chapter 8 the routing table: a closer look cis 82 routing protocols and concepts rick graziani...

Chapter 8The Routing Table: A Closer Look

CIS 82 Routing Protocols and Concepts

Rick Graziani

Cabrillo College

[email protected]

Last Updated: 4/14/2008

2

Note My web site is www.cabrillo.edu/~rgraziani. For access to these PowerPoint presentations and other

materials, please email me at [email protected].

3

For further information This presentation is an

overview of what is covered in the curriculum/book.

For further explanation and details, please read the chapter/curriculum.

Book: Routing Protocols

and Concepts By Rick Graziani and

Allan Johnson ISBN: 1-58713-206-0 ISBN-13: 978-58713-

206-3

4

Topics The Routing Table Structure

Topology Routing Table Entries Level 1 Routes Parent and Child Routes:

Classful Networks Parent and Child Routes:

Classless Networks Routing Table Lookup Process

Steps in the Route Table Lookup Process

Longest Match: Level 1 Routes


Routing Behavior Classful and Classless

Routing Behavior Classful Routing Behavior: no

ip classless Classless Routing Behavior: ip

classless

The Routing Table Structure

6

Understanding the Routing Table

Understanding the structure and lookup process of the routing table Help you diagnose any routing table issue

Regardless of your level of familiarity with the routing protocol. Is the packet is being forwarded as expected? Why or why not? Forwarded elsewhere? Discarded? For more details: Cisco IP Routing, by Alex Zinin (ISBN 0-201-60473-6)

7

The Routing Table Structure

The structure of the routing table might seem obvious. Help you verify and troubleshoot routing issues because you will

understand the routing table lookup process. You will know exactly what the Cisco IOS software does when it

searches for a route.

Router# show ip route

172.16.0.0/24 is subnetted, 4 subnets

S 172.16.4.0 is directly connected, Serial0/0/1

R 172.16.1.0 [120/1] via 172.16.2.1, 00:00:08, Serial0/0/0

C 172.16.2.0 is directly connected, Serial0/0/0

C 172.16.3.0 is directly connected, FastEthernet0/0



C 192.168.1.0/24 is directly connected, Serial0/0/1

S 192.168.100.0/24 is directly connected, Serial0/0/1

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Notice that R3 also has a 172.16.4.0/24 subnet that is disconnected, or discontiguous, from the 172.16.0.0 network that R1 and R2 share.

The effects of this discontiguous subnet are examined later in this chapter when you look at the route lookup process.

Topology

9

Topology: Interface Configurations for R1 and R3R1(config)# interface FastEthernet0/0

R1(config-if)# ip address 172.16.1.1 255.255.255.0

R1(config-if)# no shutdown

R1(config-if)# interface Serial0/0/0


R1(config-if)# clock rate 64000


R3(config)# interface FastEthernet0/0



R3(config-if)# interface Serial0/0/1



10

Sample Routing Table Entries

Route entries from the following sources: Directly connected networks Static routes Dynamic routing protocols

The source of the route does not affect the structure of the routing table




R 172.16.1.0 [120/1] via 172.16.2.1, 00:00:08, Serial0/0/0







11

Sample Routing Table Entries

The routing table hierarchy in Cisco IOS software was originally implemented with the classful routing scheme.

Although the routing table incorporates both classful and classless addressing, the overall structure is still built around this classful scheme.




R 172.16.1.0 [120/1] via 172.16.2.1, 00:00:08, Serial0/0/0







12

Level 1 Routes

Serial 0/0/1 interface for R2 is configured with the 192.168.1.1/24 address As soon as no shutdown is entered, the output from debug ip routing

shows that this route has been added to the routing table.

R2# debug ip routing

IP routing debugging is on

R2# conf t

R2(config)# interface serial 0/0/1


R2(config-if)# clock rate 64000


R2(config-if)#

00:11:06: %LINK-3-UPDOWN: Interface Serial0/0/1, changed state to up

R2(config-if)#

RT: add 192.168.1.0/24 via 0.0.0.0, connected metric [0/0]

RT: interface Serial0/0/1 added to routing table

R2(config-if)# end

R2# undebug all

All possible debugging has been turned off

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Level 1 Routes

The routing table is actually a hierarchical structure that is used to speed up the lookup process when locating routes and forwarding packets.

Within this structure, the hierarchy includes several levels. For simplicity, we discuss all routes as one of two levels: level 1

or level 2.

RT: add 192.168.1.0/24 via 0.0.0.0, connected metric [0/0]

RT: interface Serial0/0/1 added to routing table

R2(config-if)# end

R2# show ip route

<output omitted>


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Level 1 Routes

A level 1 route is a route with a subnet mask equal to or less than the classful mask of the network address. 192.168.1.0/24 is a level 1 network route because the subnet mask is

equal to the network’s classful mask. /24 is the classful mask for Class C networks, such as the 192.168.1.0

network.

R2# show ip route

<output omitted>


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Level 1 Routes

A level 1 route can function as any of the following: Default route: A default route is a static route with the address 0.0.0.0/0. Supernet route: A supernet route is a network address with a mask less than

the classful mask. Network route: A network route is a route that has a subnet mask equal to that

of the classful mask. A network route can also be a parent route (next).

We will be using this chart throughout this chapter.

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Level 1 Routes

The level 1 route 192.168.1.0/24 can be further defined as an ultimate route.

An ultimate route is a route that includes one or both of the following: A next-hop IP address (another path) An exit interface

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Level 1 Routes

Directly connected network 192.168.1.0/24 is a level 1 network route - subnet mask that is the same as its

classful mask. ultimate route - contains the exit interface Serial 0/0/1.

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Parent and Child Routes: Classful Networks

Another type of level 1 network route, a parent route. When the 172.16.3.0 subnet was added to the routing table,

Another route, 172.16.0.0 also added. First entry: no next-hop IP address or exit interface information.

This route is known as a level 1 parent route.

R2(config)# interface fastethernet 0/0



R2(config-if)# end

R2# show ip route

Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile,

<text omitted>

Gateway of last resort is not set




Level 1 Parent RouteLevel 2 Child Route

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A parent route is a heading: Indicates the presence of level 2 routes, also known as child routes.

A level 1 parent route is automatically created any time a subnet is added to the routing table.

A parent route is created whenever a route with a mask greater than the classful mask is entered into the routing table. The subnet 172.16.3.0 is the level 2 child route of the parent route

172.16.0.0.

R2# show ip route

Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile,

<text omitted>

Gateway of last resort is not set





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A level 2 route is a route that is a subnet of a classful network address. Like a level 1 route, the source of a level 2 route can be a :

directly connected network static route dynamic routing protocol.

Note: This is the case even if a classless routing protocol is the source of the subnet route.


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Parent route: 172.16.0.0: The classful network address for our subnet. /24: The subnet mask for all the child routes. If the child routes have

variable-length subnet masks (VLSM), the subnet mask will be excluded from the parent route and included with the individual child routes. (later).

is subnetted, 1 subnets: This part of the route specifies that this is a parent route and in this case has one child route (that is, one subnet).

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Child route: C: The route code for a directly connected network. 172.16.3.0: The specific route entry. is directly connected: Along with the route code of C, this specifies that

this is a directly connected network with an administrative distance of 0. FastEthernet0/0: The exit interface for forwarding packets that match this

specific route entry.

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A level 2 child route contains the route source and the network address of the route.

Notice that the subnet mask is not included with the subnet, the level 2 child route.

The subnet mask for this child route is the /24 mask included in its parent route, 172.16.0.0.

Level 2 child routes are also considered ultimate routes because they contain the next-hop IP address or exit interface.

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Adding another child route

The routing table shows two child routes for the same 172.16.0.0/24 parent route.

172.16.2.0 and 172.16.3.0 are members both members of the 172.16.0.0/16 classful network.

Because both child routes have the same subnet mask, the parent route still maintains the /24 mask but now shows two subnets.

Later we will see the role of the parent route.

R2(config)# interface serial 0/0/0



R2(config-if)# end

R2# show ip route





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Adding another child route

If there is only a single level 2 child route and that route is removed, the level 1 parent route is automatically deleted.

A level 1 parent route exists only when there is at least one level 2 child route.

If child routes are deleted there is no parent route.

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Parent and Child Routes: Classless Networks

For this discussion, we switch briefly to the RouterX topology.

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All three subnets belong to the classful network 172.16.0.0/16 and are therefore level 2 child routes.

Notice that the child routes do not share the same subnet mask, as was the case in the classful example.

Implementing a network addressing scheme with VLSM. Whenever there are two or more child routes with different subnet masks

belonging to the same classful network, the routing table presents a slightly different view, which states that this parent network is variably subnetted.

RouterX# show ip route

172.16.0.0/16 is variably subnetted, 3 subnets, 2 masks



C 172.16.3.0/24 is directly connected, FastEthernet0/0

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The parent route of 172.16.0.0 now contains the classful mask /16. In the classful example shown earlier the classful mask was not displayed.

Parent route states that the child routes are variably subnetted. Includes the number of different masks of the child routes (2 masks). Each child route now contains the subnet mask for that specific route.

In the non-VLSM example both child routes shared the same subnet mask, and the parent displayed their common subnet mask.

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Parent route: 172.16.0.0: The parent route, the classful network address associated with

all child routes /16: The classful subnet mask of the parent route variably subnetted: States that the child routes are variably subnetted

and that there are multiple masks for this classful network 3 subnets, 2 masks: Indicates the number of subnets and the number of

different subnet masks for the child routes under this parent route

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Child route: C: The route code for a directly connected network 172.16.1.4: The specific route entry /30: The subnet mask for this specific route is directly connected: Along with the route code of C, specifies that this

is a directly connected network with an administrative distance of 0 Serial0/0/0: The exit interface for forwarding packets that match this

specific route entry

Routing Table Lookup Process

32

Routing Table Lookup Process

When a router receives a packet on one of its interfaces. The routing table lookup process compares the destination IP

address of the incoming packet with the entries in the routing table. The best match between the packet’s destination IP address and

the route in the routing table is used to determine to which interface to forward the packet.

IP Packet

Routing Table

Find “best match”

33

We have specifically chosen a classful routing protocol with our discontiguous 172.16.0.0 subnets.

The reason for this will become evident in a later section.

R1(config)# router rip

R1(config-router)# network 172.16.0.0








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Steps in the Route Table Lookup ProcessR1# show ip route




R 172.16.3.0 [120/1] via 172.16.2.2, 00:00:25, Serial0/0/0

R 192.168.1.0/24 [120/1] via 172.16.2.2, 00:00:25, Serial0/0/0

R2# show ip route


R 172.16.1.0 [120/1] via 172.16.2.1, 00:00:07, Serial0/0/0




R3# show ip route




Neither R1 nor R2 has a route to 172.16.4.0. Also, R3 does not have routes to subnets 172.16.1.0/24, 172.16.2.0/24, or

172.16.3.0/24.

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The Route Lookup Process

Step 1. The router examines level 1 routes, including network routes and

supernet routes, for the best match with the destination address of the IP packet

Routing Table

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Step 1a. If the best match is a level 1 ultimate route—a classful network,

supernet, or default route—this route is used to forward the packet

The Route Lookup Process Routing Table

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Step 1b. If the best match is a level 1 parent route, proceed to Step 2


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Step 2. The router examines child routes (the subnet routes) of the parent

route for a best match.


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Step 2a. If there is a match with a level 2 child route, that subnet is used to

forward the packet.


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Step 2b. If there is not a match with any of the level 2 child routes, proceed to

Step 3.


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Step 3. Is the router implementing classful or classless routing behavior.

(later) Router(config)# no ip classless Router(config)# ip classless


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Step 3a. If classful routing behavior is in effect, terminate the lookup

process and drop the packet. Router(config)# no ip classless


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Step 3b. If classless routing behavior is in effect, continue searching level

1 supernet routes in the routing table for a match, including the default route, if there is one.


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Step 4. If there is now a lesser match with a level 1 supernet or default

routes, the router uses that route to forward the packet.


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Step 5. If there is not a match with any route in the routing table, the router

drops the packet.


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Longest Match

(Digressing from topology) For there to be a match between the destination IP address of a packet

and a route in the routing table, a minimum number of leftmost bits must match between the IP address of the packet and the route in the routing table.

The subnet mask of the route in the routing table is used to determine the minimum number of leftmost bits that must match.

IP packet only contains the IP address and not the subnet mask.

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Longest Match

The best match or longest match is the route in the routing table that has the greatest number of leftmost matching bits with the destination IP address of the packet. The route with the greatest number of equivalent leftmost bits, or the

longest match, is always the preferred route. Subnet mask in routing table specifies the minimum number of leftmost

matching bits – the only bits that are considered.

All three routes match but Route 3 has the longest match.

48

Example: Level 1 Ultimate Route

49 PC1 sends a ping to 192.168.1.2, the serial interface on R3.


R1# show ip route




R 172.16.3.0 [120/1] via 172.16.2.2, 00:00:25, Serial0/0/0

R 192.168.1.0/24 [120/1] via 172.16.2.2, 00:00:25, Serial0/0/0

50

The router first examines level 1 routes for the best match.

R1# show ip route


R 192.168.1.0/24 [120/1] via 172.16.2.2, 00:00:25, Serial0/0/0

192.168.1.2

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There is a match between the destination IP address 192.168.1.2 and the level 1 ultimate route of 192.168.1.0/24.

R1 uses this route and forwards the packet out interface Serial 0/0/0.

R1# show ip route


R 192.168.1.0/24 [120/1] via 172.16.2.2, 00:00:25, Serial0/0/0

192.168.1.2

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Why is there a match with the 192.168.1.0/24 level 1 route and not with one of the 172.16.0.0 subnets?

Why is there not a match with any of the 172.16.0.0/24 subnets in the routing table?

192.168.1.2

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172.16.0.0/24 is a parent route of three subnets or child routes. Before a child route is examined for a match, there must be

at least a match between the destination IP address of the packet and the classful address of the parent route, or 172.16.0.0/16.

16 bits (/16) must match the route 172.16.0.0/16 – no match!

192.168.1.2

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The route, 192.168.1.0, is a level 1 ultimate route and, therefore, it also contains the subnet mask, /24.

Not only does the minimum of 24 bits match, but a total of 30 bits match.

Because there is not a longer, more specific match, the packet is forwarded out the exit interface Serial 0/0/0.

192.168.1.2

55 R1 forwards the packet out Serial0/0/0.


R1# show ip route




R 172.16.3.0 [120/1] via 172.16.2.2, 00:00:25, Serial0/0/0

R 192.168.1.0/24 [120/1] via 172.16.2.2, 00:00:25, Serial0/0/0

No match

Match

192.168.1.2

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Longest Match: Level 1 Parent and Level 2 Child Routes

57


Examine what happens when there is a match with a level 1 parent route.

Before any level 2 child routes are examined for a match, there must be a match between the classful address of the level 1 parent route and the destination IP address of the packet.

R1# show ip route




R 172.16.3.0 [120/1] via 172.16.2.2, 00:00:25, Serial0/0/0

R 192.168.1.0/24 [120/1] via 172.16.2.2, 00:00:25, Serial0/0/0

Match

172.16.3.10

58


PC1 sends a ping to PC2 at 172.16.3.10. R1 receives the packet and forwards it to R2.

59


PC1 sends a ping to PC2 at 172.16.3.10. R1 receives the packet and forwards it to R2.

172.16.3.10

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The first match that occurs is with the level 1 parent route, 172.16.0.0. With non-VLSM subnets, the classful mask of the parent is not

displayed. Before any child routes (subnets) are examined for a match, there must be

a match with the classful address of the parent route. Because 172.16.0.0 is a Class B address, 16 leftmost bits must match. 16 bits match the parent route, 172.16.0.0.

R1# show ip route




R 172.16.3.0 [120/1] via 172.16.2.2, 00:00:25, Serial0/0/0

R 192.168.1.0/24 [120/1] via 172.16.2.2, 00:00:25, Serial0/0/0

Match

172.16.3.10

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Because there is a match with the parent route, the level 2 child routes will be examined for a match.

The actual subnet mask of /24 is used for the minimum number of leftmost bits that must match.

R1# show ip route




R 172.16.3.0 [120/1] via 172.16.2.2, 00:00:25, Serial0/0/0

R 192.168.1.0/24 [120/1] via 172.16.2.2, 00:00:25, Serial0/0/0

172.16.3.10

Examine child routes

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The route lookup process searches the child routes for a match. In this case, there must be a minimum of 24 bits that match. Of the three subnets there is only one route that has 24 bits that

match: 172.16.3.0

R1# show ip route




R 172.16.3.0 [120/1] via 172.16.2.2, 00:00:25, Serial0/0/0

R 192.168.1.0/24 [120/1] via 172.16.2.2, 00:00:25, Serial0/0/0Match

172.16.3.10

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The router checks the last child route for 172.16.3.0/24 and finds a match.

The first 24 bits do match.

172.16.3.10

Match Parent

No match child

No match child

Match child

64

R1# show ip route




R 172.16.3.0 [120/1] via 172.16.2.2, 00:00:25, Serial0/0/0

R 192.168.1.0/24 [120/1] via 172.16.2.2, 00:00:25, Serial0/0/0

Match Parent

No match

No match

Match child

172.16.3.10

If this child route did not have an exit interface and only included a next-hop IP address, the next-hop IP address would need to be resolved to an exit interface.

The lookup process would need to start from the beginning, this time searching the routing table for the nexthop IP address.

What happens if the router does not have a route? In this scenario, it discards the packet.

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Example: Route Lookup Process with VLSM

66

Example: Route Lookup Process with VLSM

Packet’s destination IP Address: 172.16.1.5 Using VSLM does not change the lookup process. The only difference with VLSM is that child routes display their own specific

subnet masks. 16 bits match the parent route, 172.16.0.0. For there to be a match with the 172.16.1.4 child route, a minimum of 30

leftmost bits must match because the subnet mask is /30

RouterX# show ip route





172.16.1.5

Match ParentMatch child

Routing Behavior

68

Classful and Classless Routing Behavior

What happens when there is a match between the packet’s destination IP address and a level 1 parent route, but there is not a match with any of the level 2 child routes? We might assume the routing table lookup process continues looking for

a less-specific match in the routing table. However, you will see that this might or might not be the case

depending on the configuration of the router.

R2# show ip route


R 172.16.1.0 [120/1] via 172.16.2.1, 00:00:00, Serial0/0/0




S* 0.0.0.0/0 is directly connected, Serial0/0/1

Match Parent

No match

No match

No match

Now what?

172.16.4.10

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Classful and Classless Routing Behavior

Classless and classful routing behaviors are not the same as classless and classful routing protocols.

Classful and classless routing protocols affect how the routing table is populated.

Classful and classless routing behaviors determine how the routing table is searched after it is populated.

70

Topology Changes

Classful routing protocols such as RIPv1 do not support discontiguous networks.

Even though our current topology has discontiguous networks, we can configure static routes to reach those networks.

71

Topology ChangesR2(config)# ip route 0.0.0.0 0.0.0.0 s0/0/1


R2(config-router)# default-information originate

R2(config-router)# no network 192.168.1.0

R2(config-router)# end

R2# show ip route


R 172.16.1.0 [120/1] via 172.16.2.1, 00:00:00, Serial0/0/0





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Topology Changes

Notice that the static route for the same network as the parent route is a child route in the routing table.

This is because there is a child route for that parent. If there wasn’t a 172.16.4.0/24 child route, then this 172.16.0.0/16 static

route would be a level 1 ultimate route.

R3(config)# ip route 172.16.0.0 255.255.0.0 s0/0/1

R3(config)# no router rip

R3(config-router)# end

R3# show ip route





73

Step 3. Is the router implementing classful or classless routing behavior.

(later) Router(config)# no ip classless Router(config)# ip classless

Routing Behavior Routing Table

74

Classful Routing Behavior: no ip classless

What happens when there is a match with a parent, but none of the child routes match?

Before Cisco IOS Software Release 11.3, no ip classless was the default behavior for Cisco routers.

The command no ip classless means that the route lookup process uses classful routing table lookups by default.

R2# show ip route


R 172.16.1.0 [120/1] via 172.16.2.1, 00:00:00, Serial0/0/0





Match Parent

No match

No match

No match

Now what?

R1(config)# no ip classless



75

Classful Routing Behavior: Search Process

R2 receives a packet destined for PC3 at 172.16.4.10

76


The routing process searches the routing table and finds a 16-bit match with the parent route 172.16.0.0.

None of the child routes have 24 leftmost bits that match the 24 leftmost bits of the destination IP address 172.16.4.10.

R2# show ip route


R 172.16.1.0 [120/1] via 172.16.2.1, 00:00:00, Serial0/0/0





Match Parent

No match

No match

No match

Now what?

172.16.4.10

77

So, what happens next? Router R2 drops the

packet. Default route is never

checked or used.

Classful Routing Behavior: no ip classlessR2# show ip route


R 172.16.1.0 [120/1] via 172.16.2.1, 00:00:00, Serial0/0/0





Match Parent

No match

No match

No match

Now what?

172.16.4.10

DROP PACKET

78


Why does classful routing behavior perform like this? The general idea of classful routing behavior comes from the time when all

networks were of a classful nature. At the beginning of the Internet’s growth, an organization received a Class

A, Class B, or Class C major network address. When an organization had a classful IP major network address, that

organization would also administer all the subnets for that classful address. All routers belonging to the organization would know about all the subnets

for the major network. If a subnet was not in the routing table, the subnet did not exist

172.16.4.10

79

Classless Routing Behavior: ip classless

Remember: R2 receives a packet destined for PC3 at 172.16.4.10

172.16.4.10

80


Cisco IOS 11.3, Cisco changed the default routing behavior from classful to classless. ip classless command is configured by default. Classless routing behavior means that the routing process no longer assumes that all

subnets for a major classful network can be reached only within the child routes of the parent.

Classless routing behavior works well for discontiguous networks and classless interdomain routing (CIDR) supernets.

R2# show ip route


R 172.16.1.0 [120/1] via 172.16.2.1, 00:00:00, Serial0/0/0





Match Parent

No match

No match

No match

Now what?

R1(config)# ip classless



172.16.4.10

81

Step 3b. If classless routing behavior is

in effect, continue searching level 1 supernet routes in the routing table for a match, including the default route, if there is one.

Step 4. If there is now a lesser match

with a level 1 supernet or default routes, the router uses that route to forward the packet.

172.16.4.10

82

The 192.168.1.0/24 route does not have 24 leftmost bits that match the destination IP address.

/0, which means that zero or no bits need to match. A default route will be the lowest-bit match. In classless routing behavior, if

no other route matches, the default route will match.

R2# show ip route


R 172.16.1.0 [120/1] via 172.16.2.1, 00:00:00, Serial0/0/0





Match Parent

No match

No match

No match

Now what?

172.16.4.10


83

R2# show ip route


R 172.16.1.0 [120/1] via 172.16.2.1, 00:00:00, Serial0/0/0





Match Parent

No match

No match

No match

Now what?

172.16.4.10

Continue searching level 1 supernet routes. The default route is a match, so I is used to forward the packet.


84

Step 5. If there is not a match with any route in the routing table, the router

drops the packet.

172.16.4.10


85

What does R3 do with return traffic back to PC2 at 172.16.2.10? R3 uses the 172.16.0.0/16 child route and forwards the traffic out

Serial 0/0/1 back to R2

172.16.4.10


86


172.16.4.10


87

Classful vs. Classless Routing Behavior in the Real World

Remember that classful and classless routing behaviors are independent from classful and classless routing protocols.

A router could be configured with classful routing behavior (no ip classless) and a classless routing protocol, such as RIPv2.

A router could also be configured with classless routing behavior (ip classless) and a classful routing protocol, such as RIPv1.

In today’s networks, it is recommended to use classless routing behavior so that supernet and default routes can be used whenever needed.

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Topics The Routing Table Structure

Topology Routing Table Entries Level 1 Routes Parent and Child Routes:

Classful Networks Parent and Child Routes:

Classless Networks Routing Table Lookup Process




Routing Behavior Classful and Classless

Routing Behavior Classful Routing Behavior: no

ip classless Classless Routing Behavior: ip

classless

Chapter 8The Routing Table: A Closer Look

CIS 82 Routing Protocols and Concepts

Rick Graziani

Cabrillo College

[email protected]

Last Updated: 4/14/2008

chapter 8 the routing table: a closer look cis 82 routing protocols and concepts rick graziani...

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