Routing Protocols and Concepts – Chapter 8 pptx

Parent and Child Routes A parent route is a level 1 route Parent and Child Routes – A parent route does not contain any next-hop IP address or exit interface information When the 17

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The Routing Table: A

Closer Look

Routing Protocols and Concepts – Chapter 8

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Describe the various route types found in the routing esc be e a ous ou e ypes ou d e ou g

table structure

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– 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)

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

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Routing Table Structure

3 router setup

-R1 and R2 share a common 172.16.0.0/16 network with

172 16 2 0/24 subnets 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 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

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The figure shows routing table entries come from the

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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 g

debug ip routing

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Level 1 Routes

–Have a subnet mask equal to or less than the classful q

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

The source of the level 1 route can be a directly

connected network , static route , or a dynamic routing

protocol.

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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

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Parent and Child Routes

A parent route is a level 1 route

Parent and Child Routes

– 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.

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.

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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 i b tt d 1 b t

–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

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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

– 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

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configuration of the Serial

0/0/0 interface on R2

– The routing table shows 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

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-This means the parent route maintains the /24 mask

Note: If there is only a single level 2 child route and that route is

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

child route.

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

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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

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.

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R ti T bl St t

Parent & Child Routes: Classless Networks

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Parent & Child Routes: classful and classless Networks Parent & Child Routes: classful and classless Networks

Network

Type

Parent route’s

Term

variably

Includes the # of

Subnet mask

classful

Classful mask is Displayed

subnetted

is seen in parent route in routing

different masks of child routes

included with each child route entry routing

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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

• 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 R t d t i l f l

3 Router determines classful or

classless routing behavior

• If classful then packet is dropped

• If classless then router searches level

• 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

f d d forwarded

5 If not packet is dropped

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R ti T bl L k P

Best match is also known as the longest match

– 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

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

172.16.0.0/26 has the longest match

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Finding the subnet mask

used to determine the

route for best match

– There exist a match

between192.168.1.2 &

192.168.1.0 / 24

– Router forwards packets out

s0/0/0

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– 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

• 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 p first 16 bits of the packet's destination IP address of 192.168.1.2?

– The answer, no,

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and the next route in the routing table

The figure shows a match between the destination IP of 192 168 1 0

– 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

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In the example in the figure, PC1 sends a ping to PC2 p g , p g

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

parent route and destination IP address.

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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

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

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The use of VLSM does not

-The use of VLSM does not

change the lookup process

-If there is a match between If there is a match between

destination IP address and the

level 1 parent route then

-Level 2 child routes will be

searched

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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

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What happens if there is pp

not a match with any

level 2 child routes of the

parent?

-Router must determine if

the routing behavior is g

classless or classful

-If router is utilizing classful

routing behavior then

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

terminated and

packet is dropped ip classless and no ip classless

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Using the ip classless command (cont.) g ( )

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

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 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!

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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 lookups by default

Classless routing behavior works for

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Routing Behavior “ no ip classless”

Classful Routing Behavior – Search Process

– when classful routing behavior is in effect (no ip

no ip classless

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

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The reason why the router will not search

beyond the child routes y

At the beginning of the Internet's

growth, networks were all classful

This meant an organization could

subnet a major network address and

“enlighten” all the organization’s

routers about the subnetting

Therefore, 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

route.

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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

d t d th diff b t

understand the difference between

classful and classless routing

behavior

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Ro ting Beha ior

Classless Routing

Behavior-“ ip classless”

g

ip lassless

Step 3: Step 3: If classless routing behavior in 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

match, including the default route, if there is

one.

Step 4: Match with supernet or default

Supernet 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

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