Module 8

Routing Table: A Closer Look

Objectives

2

Describe the various route types found in the routing table structure

Describe the route lookup process

Describe the routing behavior in routed networks

The Routing Table Structure

Lab Topology

4

RIPv2 Static route

Routing Table Entries

Notice that the 172.16.0.0/24 subnets have a combination of all three types of routing sources:

Directly connected networks C

Static routes S

Dynamic routing protocols R 5

R2 routing table

Routes Levels

The Cisco IP routing table is not a flat database

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

For simplicity, we will discuss all routes as one of two levels: level 1 or level 2.6

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.

Level 1 Route Examples:

Default route: 0.0.0.0/0

Supernet Route: 192.168.0.0/22

Network Route: 192.168.1.0/24 (ie. Major network)

The source of the level 1 route can be a directly connected network, static route, or a dynamic routing protocol 7

Level 1 Routes

An ultimate route is a route that includes:

Either a next-hop IP address (another path)

And/or an exit interface

8

Parent and Child Routes: Classful Network

9

Parent and Child Routes: Classful Network

A level 1 parent route is a network route that does not contain a next-hop IP address or exit interface for any network (not an ultimate route)

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

Routing Table: Parent/Child Relationship

10

Parent and Child Routes: Classful Network

11

Level 2 child routes are also considered ultimate

routes because they will contain the next-hop IP

address and/or exit interface.

Parent and Child Routes: Classful Network

R2 Routing Table: Parent/Child Relationship

12

Ultimate Route

2

Parent and Child Routes: Classless Network

Level 2 Child Route

Level 1 Parent Route

13

Parent and Child Routes: Classless Network

Routing Table Lookup Process

Steps in the Route Lookup Process

16

Steps in the Route Lookup Process

Because of

discontigous

network, neither R1

nor R2 has a route

to 172.16.4.0.

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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Steps in the Route Lookup Process

Steps in the Route Lookup Process

Steps in the Route Lookup Process

Steps in the Route Lookup Process

Steps in the Route Lookup Process

Steps in the Route Lookup Process

Steps in the Route Lookup Process

Steps in the Route Lookup Process

Steps in the Route Lookup Process

Steps in the Route Lookup Process

Steps in the Route Lookup Process

Longest Match: Level 1 Network Routes

The best match or longest match is the route in the routing table that has the most number of left-most matching bits with the destination IP address

of the packet.

29

Longest Match: Level 1 Network Routes

30

Step 1: Examines level 1 routes, including network routes and supernet routes, for the best match with the destination address (192.168.1.2) of the IP packet

Longest Match: Level 1 Network Routes

172.16.0.0/16 Level 1 Parent Route

31

Longest Match: Level 1 Network Routes

32

33

Before any level 2 child routes are examined for a match, there must first be a match between

the classful address of the level 1 parent route and the destination IP address of the packet.

Longest Match: Level 1 Parent & Level 2

Child Routes

34

Longest Match: Level 1 Parent & Level 2

Child Routes

Step 1b: The best

match is a level 1

parent route,

proceed to Step 2

Step 2a: There is a match with a level 2 child route,

that subnet will be used to forward the packet.

Step 2: The router examines child routes (the subnet routes)

of the parent route for a best match

Longest Match: Level 1 Parent & Level 2

Child Routes

35

First 16 bits match

parent route First 24 bits do not

match -> skip

Routing Behavior

Classful & Classless Routing Behavior

Routing Protocols vs Routing Behaviors

37

Classful and classless routing behaviors are independent from classful and classless routing protocols.

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

Classful & Classless Routing Behavior

38

Classful Routing Behavior: no ip classless

Step 3: Is the router

implementing classful or

classless routing behavior?

Step 3a: If classful routing

behavior is in effect, terminate the

lookup process and drop the packet.

39

Classful:

Drop packet

Classful Routing Behavior: no ip classless

Prior to IOS 11.3, no ip classless was the default behavior for Cisco routers.

40

Classful Routing Behavior: Search Process

41

R2 now check the child route

First 24 bits do not

match -> skip

Classful Routing Behavior Search Process

42

Classless Routing Behavior: ip classless

43

All routers are configured with the ip classless command.

R1(config)#ip classless

R2(config)#ip classless

R3(config)#ip classless

Classless routing behavior works well for discontiguous networks and CIDR supernets

Steps in the Route Lookup Process

Step 3: Classful or classless routing behavior?

Step 3a: If classful routing

behavior is in effect,

terminate the lookup process

and drop the packet.

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.44

Steps in the Route Lookup Process

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.

Step 5: If there is not a match with any

route in the routing table, the router

drops the packet.

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Classless Routing Behavior: Search Process

46

R2 now check the child route

First 24 bits do not

match -> skip

Classless Routing Behavior Search Process

47

Good luck with this module!