each update

describes a different route for subnet X, with a different metric. The fact that a particular

router sends the RIP message identifies the next-hop router for the route. In this case, the

three RIP updates advertise the following routes:

■ The four-hop route (distance) through R2 (vector) for subnet X

■ The three-hop route (distance) through R5 (vector) for subnet X

■ The two-hop route (distance) through R7 (vector) for subnet X

Subnet X

Subnet X,Metric 4

Subnet X,Metric 2

Subnet X,Metric 3

Routing Update

R1 R5

R2 R3

R7

R4

R6 R8

Figure G-3 Information Learned Using DV Protocols


Taking Figure G-3 a step further, imagine if R1 had learned only one route to subnet X, say

the route learned from R2. R1 would use that route because it is the only route R1 knows

for subnet X. However, having learned three routes to subnet X, R1 picks the route with the

best (lowest) metric, in this case the two-hop route through next-hop Router R7.

Full Update Messages and Split Horizon

While Figure G-3 shows a conceptual figure, Figure G-4 gets more specific about what

RIPv2 actually does. Like many DV protocols, RIPv2 sends a periodic routing update based

on a relatively short timer. The periodic part of the term refers to the fact that RIP repeats

the same update over and over on a timed basis even if nothing changes. Figure G-4 illus-

trates this concept, with a detailed breakdown of the steps following the figure.

R1 IP Routing TableSourceRIPRIPConn.Conn.

Subnet172.30.21.0/24172.30.22.0/24172.30.1.0/24172.30.11.0/24

Out Int.G0/2G0/2G0/2G0/1

Next-Hop172.30.1.2172.30.1.2N/AN/A

Metric1100

R2 IP Routing TableSourceConn.Conn.Conn.RIP

Subnet172.30.21.0/24172.30.22.0/24172.30.1.0/24172.30.11.0/24


Next-HopN/AN/AN/A172.30.1.1

Metric 0 0 0 1

RIP Update 172.30.11.0, metric 1 RIP Update172.30.21.0, metric 1172.30.22.0, metric 1

RIP Update 172.30.11.0, metric 1 RIP Update172.30.21.0, metric 1172.30.22.0, metric 1

… …

172.30.1.0 /24

30 Seconds30 Seconds

172.30.1.2

3

1

4 2

172.30.1.1R1 R2 G0/3 - 172.30.21.0 /24

G0/1G0/2

G0/2 - 172.30.22.0 /24

G0/1 - 172.30.11.0 /24

Figure G-4 Normal Steady-State RIP Operations: Full Update with Split Horizon

This figure shows a lot of information, so take the time to work through the details, follow-

ing the numbers in the figure versus the following list. For example, consider what switch

R1 learns for subnet 172.30.22.0/24, which is the subnet connected to R2’s G0/2 interface:

1. R2 interface G0/2 has an IP address and is in an up/up state.

2. R2 adds a connected route for 172.30.22.0/24, off interface G0/2, to R2’s routing

table.

3. R2 advertises its route for 172.30.22.0/24 to R1, with metric 1, in a RIP update sent to

R1. The metric of 1 means that R1’s metric to reach this subnet will be metric 1 (hop

count 1).

4. R1 adds a route for subnet 172.30.22.0/24, listing it as a RIP learned route with

metric 1.

G


Also, take a moment to focus more on the route learned at Step 4: the bold route in R1’s

routing table. This route is for 172.30.22.0/24, as learned from R2. It lists R1’s local G0/2

interface as the outgoing interface because R1 received the update on R1’s G0/2 interface.

R1’s route also lists R2’s IP address of 172.30.1.2 as the next-hop router because that’s the

IP address from which R1 learned the route. Think of R1’s outgoing interface and next-hop

router information as the forwarding instructions for this route.

Split Horizon

Figure G-4 also shows a common DV feature called split horizon. Note that both routers

list all four subnets in their IP routing tables. However, the RIP update messages do not list

four subnets. The reason? Split horizon.

Split horizon is a DV feature that tells a router to omit some routes from an update sent out

an interface. Which routes are omitted from an update sent out interface X? The routes that

would use interface X as the outgoing interface. Those routes that are not advertised on an

interface usually include the routes learned in routing updates received on that interface.

Split horizon is difficult to learn by reading words, and much easier to learn by seeing an

example. Figure G-5 continues the same example as Figure G-4, but focusing on R1’s RIP

update sent out R1’s G0/2 interface to R2. Figure G-5 shows R1’s routing table with three

light-colored routes, all of which list G0/2 as the outgoing interface. When building the RIP

update to send out that same G0/2 interface, split horizon rules tell R1 to ignore those light-

colored routes. Only the bold route, which does not have G0/2 as an outgoing interface,

can be included in R1’s RIP update sent out G0/2 .


Subnet172.30.21.0/24172.30.22.0/24172.30.1.0/24172.30.11.0/24


Next-Hop172.30.1.2172.30.1.2N/AN/A

Metric1100

RIP Update

Only Bold Route Has InterfaceOther Than G0/2

172.30.11.0, metric 1

G0/3 - 172.30.21.0 /24

G0/1G0/2

G0/2 - 172.30.22.0 /24

G0/1 - 172.30.11.0 /24 172.30.1.0 /24

172.30.1.2172.30.1.1R1 R2

Figure G-5 R1 Does Not Advertise Three Routes Due to Split Horizon

Route Poisoning

DV protocols help prevent routing loops by ensuring that every router learns that the route

has failed, through every means possible, as quickly as possible. A routing loop occurs when

the routes for some destination, on a set of routers, would cause a packet sent to that des-

tination to keep looping between those routers and never arrive at the destination. Routing

protocols attempt to prevent any routing loop. One of these features, route poisoning,

helps all routers know for sure that a route has failed.

Route poisoning refers to the practice of advertising a failed route, but with a special met-

ric value called infinity. Routers consider routes advertised with an infinite metric to have

failed.


Figure G-6 shows an example of route poisoning with RIP, with R2’s G0/2 interface failing,

meaning that R2’s route for 172.30.22.0/24 has failed. RIP defines infinity as 16.

172.30.21.0, metric 1172.30.22.0, metric 16

3

172.30.1.0 /24

172.30.1.2

1

172.30.1.1


Subnet172.30.21.0/24172.30.22.0/24172.30.1.0/24172.30.11.0/24


Next-Hop172.30.1.2172.30.1.2N/AN/A

Metric11600

R2 IP Routing TableSourceConn.Conn.Conn.RIP

Subnet172.30.21.0/24172.30.22.0/24172.30.1.0/24172.30.11.0/24


Next-HopN/AN/AN/A172.30.1.1

Metric 0 0 0 1

4 2

G0/3 - 172.30.21.0 /24

G0/1G0/2

G0/2 - 172.30.22.0 /24

G0/1 - 172.30.11.0 /24

R1 R2

Figure G-6 Route Poisoning

Figure G-6 shows the following process, again following the numbers in the figure:

1. R2’s G0/2 interface fails.

2. R2 removes its connected route for 172.30.22.0/24 from its routing table.

3. R2 advertises 172.30.22.0 with an infinite metric (which for RIP is 16).

4. R1 realizes that the route to 172.30.22.0/24 no longer works. Depending on conditions

not discussed here, R1 either removes the route from its routing table or marks the

route as unusable (with an infinite metric) for a few minutes before removing the route.

By the end of this process, Router R1 knows for sure that its old route for subnet

172.30.22.0/24 has failed, which helps R1 avoid introducing looping IP routes.

Note that all routing protocols have mechanisms to use to mark routes as expired in some

way, in some cases using an infinite metric value similar to RIP. RIP uses 16 per the RIP pro-

tocol definition; as a result, a route with hop count 15 is the longest valid route that can be

used in a RIP network, because advertising a route with hop count 16 would be considered

a poison route.

Summarizing RIPv2 FeaturesThis final section briefly mentions a few more features of RIPv2, and collects those features

into a table for easier review and study.

Of course, RIPv2 adds features beyond RIPv1. For instance, RIPv2 supports authentica-

tion, which is a feature by which routers can use a password-like mechanism to make sure

they exchange routes only with authentic other routers. RIPv2 also supports manual route

summarization, which allows an engineer to plan and reduce the size of routing tables. (The

DVD Appendix O, “Route Summarization,” copied from a previous edition of this book,

provides more detail if you are interested.) However, this book does not get into details

about these features beyond this brief mention.

For another difference, RIPv2 sends its update message—the message that lists routing

information—to the 224.0.0.9 multicast IP address. RIPv1 used the local subnet broadcast

G


address of 255.255.255.255. Using the multicast address is more efficient and causes less

impact to other hosts.

Finally, RIPv2 adds support for variable-length subnet masks (VLSM). Chapter 22, “Variable

Length Subnet Masks,” goes into detail about VLSM. To review, VLSM means that inside

one classful network (one Class A, B, or C network), more than one subnet mask is used.

For instance, the network in Figure G-7 uses VLSM because all the subnets are from Class A

network 10.0.0.0, but some subnets use a /24 mask whereas others use a /30 mask .

10.3.5.0 /24

10.3.6.0 /24

10.3.7.0 /24

10.2.2.0 /24

10.2.3.0 /24

10.2.4.0 /24Yosemite

Albuquerque

Seville

10.1.4.0 /30

S0/0 S0/110.1.6.0 /30

10.1.1.0 /24

S0/0S0/1

Figure G-7 An Example of VLSM

Table G-1 lists the features comparing RIPv1 and RIPv2. However, note that the list of fea-

tures in the table is more about emphasizing the features of RIPv2 than stressing the differ-

ences between the two versions .

Table G-1 Key Features of RIPv1 and RIPv2

Feature RIPv1 RIPv2

Hop-count metric Yes Yes

Sets 15 as the largest metric for a working route Yes Yes

Sends full routing updates Yes Yes

Uses split horizon Yes Yes

Uses route poisoning, with metric 16 to mean “infinite” Yes Yes

Sends mask in routing update, thereby supporting VLSM No Yes

Supports manual route summarization No Yes

Sends updates to 224.0.0.9 multicast address No Yes

Supports authentication No Yes

Core RIPv2 Configuration and VerificationRIPv2 requires three basic configuration commands, with just a couple of show commands

to check RIPv2 status. This second of four major sections of this chapter focuses on that

core configuration and verification.

Configuring Core RIPv2 FeaturesRIPv2 configuration is simple compared to the concepts related to routing protocols. The

configuration process uses three required commands, with only one command, the network

command, requiring any real thought. You should also know the more popular show com-

mands for helping you analyze and troubleshoot routing protocols.

The RIPv2 configuration process takes only the following three required steps, with the

possibility that the third step might need to be repeated several times on the same router:


Step 1. Use the router rip command in global configuration mode to move into RIP

configuration mode.

Step 2. Use the version 2 command in RIP configuration mode to tell the router to use

RIP Version 2 exclusively.

Step 3. Use one or more network net-number commands in RIP configuration mode

to enable RIP on the correct interfaces.

Understanding the RIP network Command

To configure RIPv2, always start with those first two commands in the configuration

checklist, and then think hard about the third step, the network command. The RIP

network indirectly identifies the interfaces on which RIP is then enabled. The command

has one parameter: some classful IP network number. IOS then compares each interface IP

address of each interface on the local router with the IP network in the network command.

IOS enables RIP on each interface whose IP address is in that same classful network.

For example, in Figure G-8, the configuration on the left uses two network commands. The

first network command happens to match one interface IP address of the four interfaces on

the right, because one of the interfaces is in classful network 10.0.0.0. The second command

matches two interfaces, because both are in classful network 172.16.0.0. Neither of the two

network commands match the fourth interface, which is in classful network 192.168.1.0.

router rip version 2 interface F0/0 network 10.0.0.0 ip address 10.1.1.1…

interface F0/1 network 172.16.0.0 ip address 172.16.1.1…

interface F0/2 ip address 172.16.2.2…

interface F0/3 ip address 192.168.1.1…

RIP Configuration Interface Configuration

Figure G-8 RIP Network Command Enabling RIP Per-Interface Logic

So , what does RIPv2 do on an interface once enabled? Well, RIP takes three separate

actions once enabled on an interface. So rather than think of enabling RIP on an interface as

one idea, break it into these three actions, which will help when you think about some later

configuration and troubleshooting topics. The following are the three actions:

■ The router sends routing updates out the interface.

■ The router listens for and processes incoming updates on that same interface.

■ The router advertises about the subnet connected to the interface.

Note that with the version 2 command configured, the updates sent and received per this

list are RIP version 2 updates.

ConfigChecklist

G


RIP Configuration Example, with Many IP Networks

Keeping these facts in mind, now consider how to configure RIP on a single router. Examine

Figure G-9 for a moment and try to apply the first three configuration steps to this router

and anticipate the configuration required on the router to enable RIP on all interfaces.

192.168.2.0 /24

G0/1

192.168.1.0 /24

S0/0/0

S0/0/1 S0/0/0

S0/0/1

S0/0/0

G0/1

G0/1

S0/0/1192.168.5.0 /24

192.

168.

4.0

/24 192.168.6.0 /24

192.168.3.0 /24

R3

R2R1192.168.5.2

192.168.4.3

Figure G-9 RIPv2 Configuration: Three Routers, Each Connected to Three Networks

Take a close look at the IP subnets listed in the figure. All links use an entire Class C net-

work. I chose to use different Class C networks on each link on purpose so that each router

connects to multiple classful networks. For instance, R2 will need network commands for

networks 192.68.2.0, 192.168.5.0, and 192.168.6.0. Example G-1 shows the configuration

for all three routers .

Example G-1 R1, R2, and R3 RIPv2 Configuration, for Figure G-9

! Router R1 configuration

router rip

version 2

network 192.168.1.0

network 192.168.4.0

network 192.168.5.0


router rip

version 2

network 192.168.2.0

network 192.168.5.0

network 192.168.6.0


router rip

version 2

network 192.168.3.0

network 192.168.4.0

network 192.168.6.0


First, focus on meeting the primary goals. All three routers have the router rip and version

2 commands, which together enable RIPv2, but without enabling RIPv2 on any interfaces.

Next, focus on the three network commands on each router. Each router has three

network commands in this example because each router connects to three different classful

networks. For example, R1 lists IP network 192.168.1.0, 192.168.4.0, and 192.168.5.0 in its

three network commands, because according to the figure, R1 connects directly to those

three IP networks. Those three commands enable RIPv2 on R1’s G0/1, S0/0/0, and S0/0/1

interfaces. The network commands on the other two routers enable RIPv2 on their inter-

faces, respectively.

This particular example configuration gives us a good backdrop to discuss a common ques-

tion about RIPv2 configuration and network commands. First, no one router has network

commands for all six classful IP network numbers. The network command does not pre-

define all classful networks in the entire topology. Instead, it triggers local logic on that

one router, matching the network commands against the interface IP addresses on that one

router, as shown earlier in Figure G-8.

Finally, on a complete side note about the RIP network command: IOS will actually accept

a parameter besides a classful network number. IOS will not even issue an error message.

However, IOS, knowing that the parameter must be a classful network number, interprets

the IP address and changes the number to the matching network number. For example, if

you were to type network 10.1.2.3 in RIP configuration mode, IOS would accept the com-

mand, with no error message, and change what you typed so that the configuration has a

network 10.0.0.0 command. Your original network 10.1.2.3 command would disappear .

RIP Configuration Example, with One IP Network

Figure G-9, used in the first RIP configuration example, purposefully used many IP net-

works so that the configuration required several RIPv2 network commands. However, often

a design will use subnets of one classful network, as shown in Figure G-10. In this case, all

six subnets are subnets of Class A network 10.0.0.0. Note that Figures G-9 and G-10 are

identical other than the IPv4 subnets used.

10.1.2.0 /24

G0/1

10.1.1.0 /24

S0/0/0

S0/0/1 S0/0/0

S0/0/1

S0/0/0

G0/1

G0/1

S0/0/110.1.5.0 /24

10.1

.4.0

/24 10.1.6.0 /24

10.1.3.0 /24

R3

R2R1

Figure G-10 Three Routers, Each Connected to Subnets of Class A Network 10.0.0.0

G


To enable RIPv2 on all interfaces, each router needs only one network command: the

network 10.0.0.0 command. That one command on a router matches all three interfaces.

Example G-2 shows the identical configuration used by all three routers .

Example G-2 The Identical RIPv2 Configuration Used for R1, R2, R3 for Figure G-10

router rip

version 2

network 10.0.0.0

RIPv2 Verification IOS includes three primary show commands that are helpful to confirm how well RIPv2 is

working. Table G-2 lists the commands and their main purpose.

Table G-2 RIP Operational Commands

Command Purpose

show ip route [rip] Routes: This command lists IPv4 routes as learned by RIP. The show ip route command lists all IPv4 routes, and the show ip route rip command lists RIP-learned routes only.

show ip protocols Configuration: This command lists information about the RIP configuration, plus the IP addresses of neighboring RIP routers from which the local router has learned routes.

show ip rip database Best routes: This command lists the prefix/length of all best routes known to RIP on this router, including routes learned from neighbors and connected routes for interfaces on which RIP has been enabled.

Examining RIP Routes in the IP Routing TableTo begin, consider Router R1’s routing table, based on Figure G-9 and the configuration

in Example G-1. That is the sample with six different Class C networks in the three-router

design. Example G-3 shows the full IP routing table, as well as just the RIP-learned routes.

Example G-3 The show ip route Command

R1# show ip route

Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B - BGP

D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area

N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2

E1 - OSPF external type 1, E2 - OSPF external type 2

i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2

ia - IS-IS inter area, * - candidate default, U - per-user static route

o - ODR, P - periodic downloaded static route, H - NHRP, l - LISP

a - application route

+ - replicated route, % - next hop override

Gateway of last resort is not set

192.168.1.0/24 is variably subnetted, 2 subnets, 2 masks

C 192.168.1.0/24 is directly connected, GigabitEthernet0/1

L 192.168.1.1/32 is directly connected, GigabitEthernet0/1


R 192.168.2.0/24 [120/1] via 192.168.5.2, 00:00:21, Serial0/0/0

R 192.168.3.0/24 [120/1] via 192.168.4.3, 00:00:05, Serial0/0/1


C 192.168.4.0/24 is directly connected, Serial0/0/1

L 192.168.4.1/32 is directly connected, Serial0/0/1




R 192.168.6.0/24 [120/1] via 192.168.5.2, 00:00:21, Serial0/0/0

[120/1] via 192.168.4.3, 00:00:05, Serial0/0/1

R1# show ip route rip

! The same lines of legend show up here – removed for brevity

R 192.168.2.0/24 [120/1] via 192.168.5.2, 00:00:21, Serial0/0/0

R 192.168.3.0/24 [120/1] via 192.168.4.3, 00:00:05, Serial0/0/1

R 192.168.6.0/24 [120/1] via 192.168.5.2, 00:00:21, Serial0/0/0

[120/1] via 192.168.4.3, 00:00:05, Serial0/0/1

First , scan around all the detail in the show ip route command. Notice the legend at the

top—about 10 lines of output—which are the same for every show ip route command. The

legend lists the routing codes, which are short codes that identify the source from which a

route is learned. In this case, Router R1 IPv4 routes with three codes—C, L, and R—mean-

ing connected, local, and RIP.

Scan farther down in the output to see the individual routes. The routes list the subnet

and then mask in prefix format, followed by other details. Ignoring the detail for another

moment, notice the three highlighted RIP-learned routes, both in the output of the show

ip route command and in the show ip route rip command at the end of the example. The

highlighted lines show the same routes, but the show ip route rip command lists only the

RIP routes, and not the connected and local routes.

Each line in the output of these commands reveals many details about a route. Using R1’s

route to 192.168.2.0/24 as an example, the details are as follows:

■ The network number and mask are listed, 192.168.2.0 and /24 in this case. (In some

cases, the mask is on a heading line just above the route.)

■ The next-hop router’s IP address is 192.168.5.2 in this case.

■ The outgoing interface is Serial0/0/0 in this case.

■ The update RIP timer that measures how long it has been since R1 has last heard about

this route in a periodic RIP update is 21 seconds ago in this case.

■ The RIP metric for this route (1 in this case), is listed as the second number in the square

brackets. For example, between R1 and subnet 192.168.2.0/24, one other router (R2)

exists, so it is a one-hop route.

■ The administrative distance of the route is 120 in this case; the first number in brackets.

Take the time now to review the other two RIP routes, noting the values for these various

items in those routes.

G


To better understand RIP metrics, think for a moment what would happen in this three-

router topology of Figure G-9 if R1’s S0/0/0 interface failed. R1’s one-hop route to

192.168.2.0/24 uses that outgoing interface, but if it were to fail, R1 would converge to

use the two-hop route that runs through R3 next. Example G-4 shows the RIP routes on R1

again after that failure.

Example G-4 The show ip route Command with New Metric 2 for Subnet 192.168.2.0


! The same lines of legend show up here – removed for brevity

R 192.168.2.0/24 [120/2] via 192.168.4.3, 00:00:01, Serial0/0/1

R 192.168.3.0/24 [120/1] via 192.168.4.3, 00:00:01, Serial0/0/1

R 192.168.6.0/24 [120/1] via 192.168.4.3, 00:00:01, Serial0/0/1

NOTE In lab, all you have to do to re-create the link failure in this example is to issue a

shutdown command under R1’s S0/0/0 interface.

Examine the highlighted route in detail, and compare it to R1’s route for 192.168.2.0 from

the previous example. In this case, the network, mask, and administrative distance remain

the same. However, the metric is now 2, because this route goes through R3 and then R2,

for two hops. It also lists forwarding directions of going through 192.168.4.3 (R3’s S0/0/0

IP address) and going out R1’s S0/0/1 interface.

Comparing Routing Sources with Administrative Distance

As you just examined, when an internetwork has redundant links and uses a single routing

protocol, each router may learn multiple routes to reach a particular subnet. That one rout-

ing protocol then uses a metric to choose the best route, and the router adds that route to

its routing table. For instance, R1 uses the metric 1 route for 192.168.2.0/24 when all links

were working, and then the metric 2 route for 192.168.2.0/24 when a link in that better

route failed.

However, some enterprises use multiple IP routing protocols. One router might learn of

multiple routes to a particular subnet using different routing protocols. In these cases, the

metric does not help the router choose which route is best, because each routing protocol

uses a metric unique to that routing protocol. For example, RIP uses the hop count as the

metric, but EIGRP uses a math formula with bandwidth and delay as inputs. A route with

RIP metric 1 might need to be compared to an EIGRP route, to the same subnet, but with

metric 4,132,768. (Yes, EIGRP metrics tend to be large numbers.) Because the numbers have

different meanings, there is no real way to compare the metrics.

The router still needs to choose the best route even between routes learned by different

routing protocols, or between routing protocols and static routes. IOS solves this problem

by assigning a numeric value to each routing protocol. IOS then chooses the route whose

routing protocol has the lower number. This number is called the administrative distance

(AD). For example, EIGRP defaults to use an AD of 90, and RIP defaults to use the value of

120, as shown in the routes in Examples G-3 and G-4. Given the lower-is-better logic used

with administrative distance, an EIGRP route to a subnet would be chosen instead of a com-

peting RIP route.


Table G-3 lists the AD values for the most common sources of routing information.

Table G-3 IOS Defaults for Administrative Distance

Route Source Administrative Distance

Connected routes 0

Static routes 1

EIGRP 90

OSPF 110

RIP (v1 and v2) 120

DHCP default route 254

Unknown or unbelievable 255

NOTE You might recall a brief mention of administrative distance in Chapter 18,

“Configuring IPv4 Addresses and Static Routes.” That chapter explained how to configure a

floating static route that was used only when the routing protocol did not learn a route for

a given subnet, using administrative distance as the key mechanism .

Revealing RIP Configuration with the show ip protocols Command

The show ip route command shows the end result of RIP’s work, but the show ip protocols

command tells us something about how RIP works and about the RIP configuration. Example

G-5 lists the output of this command, taken from Router R1 in Figure G-9, again based on the

configuration in Example G-1.

Example G-5 The show ip protocols Command Based on Example G-1 Configuration

R1# show ip protocols

Routing Protocol is "rip"

Outgoing update filter list for all interfaces is not set

Incoming update filter list for all interfaces is not set

Sending updates every 30 seconds, next due in 23 seconds

Invalid after 180 seconds, hold down 180, flushed after 240

Redistributing: rip

Default version control: send version 2, receive version 2

Interface Send Recv Triggered RIP Key-chain

GigabitEthernet0/1 2 2

Serial0/0/0 2 2

Serial0/0/1 2 2

Automatic network summarization is in effect

Maximum path: 4

Routing for Networks:

192.168.1.0

192.168.4.0

192.168.5.0

Routing Information Sources:

G


Gateway Distance Last Update

192.168.4.3 120 00:00:18

192.168.5.2 120 00:00:05

Distance: (default is 120)

The example highlights the configuration information that can be gleaned from the output,

at least for configuration commands mentioned in this chapter, as follows:

■ The version 2 RIP subcommand configured R1 to send version 2 updates only and to

process received version 2 updates only as well.

■ Automatic summarization (per the default auto-summary command) is enabled.

■ The maximum-paths command is set to 4 (also the default).

■ The “Routing for Networks” section re-lists the configuration of network commands,

listing the network numbers in those commands. In this case, it implies three RIP sub-

commands: network 192.168.1.0, network 192.168.4.0, and network 192.168.5.0.

The output also lists RIP status information. For instance, look at the bottom of the exam-

ple, to the “Routing Information Sources” section (not highlighted). It lists two gateways

(that is, routers) by IP address. This list is the list of IP addresses of neighboring routers

from which R1 has heard RIP updates. (If you look back to Figure G-9, you will see these

two IP addresses as addresses on R3 and R2, respectively.) The last update timer lists the

time since R1 last heard from the neighbor; remember, RIP relies on the periodic receipt of

RIP updates to know that the neighbor still exists. In short, this list shows that R1 is learning

RIP routes from these two routers .

Examining the Best RIP Routes Using RIP Database

One more command can reveal some important details about RIP operation on a router:

the show ip rip database command. This command lists the prefix/length of each subnet

known to the local router’s RIP process. This command lists the local router’s best route to

each known subnet. In particular, this command lists

■ Routes for subnets learned from other RIP routers

■ Routes for connected subnets for which RIP is enabled on interfaces due to the RIP

network command(s)

The fact that the show ip rip database command lists both learned routes and connected

routes for RIP-enabled interfaces makes this command unique. In comparison, Examples G-3

and G-4 show samples of the show ip route command from R1 in Figure G-10, listing RIP-

learned routes. However, you cannot tell for sure on which interfaces RIP has been enabled

based on this output. Example G-5 shows how the show ip protocols command identifies the

interfaces on which RIP is enabled, with three interfaces on that same Router R1. However,

this command does not identify any RIP-learned routes.

As shown in Example G-6, the show ip rip database command lists both connected and RIP-

learned routes. The sample is again taken from Router R1 in Figure G-10, with all interfaces

working. The output identifies the same three RIP-learned routes listed in Example G-4, with

the hop-count metrics in brackets and the next-hop router IP addresses listed (R2, address

10.1.5.2, and R3, 10.1.4.3). The output also lists the connected subnets, identifying the inter-

faces on which RIP has been enabled.


Example G-6 The show ip rip database Command on Router R1 (Figure G-10)

R1# show ip rip database

10.0.0.0/8 auto-summary

10.1.1.0/24 directly connected, GigabitEthernet0/1

10.1.2.0/24

[1] via 10.1.5.2, 00:00:00, Serial0/0/0

10.1.3.0/24

[1] via 10.1.4.3, 00:00:08, Serial0/0/1

10.1.4.0/24 directly connected, Serial0/0/1

10.1.5.0/24 directly connected, Serial0/0/0

10.1.6.0/24

[1] via 10.1.5.2, 00:00:00, Serial0/0/0

[1] via 10.1.4.3, 00:00:08, Serial0/0/1

Optional RIPv2 Configuration and VerificationThis next major section, the third of four major sections in this chapter, introduces a few

optional RIPv2 features. The features are passive interfaces, maximum (routing) paths, auto-

matic route summarization, and discontiguous networks.

Controlling RIP Updates with the passive-interface CommandIn some cases, you want to enable RIP on an interface so that you can advertise about the

connected subnet, but you do not need to advertise routes on that interface. This is typi-

cally true for any router LAN interface for which that router interface is the only interface

connected to the LAN. No other routers connect to the LAN, so the router does not need

to send updates onto the LAN.

The RIPv2 passive-interface command can be used to stop all RIPv2 updates from being

sent out the interface that is matched by a network command. By making an interface pas-

sive to RIP, the RIP process no longer sends RIP updates out that interface. RIP will still

process any received updates and will still advertise about the connected subnet.

IOS gives us two configuration methods to make interfaces passive. The first is the most

obvious: use the passive-interface type number RIP subcommand for each interface that

you want to make passive to RIP.

Example G-7 shows a revised version of the Router R1 configuration first shown in

Example G-2. In that example, all three routers (from Figure G-10) connect to subnets of

network 10.0.0.0. All three routers also have a G0/1 LAN interface that connects to a LAN

that has no other routers connected to it. Example G-7 shows the original configuration

from Example G-2, with the addition of the passive-interface command to make the LAN

interface passive.

Example G-7 Directing RIPv2 to Not Send Advertisements with passive-interface

router rip

version 2

network 10.0.0.0

passive-interface G0/1

G


The second configuration method flips the logic: Make all interfaces passive by default, with the passive-interface default RIP subcommand, and then selectively make interfaces not be passive, with the no passive-interface type number RIP subcommand. Using this method makes sense when a router has mostly passive interfaces and only a few nonpassive interfaces. However, just to show the configuration, Example G-8 converts the configura-tion of Example G-7 to this alternate style, under the assumption that R1 has three inter-faces enabled for RIP: S0/0/0 and S0/0/1 (which should not be passive), and G0/1 (which should be passive).

Example G-8 Using the passive-interface default Command Option

router rip

version 2

network 10.0.0.0

passive-interface default

no passive-interface s0/0/0

no passive-interface s0/0/1

Supporting Multiple Equal-Cost Routes with Maximum PathsWhat should a router do when it learns multiple routes for the same subnet but the metrics tie? Routing protocols use their metrics to choose the best route to each destination subnet, but with RIP’s hop-count metric, ties can easily happen. So RIP needs options for how to deal with a tie.

RIP’s default behavior when it learns more than one route that ties is to put multiple routes into the routing table and use them all. Once in the routing table, the router’s forwarding logic balances the packets across those equal-metric routes.

For instance, consider Figure G-11. R1 will learn two different 2-hop routes to subnet 10.1.4.0 (a router with R3 as the next-hop router and a route with R2 as the next-hop rout-er). R1 will then place both routes in its routing table, with equal metric (2). When R1 needs to forward a packet to subnet 10.1.4.0, R1 can send some packets over one route and some over the other, balancing the load.

2-Hop Route

2-Hop Route

R1

R3

R2

R4

10.1.4.0

Figure G-11 Equal-Cost Routes with RIP

Cisco refers to this feature of using multiple equal-metric routes to the same destination as

equal-cost load balancing. RIP controls this behavior with the maximum-paths number-of-paths RIP subcommand, whose default setting of 4 means that RIP will use the feature by

default, with up to 4 equal cost routes for each subnet. This command can be set larger (the

maximum setting is dependent on the route model and IOS version), and it can be set as low


as 1. Setting the value to 1 disables the feature, so that RIP places the first-learned equal-

metric route into the routing table .

Understanding Autosummarization and Discontiguous Classful Networks

Older routing protocols, namely RIPv1 and IGRP, were classified as classful routing proto-cols. These older classful routing protocols had to use a more careful and cautious subnet

design plan to avoid a problem called a discontiguous classful network. These simpler old

routing protocols just got confused when a classful network became discontiguous, because

of a required feature of classful routing protocols called autosummarization.

Today, most enterprises use OSPF or EIGRP, or in a few cases, RIPv2. All these proto-

cols are classless routing protocols. These classless routing protocols either do not have a

problem with discontiguous classful networks or can be configured so they do not have a

problem. In fact, when using RIPv2, if you simply configure the no auto-summary RIP sub-

command on every router, you can avoid the problem altogether. But of course, it helps to

understand what the problem is as well, so this section looks at both autosummarization and

the discontiguous network problem.

A routing protocol that uses autosummarization automatically creates a summary route

under certain conditions. That automatic process happens when

■ That one router connects to subnets of multiple different classful networks

■ That router uses a routing protocol that uses the autosummary feature. (Note that class-

ful routing protocols had to use this feature and could not disable it.)

To see specifically what that means, consider Figure G-12, which shows one router (R3) that

connects to several subnets of network 10.0.0.0 on the right and to a serial link in network

172.16.0.0 on the left. In other words, R3 meets the first criteria in the list. If using a rout-

ing protocol with the autosummarization feature, R3, when advertising a route to the left

toward R2, would automatically create a summary route: a route for the entire Class A net-

work 10.0.0.0, as shown in the figure .

R3R2

S0/0/0

I Only Learned About 10.0.0.0/8 - No Subnets!

172.16.3.0/24Not in 10.0.0.0

I Have a Routeto 10.0.0.0/8

172.16.1.0/24Not in 10.0.0.0

3.3

1

23

10.3.5.0/24

10.3.6.0/24

10.3.7.0/24

Figure G-12 Autosummarization Example

Following the steps in the figure:

1. R3 has autosummary enabled, with the RIPv2 auto-summary router subcommand.

2. R3 advertises a route for all of Class A network 10.0.0.0, instead of advertising routes

for each subnet inside network 10.0.0.0, because the link to R2 is a link in another

network (172.16.0.0).

G


3. R2 learns one route in network 10.0.0.0: a route to 10.0.0.0/8, which represents all of

network 10.0.0.0, with R3 as the next-hop router.

Autosummarization is not necessarily bad by itself, but when combined with a design that

creates a discontiguous classful network, the combination is bad. Which of course begs the

question of what is a discontiguous classful network? First, more formally:

Contiguous network: A network topology in which subnets of network X are not sepa-

rated by subnets of any other classful network

Discontiguous network: A network topology in which subnets of network X are sepa-

rated by subnets of some other classful network

Figure G-13 makes the idea more obvious. First, look to the far left and right and see the

subnet IDs for several subnets of network 10.0.0.0. Then look in the center, and see subnets

of network 172.16.0.0. The subnets of network 10.0.0.0 are separated by subnets of another

network, so network 10.0.0.0 is discontiguous.

10.3.5.0/24

10.3.6.0/24

10.3.7.0/24172.16.3.0/24

R1 R3

10.2.2.0/24

10.2.3.0/24

10.2.4.0/24

R2 S0/0/0S0/0/1172.16.2.0/24

2.1 3.3

Which Route To10.0.0.0/8 Do I Believe? I Have a Route

to 10.0.0.0/8I Have a Routeto 10.0.0.0/8

Figure G-13 Discontiguous Network 10.0.0.0

The figure also points out the issue when combining autosummarization and discontiguous

networks. With autosummarization on both R1 and R3, R1 declares with its routing update

“All of network 10.0.0.0 is over here.” R3 does the same. R2, in the middle, is confused

and does not even realize that it is confused. If R2 sends all packets destined for network

10.0.0.0 to the left, the hosts in the subnets on the right will be unreachable, and vice versa.

If R2 balances the traffic over the two routes to network 10.0.0.0, sending some to the left

and some to the right, then it appears that random hosts work for short periods of time.

This problem has two solutions. The old-fashioned solution is to create IP addressing plans

that do not create discontiguous classful networks. In other words, keep all subnets of each

classful network together in a design. For instance, in this case, use subnets of network

10.0.0.0 for those links in the middle of the figure, or use subnets of a third classful net-

work instead of the 172.16.0.0 subnets on the left.

The other solution solves the problem by disabling autosummarization with the no auto-

summary RIPv2 subcommand. This command is needed on the router that connects to both

classful networks (Routers R1 and R3 in Figure G-13), because those are the routers that

automatically create summary routes. Figure G-14 shows the results with this solution: R1

and R3 advertise about all their subnets to R2, and R2 knows specific routes for each sub-

net, solving the problem .


10.3.5.0/24

10.3.6.0/24

10.3.7.0/24172.16.3.0/24

R1 R3

10.2.2.0/24

10.2.3.0/24

10.2.4.0/24

R2 S0/0/0S0/0/1172.16.2.0/24

2.1 3.3

I Know ExactlyWhere to SendEach Packet!

Routes for:10.3.5.0/2410.3.6.0/2410.3.7.0/24

Routes for:10.2.2.0/2410.2.3.0/2410.2.4.0/24

Figure G-14 The Effect of no auto-summary on the Network from Figure G-13

Verifying Optional RIP FeaturesAll the configuration settings for these optional features can be seen in the output of the

show ip protocols command. Example G-9 shows a sample from Router R1, based on the

original Figure G-9 and Example G-1. (That is the example with six different IP networks,

with each router needing three different network commands.) The RIP configuration is list-

ed at the top of the example for perspective, with each of these optional settings changed

from their default values to make it more obvious in the output of the example.

Example G-9 Verifying RIPv2 Optional Configuration in show ip protocols Output

R2# show running-config

! Lines other than the RIP configuration are omitted

router rip

version 2

network 192.168.1.0

network 192.168.4.0

network 192.168.5.0

no auto-summary

maximum-paths 3

passive-interface gigabitethernet0/1







Redistributing: rip



Serial0/0/0 2 2

Serial0/0/1 2 2

Automatic network summarization is not in effect

Maximum path: 3


192.168.1.0

192.168.4.0

G


192.168.5.0

Passive Interface(s):

GigabitEthernet0/1



192.168.4.3 120 00:00:03

192.168.5.2 120 00:00:09


On a final note regarding the passive interfaces, note that the show ip protocols output lists

an interface either in the primary list of interfaces (those that are not passive) or in the list

of passive interfaces, but not both. For instance, interface G0/1 is in the list under the head-

ing “Passive Interface(s):” but is not under the heading “Interface.”

Example G-10 closes this section by showing an example in which a router has learned

multiple equal-cost routes. In this case, R1 has learned two 1-hop routes to subnet

192.168.6.0/24, just as was shown in Figure G-11. Note that the show ip route command

lists the subnet ID on one line, with two different sets of forwarding instructions (outgoing

interface and next-hop IP address) listed on the two lines.

Example G-10 Evidence of Equal-Cost Load Balancing


! Legend omitted for brevity


C 192.168.1.0/24 is directly connected, GigabitEthernet0/1

L 192.168.1.1/32 is directly connected, GigabitEthernet0/1

R 192.168.2.0/24 [120/1] via 192.168.5.2, 00:00:21, Serial0/0/0

R 192.168.3.0/24 [120/1] via 192.168.4.3, 00:00:05, Serial0/0/1







R 192.168.6.0/24 [120/1] via 192.168.5.2, 00:00:21, Serial0/0/0

[120/1] via 192.168.4.3, 00:00:05, Serial0/0/1

That concludes this chapter’s examination of a few optional RIPv2 features. The following

list summarizes these options and their configuration:

Step 1. Disable RIP sent updates on some interfaces as follows:

A. Use the passive-interface type number command in RIP configuration

mode to make RIP not send RIP updates out that RIP-enabled interface.

B. Use the passive-interface default command in RIP configuration mode

to make RIP not send RIP updates out all interfaces by default, and then

selectively use the no passive-interface type number command in RIP

configuration mode to enable RIP sent updates on some interfaces.

ConfigChecklist


Step 2. Use the no auto-summary command in RIP configuration mode to disable

automatic summarization on routers that connect to multiple classful networks,

as needed.

Step 3. Use the maximum-paths number command in RIP configuration mode to set

the number of equal-metric routes for a single destination subnet to add to the

IP routing table.

RIPv2 Default RoutesIn network designs that use a single router at a remote site, the remote router can use a

default route rather than using a routing protocol. The section “Static Default Routes” in

Chapter 18 shows that exact scenario, with branch offices, each with a single router. Each

branch office router could use a static default route that forwards packets over the single

WAN link toward the core of the enterprise network.

In some designs, the network engineer might want to use the default route concept, except

that multiple routers may exist in that part of the network. All the routers in that part of the

network would need to forward their packets to the one router that has a WAN link con-

nected to another part of the enterprise or to the Internet.

Figure G-15 shows an example, with enterprise routers R1, B01, and B02 all wanting to

use the one link to the Internet that is connected to R1. To make that work, R1 uses a

default route that points directly over that link to the Internet. Routers B01 and B02 have

default routes that forward packets to R1, so R1 will then forward packets by default to the

Internet. (The figure shows the default routes on each router as arrow lines.)

Internet

Default

Default

Default

ISP1

B02

B01

R1

Figure G-15 Branch Routers with Default Route to R1; R1 with Default Route to Internet

Learning Default Routes Using Static Routes and RIPv2

The enterprise routers in this design could each use a static default route, but RIPv2 pro-

vides an alternative that uses a static default route on only one router. One router, directly

connected to the link of the true default route, configures a static default route as normal.

That router then uses RIPv2 to advertise a default route—a route to 0.0.0.0, mask /0—to

the other routers. Basically, the remote routers learn default routes like those shown for

routers B01 and B02 in Figure G-15, pointing to the router that originally advertised the

default route.

Figure G-16 shows the basic idea of what happens on the router on which the static default

route is configured. R1 configures a static route to the Internet at Step 1, and then adver-

tises that static route to the other routers with RIPv2 at Step 2.

G


1

2

2

Internet

RIPv2 Advertises Default

RIPv2 Advertises Default

192.0.2.1

G0/3

G0/2

G0/1

G0/1

G0/1ip route 0.0.0.0 0.0.0.0 192.0.2.1

ISP1

B02

B01

R1

Figure G-16 A Scenario for RIPv2 to Advertise Default Routes

The key to making the process work is the addition of the default-information originate

command to the RIP configuration on the router where the static default route is configured,

as demonstrated in Example G-11. This new RIP subcommand tells the router simply this:

If the IPv4 routing table has a default route in it, advertise a default route with RIP, with

this local router as the eventual destination of those default routes.

Note that only the router that originally advertises the default route—Router R1 in the

example shown in the figure—needs the default-information originate command. Other

RIPv2 routers, like B01 and B02 in the example, learn the default route like any other RIP-

learned route.

Example G-11 Configuring Router R1 to Advertise a Default Route with RIPv2

R1# configure terminal

R1(config)# ip route 0.0.0.0 0.0.0.0 192.0.2.1

R1(config)# router rip

R1(config-router)# default-information originate

R1(config-router)# end

R1#

To verify the configuration, first check on the static default route. The configuration and

verification of the static default route on Router R1 looks the same as it did in the “Static

Default Routes” section in Chapter 18. Example G-12 shows a sample from Router R1; note

the static route to prefix/length 0.0.0.0/0 and the Gateway of Last Resort set to the next-

hop address of 192.0.2.1.

Example G-12 Displaying the Static Default Route Configured on Router R1

R1# show ip route static

! Legend omitted

Gateway of last resort is 192.0.2.1 to network 0.0.0.0

S* 0.0.0.0/0 [1/0] via 192.0.2.1


The other routers list default routes in their routing table, but of course the routes show up

as RIP-learned routes instead of static routes. Note that the route for 0.0.0.0/0 shows up

with a code of R, meaning that it is a RIP-learned route. The route also has an * beside it,

meaning that this route is a candidate to be the default route for this router. The Gateway

of Last Resort (which is the chosen default route for this router) lists the same next-hop

IP address listed in the RIP-learned default route. Example G-13 shows an example from

router B01, which will use a default route with next-hop address 10.1.12.1, which is R1’s IP

address on the WAN link.

Example G-13 Router B01 with Default Route Learned Using RIPv2


! Legend omitted


R* 0.0.0.0/0 [120/1] via 10.1.12.1, 00:00:06, GigabitEthernet0/1

Learning a Default Route Using DHCP

Chapter 20, “DHCP and IP Networking on Hosts,” discusses how Dynamic Host

Configuration Protocol (DHCP) can be used by hosts to learn their IP addresses. Hosts can

also learn other important details using DHCP, like the subnet mask to use, the DNS server

IP addresses, and the IP address of the router on the subnet to use as the default gateway.

Routers that connect to the Internet can use DHCP as well. In particular, a router with a link

to the Internet can dynamically learn the interface IPv4 address it should use. Additionally,

DHCP announces the IP address of the ISP’s router on the other end of the Internet con-

nection, listed as the default gateway in the DHCP messages. And that default gateway IP

address is the address the enterprise router would normally use as the next-hop address on

its default route to the Internet.

To pull those ideas together, examine the details in Figure G-17. The figure repeats the same

design shown in the previous two figures, with one change. In this case, enterprise Router R1

uses DHCP to learn its IP address (192.0.2.2). The DHCP process also lists a default gateway,

192.0.2.1, which in this case identifies the ISP router’s IP address on the link.

1

2

Use Address 192.0.2.2Use Gateway 192.0.2.1

DHCP

Internet

192.0.2.1

ISP1

B02

B01

R1

DHCP-learned: 0.0.0.0 /0, Next-hop 192.0.2.1

Figure G-17 Enterprise Router Building and Advertising Default Routes with DHCP Client

G


By dynamically learning the IP address of the ISP router, Router R1 can dynamically add

a default route to its routing table. R1’s new default route will use the default gateway IP

address from the DHCP message—which is the ISP router’s IP address—as the next-hop

address. Then, using the same RIPv2 methods and the default-information originate RIP

subcommand configured on Router R1, R1 will advertise a default route to the other routers.

Example G-14 shows the configuration on Router R1. Note that it begins with R1 configur-

ing its G0/1 interface to use DHCP to learn the IP address to use on the interface, using the

ip address dhcp command.

Example G-14 Learning an Address and Default Static Route with DHCP

R1# configure terminal

R1(config)# interface gigabitethernet0/1

R1(config-if)# ip address dhcp

R1(config-if)# end

R1#

R1# show ip route static

! Legend omitted


S* 0.0.0.0/0 [254/0] via 192.0.2.1

The end of the example shows the default route added to R1’s routing table as a result of

learning a default gateway address of 192.0.2.1 from DHCP. Oddly, the route shows this

route as a static route, although the route is learned dynamically. In fact, the only difference

in the output of the show ip route static command in this case, versus the case with the

static route as configured with the ip route command, is the administrative distance of 254.

IOS uses a default administrative distance of 1 for static routes configured with the ip route

configuration command (as seen in Example G-12). When adding a route to the default

gateway, as learned with DHCP, IOS uses a default administrative distance of 254, as shown

here in Example G-14.

Finally, not shown in the example, the remote routers still learn the route, so R1 also needs

the default-information originate RIP subcommand. On those remote routers like B01 and

B02, there is no difference in the show command output for the default routes.

Troubleshooting RIPv2Welcome to this fourth and final major section of this chapter, a section that focuses on

troubleshooting RIPv2.

Troubleshooting on the ICND1, ICND2, and CCNA R&S exams requires thinking about

question types. First, Sim questions typically begin with a broken configuration, and to

answer the question, you must reconfigure one or more devices to fix the configuration. To

prepare for Sim questions, you need to master the art of correct configuration so that you

quickly recognize the incorrect configuration. The earlier topics have already covered con-

figuration in some depth.

However, Simlet questions make you exercise your verification and troubleshooting skills,

and this final section hopes to focus more on those skills. With Simlet questions, you do


see a Simulator where you can issue commands. The lab may or may not be configured cor-

rectly, and you may or may not be able to see the configuration.

To be ready for these types of questions, you must be ready to predict what different

symptoms would be when a network was misconfigured in a particular way, and what the

available show commands would look like. With RIPv2, that primarily means that you have

to think about the output of the show ip route and show ip protocols commands, in cases

of misconfiguration, which is the focus of this section.

This section examines the most common misconfiguration items with the RIP settings

included in this chapter, with a discussion of typical symptoms. The end of the section sum-

marizes these common issues for easier review and study.

All the examples in this section use the same small sample topology, specifically chosen as a

good backdrop to discuss the issues in this section. Figure G-18 shows the topology, inter-

faces, and IP subnets, and Example G-15 that follows shows the correct configuration on

both routers.

192.168.1.96/27 192.168.12.0/27 192.168.12.96/27

R1 R2G0/1 G0/2 G0/1G0/2

Figure G-18 Sample Network for Troubleshooting Examples

Example G-15 R1 and R2 Correct RIPv2 Configuration

! Configuration on router R1

interface G0/1

ip address 192.168.1.101 255.255.255.224

interface G0/2

ip address 192.168.12.1 255.255.255.224

!

router rip

version 2

no auto-summary

network 192.168.1.0

network 192.168.12.0

! Configuration on router R2

interface G0/1

ip address 192.168.12.102 255.255.255.224

interface G0/2

ip address 192.168.12.2 255.255.255.224

!

router rip

version 2

network 192.168.12.0

G


Symptoms with Missing and Incorrect network CommandsAs a first example, think about the symptoms that should occur when a router is missing a

network command, or the command was incorrect so that it does not match an interface or

interfaces. Basically, two things happen:

■ The router does not advertise about the subnets on those interfaces.

■ The router does not exchange routing information with other routers on those interfaces.

Imagine that the engineer left out either of the network commands on Router R1 in

Example G-15. Can you imagine the results? Can you predict the output of the show ip

protocols command—the one command that reveals RIPv2 configuration details other than

the show running-config command?

Consider the omission of the network 192.168.12.0 command from R1 first. Per the figure,

that means that R1 would not enable RIPv2 on the link between R1 and R2, so that R1 and

R2 would not exchange RIPv2 routes at all. The show ip protocols command on R1 would

not list R2 as an information source, and that output on R1 would omit R1’s G0/2 interface in

the list of interfaces. Example G-16 shows those facts with R1’s show ip protocols command

output for just this example with a missing network 192.168.12.0 command.

Example G-16 show ip protocols on R1 with Missing network 192.168.12.0 Command







Redistributing: rip





Maximum path: 4


192.168.1.0




In particular, note that R1 lists only one interface in its list of interfaces (G0/1) and no infor-

mation sources, per the highlighted sections. It also lists only one network under the head-

ing “Routing for Networks,” which is the section that lists the networks in the configured

network commands.

Now think about the opposite case, in which the configuration of Example G-17 omits the

network 192.168.1.0 command but includes the network 192.168.12.0 command. In that

case, R1 and R2 will communicate with RIP. But what symptoms might you expect to see?

In this case, R1 will not advertise about the subnet off R1’s G0/1 interface, 192.168.1.96/27,

because there is no network command matching that interface. As a result, R2 will not list


192.168.1.96/27 in its IP routing table, and R1 will not list its interface G0/1 in the list of

enabled interfaces in the output of the show ip protocols command. Example G-17 shows

the output from show ip protocols on R1 in this case .

Example G-17 show ip protocols on R1 with Missing network 192.168.1.0 Command







Redistributing: rip





Maximum path: 4


192.168.12.0



192.168.12.2 120 00:00:12


Again, this example provides another chance to practice an important troubleshooting skill:

to re-create the RIP configuration’s network commands based on the output of the show

ip protocols command. The section with the heading “Routing for Networks” in Example

G-17 lists only 192.168.12.0, meaning that only the network 192.168.12.0 command

exists. Earlier, Example G-16 lists only 192.168.1.0, meaning only the network 192.168.1.0

command exists in that case.

Issues Related to Passive InterfacesThe passive-interface command has a basic and useful function, but when used incorrectly,

not only does it cause problems but the problem has an odd symptom. The passive interface

feature should never be used on an interface that connects to another RIP router. However,

if one router is incorrectly configured to be passive, whereas a neighboring router is not, the

passive router still hears RIP updates, while the correctly configured router does not hear

updates.

For example, imagine you start with the correct configuration shown in Example G-15

again. Then imagine that the engineer misreads the figure and adds a passive-interface g0/2

command to R1, instead of a passive-interface g0/1 command (which would make sense).

Figure G-19 shows the results: R1 quits sending RIP updates to R2, but R2 keeps sending

them to R1. R1 has the incorrect configuration but R2 suffers.

G


Passive

R1 R2

G0/1 G0/2 G0/1G0/2

I Heard Nothing.

I Heard an Update!

RIP

RIP

Figure G-19 One-Way Exchange of Routes with Incorrect Passive Interface

Anytime you see a RIP problem in which routes are not being learned, make sure to check

for passive interfaces in the configuration and with show ip protocols.

Issues Related to auto-summaryThe earlier section “Understanding Autosummarization and Discontiguous Classful

Networks” discussed when a network needs to disable the use of autosummarization. But

when you do need to disable automatic summarization, there is one common mistake: to

put the no auto-summary command on the wrong router.

As noted earlier, the auto-summary and no auto-summary setting only impacts routers

that connect directly to multiple classful networks. In the small network topology used in

this troubleshooting section (Figure G-18), R1 directly connects to subnets of two different

classful networks, so a no auto-summary command would affect its operation. R2 does not

connect to subnets of two different classful networks, so a no auto-summary command on

R2 would have no effect.

First, the original configuration in Example G-15 lists a no auto-summary command on

Router R1. As a result, R1 does not perform automatic summarization, instead advertising

a route for 192.168.1.96/27 to R2. Example G-18 shows that route on Router R2 in the top

half of the example.

Example G-18 show ip route rip on R2 with Different auto-summary Settings on R1

! R2's RIP route with no auto-summary configured on R1 (per Example G-12)

R2# show ip route | section 192.168.1.0

! lines omitted for brevity

192.168.1.0/27 is subnetted, 1 subnets

R 192.168.1.96 [120/1] via 192.168.12.1, 00:00:16, Serial0/0/1

! R2's RIP route with auto-summary configured on R1


! lines omitted for brevity

R 192.168.1.0/24 [120/1] via 192.168.12.1, 00:00:03, Serial0/0/1

Now consider the mistake of putting the no auto-summary command on R2 in this case and

leaving R1 with a default setting of auto-summary. In this case, the design does not create a

discontiguous classful network, so no real problem exists. However, changing R1 to use the

auto-summary setting would cause R2 to learn a different route: R1 would advertise a route

for Class C network 192.168.1.0/24 to R2, instead of a route for subnet 192.168.1.96/27.

The second half of Example G-18 shows that route, after that change to the configurations

of R1 and R2 .


RIP Issues Caused by Other Router FeaturesRIP could be configured perfectly but still not work due to other issues. This last part of the

troubleshooting section takes a brief look at these other features.

First, RIP operates only on working interfaces; that is, interfaces that are in an up and up

status per the show interfaces and show ip interfaces commands. Chapter 17, “Operating

Cisco Routers,” and Chapter 24, “Troubleshooting IPv4 Routing,” go into more detail about

issues that can cause an interface to fail.

RIP requires that all neighbors on a link be in the same subnet. That makes good sense from

an IP design perspective, but it is the kind of error that can be easily created for an exam

question. For instance, Example G-15—the correct configuration to go along with Figure

G-15 and the examples in this troubleshooting section—shows R1 and R2 with addresses

192.168.12.1 and 192.168.12.2, both with mask /27 on their shared Ethernet link. If R2 had

been misconfigured with 192.168.12.202/27, R1’s 192.168.12.1/27 address/mask would

be in a different subnet, and R1 and R2 would ignore the RIP updates received from each

other.

Finally, you have not yet gotten to the chapters about access control lists (ACL) yet

(Chapter 25, “Basic IPv4 Access Control Lists,” and Chapter 26, “Advanced IPv4 Access

Control Lists,” cover ACLs), but ACLs act as packet filters. The router can watch packets as

they pass through the router, match the packet header, and decide to filter some packets

based on those matches. The ACL configuration could inadvertently match and discard RIP

messages. As it turns out, the RIP uses UDP as a transport protocol, with well-known UDP

port 520. Any ACL that matches and discards these packets cause the symptom in which

the RIP configuration is correct but the routers do not hear from each other.

Summary of RIP Troubleshooting IssuesSummarizing the troubleshooting issues mentioned in this section, for easier review and study:

Step 1. The RIP network command controls where RIP operates. If a missing network

command fails to enable RIP on an interface:

A. RIP will not advertise about that connected subnet.

B. RIP will not send advertisements out that interface or process received

advertisements in that interface.

Step 2. The passive-interface command should not be used for interfaces that connect

to other routers. If configured, the passive router does not advertise routes to

neighboring routers, even though the passive router can still learn RIP routes

from RIP messages entering that interface.

Step 3. The no auto-summary command has an impact only on routers that directly

connect to more than one classful network. However, the command is needed

only if a discontiguous classful network exists.

Step 4. Some non-RIP features impact RIP operation, namely

A. Interfaces must be working for RIPv2 to use the interfaces.

B. Two routers on the same link must have IP addresses in the same subnet for

RIPv2 to exchange routing information.

C. Note that ACLs can filter RIP update messages and therefore break RIP .

G


Chapter ReviewOne key to doing well on the exams is to perform repetitive spaced review sessions. Review this chapter’s material using either the tools in the book, DVD, or interactive tools for the same material found on the book’s companion website. Refer to the “Your Study Plan” ele-ment for more details. Table G-4 outlines the key review elements and where you can find them. To better track your study progress, record when you completed these activities in the second column.

Table G-4 Chapter Review Tracking

Review Element Review Date(s) Resource Used

Review key topics Book, DVD/website

Review key terms Book, DVD/website

Repeat DIKTA questions Book, PCPT

Review memory tables Book, DVD/website

Review config checklists Book, DVD/website

Do labs Blog

Review command tables Book

Review All the Key TopicsTable G-5 Key Topics for Appendix G

Key Topic Element

Description Page Number

List Comparisons of IGPs 4

Table G-1 A list of RIPv2 features in comparison to RIPv1 9

Figure G-8 Matching logic on one router with the RIP network command 10

List The three actions RIP takes when enabled on an interface 10

Table G-2 RIP verification commands 13

List Analysis of the contents of a single IP route in the output of the show ip route command

14

Table G-3 List of administrative distance values 16

List Common troubleshooting issues 32

Key Terms You Should Knowadministrative distance, distance vector, exterior gateway protocol (EGP), interior gateway protocol (IGP), metric, routing update, contiguous network, discontiguous network, auto-summarization, passive interface, IP routing table, hop count

Command ReferencesTables G-6 and G-7 list configuration and verification commands used in this chapter. As an easy review exercise, cover the left column in a table, read the right column, and try to recall the command without looking. Then repeat the exercise, covering the right column, and try to recall what the command does.


Table G-6 Appendix G Configuration Command Reference

Command Description

router rip Global command that moves the user into RIP configuration mode

network network-number RIP subcommand that lists a classful network number, enabling RIP on all of that router’s interfaces in that classful network

version 2 RIP subcommand that sets the RIP version

passive-interface {interface-type interface-number}

RIP subcommand that tells RIP to no longer advertise RIP updates on the listed interface

passive-interface default RIP subcommand that changes the default setting on RIP-enabled interfaces to be passive instead of not passive

no passive-interface {interface-type interface-number}

RIP subcommand that overrides a default passive setting per the passive-interface default command for the listed interface

[no] auto-sunmmary RIP subcommand that toggles on (auto-summary) and off (no auto-summary) the autosummarization feature of RIP

maximum-paths number RIP subcommand that sets the number of equal-metric routes for the same subnet that RIP will add to the IP routing table

default-information originate

RIP subcommand that causes RIP to advertise a default route—a route for prefix 0.0.0.0, mask 0.0.0.0—if the local router has a default route in its routing table already

ip address dhcp Interface subcommand that causes a router to act as a DHCP client, learning the IPv4 address to use on the interface and dynamically learning a default route that uses the DHCP-announced default gateway address as the next-hop IP address in a static route

Table G-7 Appendix G EXEC Command Reference

Command Purpose

show ip interface brief Lists one line per router interface, including the IP address and interface status; an interface must have an IP address and be in an “up and up” status before RIP begins to work on the interface

show ip route [rip] Lists the routing table, including RIP-learned routes, and optionally just RIP-learned routes

show ip route ip-address Lists details about the route the router would match for a packet sent to the listed IP address

show ip protocols Lists information about the RIP configuration, plus the IP addresses of neighboring RIP routers from which the local router has learned routes

show ip rip database Lists one line per router interface, including the IP address and interface status; an interface must have an IP address and be in an “up and up” status before RIP begins to work on the interface

learning ipv4 routes with ripv2 -...

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