the routing table: a closer look

© 2007 Cisco Systems, Inc. All rights reserved. Cisco PublicITE PC v4.0Chapter 1 1

The Routing Table: A Closer Look

Routing Protocols and Concepts – Chapter 8

Modified by Tony Chen

04/01/2008

ITE PC v4.0Chapter 1 2© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public

Notes: If you see any mistake on my PowerPoint slides or if

you have any questions about the materials, please feel free to email me at [email protected].

Thanks!

Tony Chen

College of DuPage

Cisco Networking Academy


Objectives Describe the various route types found in the routing

table structure

Describe the routing table lookup process.

Describe routing behavior in routed networks.


Introduction

Chapter Focus– Structure of the routing table

•Will examine the format of the routing table and learn about level 1 and level 2 routes.

– Lookup process of the routing table

– Classless and classful routing behaviors

Cisco IP Routing, by Alex Zinin (ISBN 0-201-60473-6).


Routing Table Structure Lab Topology

3 router setup-R1 and R2 share a common 172.16.0.0/16 network with 172.16.2.0/24 subnets.

-R2 and R3 are connected by the 192.168.1.0/24 network.

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

In a later section, we will configure the interfaces for R2


Routing Table Structure The figure shows routing table entries come from the

following sources

-Directly connected networks

-Static routes

-Dynamic routing protocols


Routing Table Structure The figure shows what happens as the Serial 0/0/1 interface for R2 is

configured with the 192.168.1.1/24 address. – R1 and R3 already have their interfaces configured with the appropriate IP addresses and subnet masks.

–We will now configure the interfaces for R2 and use debug ip routing to view the routing table process that is used to add these entries.

As soon as the “no shutdown” command is issued the route is added to routing table

debug ip routing


Routing Table Structure Cisco IP routing table

is a hierarchical structure

–The reason for this is to speed up lookup process

–The hierarchy includes several levels.

•level 1

•level 2


Routing Table Structure Level 1 Routes

–Have 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 for class C networks, such as the 192.168.1.0 network.

Level 1 route can function as

–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.

The source of the level 1 route can be a directly connected network, static route, or a dynamic routing protocol.


Routing Table Structure The level 1 route 192.168.1.0/24 can be further defined as an

ultimate route.ultimate route includes either:

-A next-hop ip address (another path) OR

-An exit interface The directly connected network 192.168.1.0/24

–It is a level 1 network route because it has a subnet mask that is the same as its classful mask.

–This same route is also an ultimate route because it contains the exit interface Serial 0/0/1.


A parent route is a level 1 route

–A parent route does not contain any next-hop IP address or exit interface information

When the 172.16.3.0 subnet was added to the routing table, another route, 172.16.0.0, was also added.

–The first entry, 172.16.0.0/24, does not contain any next-hop IP address or exit interface information.

–This route is known as a level 1 parent route.

–A parent route is actually a heading that indicates the presence of level 2 routes, also known as child routes.

Parent and Child Routes


Routing Table Structure A level 1 parent route is automatically

created any time a subnet is added to the routing table.

–In other words, a parent route is created whenever a route with a mask greater than the classful mask is entered into the routing table.

–172.16.0.0/24 is subnetted, 1 subnets

A level 2 route is a route that is a subnet of a classful network address.

– Child routes are level 2 routes

– Child routes are a subnet of a classful network address

–C 172.16.3.0 is directly connected, FastEthernet0/0


Routing Table Structure The parent route contains the 172.16.0.0 - The classful network

address for our subnet.

Level 2 child routes contain 172.16.3.0, route source & 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 (subnet) is the /24 mask included in its parent route, 172.16.0.0

Level 2 child routes are also considered ultimate routes–Reason: they contain the next hop address &/or exit interface


Routing Table Structure The figure shows the

configuration of the Serial 0/0/0 interface on R2.

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

•Both 172.16.2.0 and 172.16.3.0 are members of the same parent route,

•because they are both members of the 172.16.0.0/16 classful network


Routing Table Structure Both child routes have the same subnet mask

-This means the parent route maintains the /24 mask

Note: If there is only a single level 2 child route and that route is removed, the level 1 parent route will be automatically deleted. A level 1 parent route exists only when there is at least one level 2 child route.

The role of the parent route will be examined when we discuss the route lookup process.


Routing Table Structure In classless networks, child routes do not have to share

the same subnet mask–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.


Routing Table Structure

Parent & Child Routes: Classless Networks


Routing Table Structure Parent & Child Routes: classful and classless Networks

NetworkType

Parent route’s

Classful mask is

Displayed

Term variably

subnetted

is seen in parent

route in routing table

Includes the # of different masks of

child routes

Subnet mask

included with each child route

entry

Class-ful

No No No No

Class-less

Yes Yes Yes Yes

classful

classless


Routing Table Lookup Process The Route Lookup Process

1. Examine level 1 routes

• If best match a level 1 ultimate route and is not a parent route this route is used to forward packet

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

2. Router examines level 2 (child) routes

• If there is a match with level 2 child route then that subnet is used to forward packet

• If no match then proceed to Step 3

3. Router determines classful or classless routing behavior

• If classful then packet is dropped

• If classless then router searches level one supernet and default routes

4. If there exists a level 1 supernet or default route match then Packet is forwarded

5. If not packet is dropped


Routing Table Lookup Process Longest Match: Level 1 Network Routes

–Best match is also known as the longest match

–The best match is the one that has the most number of left most bits matching between the destination IP address and the route in the routing table.

For example, in the figure we have a packet destined for 172.16.0.10. Many possible routes could match this packet. Three possible routes are shown that do match this packet: 172.16.0.0/12, 172.16.0.0/18, and 172.16.0.0/26. Of the three routes, 172.16.0.0/26 has the longest match.


Routing Table Lookup Process Finding the subnet mask

used to determine the longest match

Scenario:

–PC1 pings 192.168.1.2

–Router examines level 1 route for best match

–There exist a match between192.168.1.2 & 192.168.1.0 / 24

–Router forwards packets out s0/0/0


Routing Table Lookup Process The process of matching

–1st there must be a match made between the parent route & destination IP

•If a match is made then an attempt at finding a match between the destination IP and the child route is made. •Do at least 16 of the left-most bits of the parent route match the first 16 bits of the packet's destination IP address of 192.168.1.2?

–The answer, no,


Routing Table Lookup Process Finding a match between packet’s destination IP address

and the next route in the routing table–The figure shows a match between the destination IP of 192.168.1.0 and the level one IP of 192.168.1.0 / 24 then packet forwarded out s0/0/0

–Not only does the minimum of 24 bits match, but a total of 30 bits match, as shown in the figure.


Routing Table Lookup Process In the example in the figure, PC1 sends a ping to PC2

at 172.16.3.10. What happens when there is a match with a level 1 parent route?

Before level 2 child routes are examined

-There must be a match between classful level one parent route and destination IP address.


Routing Table Lookup Process After the match with parent route has been made Level 2 child

routes will be examined for a match

-Route lookup process searches for child routes with a match with destination IP


Routing Table Lookup Process How a router finds a match with one of the level 2

child routes–First router examines parent routes for a match

–If a match exists then:

•Child routes are examined

•Child route chosen is the one with the longest match

First, the router examines the parent route for a match.

The router checks the last child route for 172.16.3.0/24 and finds a match. The first 24 bits do match. The routing table process will use this route, 172.16.3.0/24, to forward the packet with the destination IP address of 172.16.3.10 out the exit interface of Serial 0/0/0.

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


Routing Table Lookup Process

Example: Route Lookup Process with VLSM

-The use of VLSM does not change the lookup process

-If there is a match between destination IP address and the level 1 parent route then

-Level 2 child routes will be searched


Routing Behavior Classful & classless routing protocols

Influence how routing table is populated

Classful & classless routing behaviors

Determines how routing table is searched after it is filled


Routing Behavior Classful Routing

Behavior: no ip classless

What happens if there is not a match with any level 2 child routes of the parent?

-Router must determine if the routing behavior is classless or classful

-If router is utilizing classful routing behavior then

-Lookup process is terminated and packet is dropped ip classless and no ip classless


Using the ip classless command (cont.)

What is IP Classless?

The "ip classless" command prevents the existence of a single "subnet" route from blocking access via the default route to other subnets of the same old-style network. Default only works with single-homed ISPs.

RFC 1879

IP classless command is not easy to understand, we know that. But I bet, after you read the following lines, you will understand what it is all about.

First, you must understand a very simple logic. Here is the logic: Me and you are on a journey. If you break my leg, then you must carry me all the way! If you understand this logic, you will understand "IP classless".

RIP is telling you: I am classful, if you break my class, then you have to show me every route there is, or I will drop your packet. I will drop it even though there is a default route (0.0.0.0).

What is classful? Classful means that a class A subnet should be shown as x.0.0.0 such as 10.0.0.0 255.0.0.0

If you show it as 10.44.0.0 255.255.0.0, you are breaking its class.

Or, a class B subnet should be shown as x.x.0.0 255.255.0.0 such as 172.29.0.0 255.255.0.0

If you show it as 172.29.26.0 255.255.255.0, you are breaking its class.

Let’s assume RIP knows about 10.0.0.0

If you break 10.0.0.0 into three, for example to 10.1.0.0 and 10.2.0.0 and 10.3.0.0, and then give RIP a packet with a destination of 10.4.0.1, RIP will drop it. Why? Why doesn’t RIP send the packet to the default route?

Because RIP told you, if you break my class, then you have to show me every damn route, otherwise I will drop it. Here, you broke RIP's class so you must show him the way to 10.4.0.1 and every other 10.x.x.x route in the universe. Otherwise RIP will drop the packet, even if there is a default route. RIP will not care about your default route or last resort gateway; it will drop your packet.

How do you ask RIP not to drop your packet and send the unknown destinations to the default route, although you have been so mean to him and have broken its class? You tell him: please, please, ip classless!

If no ip classless, drop the packet

If ip classless, send the packet to the default.

http://www.networkking.net/out/IPClassless.htm


Routing Behavior ip Classless

Beginning with IOS 11.3, “ip classless” was configured by default

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

Classless routing behavior works for

-Discontiguous networks

And

-CIDR supernets


Routing Behavior Classful Routing Behavior – Search Process

–when classful routing behavior is in effect (no ip classless) the process will not continue searching level 1 routes in the routing table. If a packet doesn't match a child route for the parent network route, then the router drops the packet.

R2 receives a packet destined for PC3 at 172.16.4.10.

–Even with the default route configured.

–The destination’s subnet mask is a /24 and none of the child routes left most bits match the first 24 bits. This means packet is dropped

“no ip classless”


Routing Behavior Classful Routing Behavior – Search

Process

The reason why the router will not search beyond the child routes

At the beginning of the Internet's growth, networks were all classfulThis meant an organization could subnet a major network address and “enlighten” all the organization’s routers about the subnettingTherefore, if the subnet was not in the routing table, the subnet did not exist and packet was dropped

The routing table process will not use the default route, 0.0.0.0/0, or any other route.



Routing Behavior The routing table process will not

use the default route, 0.0.0.0/0, or any other route.

A common error is to assume that a default route will always be used if the router does not have a better route.

In our example, R2's default route is not examined nor used, although it is a match.

This is often a very surprising result when a network administrator does not understand the difference between classful and classless routing behavior.



Routing Behavior Classless Routing Behavior-

ip lassless

Step 3: If classless routing behavior in effect then, continue searching level 1 supernet routes in the routing table for a match, including the default route, if there is one.

Step 4: Match with supernet or defaultSupernet routes Checked first

–If a match exists then forward packet

Default routes Checked second

Step 5: If there is no match or no default route then the Packet is dropped

“ip classless”


Routing Behavior Classless Routing Behavior – Search Process

Router begins search process by finding a match between destination IP and parent route

After finding the above mentioned match, then there is a search of the child route

There is no match with the level 2 child routes.

“ip classless”


Routing Behavior If no match is found in child routes of

previous slide then

–Router continues to search the routing table for a match that may have fewer than 16 bits in the match

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

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

“ip classless”


Routing BehaviorS* 0.0.0.0/0 is directly connected, Serial0/0/1

The mask is /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.

–In this case the router will use the default route, because it is the best match. The packet will be forwarded out the Serial 0/0/1 interface.

“ip classless”


Routing Behavior

What does R3 do with return traffic back to PC2 at 172.16.2.10?

In this case, R3 uses the 172.16.0.0/16 child route and forwards the traffic out Serial 0/0/1 back to R2.


Routing Behavior Classful vs. Classless Routing Behavior

-It is recommended to use classless routing behavior

Reason: so supernet and default routes can be used whenever needed


Longest Match http://www.cisco.com/warp/public/105/21.html

Let's look at the three routes we just installed in the routing table, and see how they look on the router.

router# show ip route ....

D 192.168.32.0/26 [90/25789217] via 10.1.1.1 ---- (192.168.32.0 to 192.168.32.63)

R 192.168.32.0/24 [120/4] via 10.1.1.2 ---- (192.168.32.0 to 192.168.32.255)

O 192.168.32.0/19 [110/229840] via 10.1.1.3 ---- (192.168.32.0 to 192.168.63.255)

....

If a packet arrives on a router interface destined for 192.168.32.1, which route would the router choose?

If a packet arrives on a router interface destined for 192.168.32.100, which route would the router choose? Answers are on the next page

POP QUIZ


Longest Match http://www.cisco.com/warp/public/105/21.html

Let's look at the three routes we just installed in the routing table, and see how they look on the router.

router# show ip route .... D 192.168.32.0/26 [90/25789217] via 10.1.1.1 ---- (192.168.32.0 to 192.168.32.63)R 192.168.32.0/24 [120/4] via 10.1.1.2 ---- (192.168.32.0 to 192.168.32.255)O 192.168.32.0/19 [110/229840] via 10.1.1.3 ---- (192.168.32.0 to 192.168.63.255)....

If a packet destined to 192.168.32.1 is directed toward 10.1.1.1, because 192.168.32.1 falls within the 192.168.32.0/26 network (192.168.32.0 to 192.168.32.63). It also falls within the other two routes available, but the 192.168.32.0/26 has the longest prefix within the routing table (26 bits verses 24 or 19 bits).

if a packet destined for 192.168.32.100 arrives on one of the router's interfaces, it's forwarded to 10.1.1.2, because 192.168.32.100 doesn't fall within 192.168.32.0/26 (192.168.32.0 through 192.168.32.63), but it does fall within the 192.168.32.0/24 destination (192.168.32.0 through 192.168.32.255). Again, it also falls into the range covered by 192.168.32.0/19, but 192.168.32.0/24 has a longer prefix length


SummaryContent/structure of a routing table Routing table entries

-Directly connected networks

-Static route

-Dynamic routing protocols

Routing tables are hierarchical

-Level 1 route

Have a subnet mask that is less than or equal to classful subnet mask for the network address

-Level 2 route

These are subnets of a network address


SummaryRouting table lookup process Begins with examining level 1 routes for best match with packet’s destination IP

If the best match = an ultimate route then-Packet is forwarded -Else--Parent route is examined

If parent route & destination IP match then Level 2 (child) routes are examinedLevel 2 route examination If a match between destination IP and child route found then

Packet forwarded -Else If Router is using classful routing behavior then Packet is dropped -Else

If router is using classless routing behavior thenRouter searches Level 1 supernet & default routes for a match

If a match is found then Packet if forwarded -Else Packet is dropped


Summary Routing behaviors

-This refers to how a routing table is searched

Classful routing behavior

-Indicated by the use of the no ip classless command

-Router will not look beyond child routes for a lesser match

Classless routing behavior

-Indicated by the use of the ip classless command

-Router will look beyond child routes for a lesser match

the routing table: a closer look

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