CA Ex S2M8 the Routing Table

8/3/2019 CA Ex S2M8 the Routing Table

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Chapter 8:

The Routing Table: A Closer Look

CCNA Ex loration version 4.0

Please purchase apersonal license.


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Objectives

Describe the various route types found in the routingtable structure

Describe the routing table lookup process. Describe routing behavior in routed networks.

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Routing Table Structure



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Lab Topologies



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Routing table entries come 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. Although the routing table incorporates both classful and classless addressing,

the overall structure is still built around this classful scheme.


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Routing Table StructureRIP v1Classful address

RIP v2

Classful address


RIP v2

Classless address


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Level 1 Routes

As soon as the no shutdown command is issued the routeis added to routing table


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Cisco IP routing tableis a hierarchicalstructure

The reason for this


lookup process


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A level 1 route is a route with a subnet mask equal to or less thanthe classful mask of the network address. 192.168.1.0/24 is a level1 network route, because the subnet mask is equal to thenetwork's classful mask. /24 is the classful mask for class Cnetworks, such as the 192.168.1.0 network.



A level 1 route can function as a: 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.


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Ultimate Route

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

An ultimate route is a route that includes: either a next-hop IP address (another path)

and/or an exit interface


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Parent and ChildRoutes

A parent route is alevel 1 route

A parent route does

-



hop IP address or exitinterface information

The subnet is thelevel 2 child route of

the parent route.


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A level 1 parent route is

automatically created any timea subnet is added to the routingtable.

A parent route is created

Parent and Child Routes: Classful Networks


greater than the classful maskis entered into the routing table.

Child routes Child routes are level 2

routes Child routes are a subnet ofa classful network address


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


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Both child routes have the same subnet mask

This means the parent route maintains the /24 mask


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Parent and Child Routes: Classless Networks


In classless networks, child routes do not have to share the same subnetmask.

Regardless of the addressing scheme used by the network (classless orclassful), the routing table will use a classful scheme.


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Parent and Child Routes: Classless Networks



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NetworkParent routesClassful mask

Term variablysubnetted is

seen in parent

Includes the #

of different

Subnet maskincludedwith each



is displayed route in routing

table

routeschild route

entry

Classful No No No No

Classless Yes Yes Yes Yes


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Routing Table Lookup Process



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Result of Routing



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Let's examine in more depth how the routers determinethe best routes to use when sending packets and whyclassful routing protocols do not work with discontiguous

designs. We will consider:

1. What happens when a router receives an IP packet,


,address up in the routing table?

2. How does the router decide which route in the routingtable is the best match?

3. What effect does the subnet mask have on the routingtable lookup process?

4. How does the router decide whether or not to use asupernet or default route if a better match is not found?


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Examine level 1 routes

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

If the best match is a level 1 parent route examines level 2routes

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

Determine routing behavior type

Router determines classful or classless routing behavior

If classful then packet is dropped If classless then router searches level one supernet and

default routes

If there exists a level 1 supernet or default route match thenpacket is forwarded. If not packet is dropped.


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Steps in the Route Lookup Process: 8.2.1.2



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Longest Match: Level 1 Network Routes

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 routein the routing table.



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Finding the subnet maskused to determine the

longest match

Scenario:

PC1 pings 192.168.1.2

Router examines level 1


route for best match

There exist a matchbetween192.168.1.2 &192.168.1.0 / 24

Router forwards packetsout s0/0/0


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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 matchbetween the destination IP and the child route is made.


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Finding a match between packets destination IP address and thenext 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


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Longest Match: Level 1 Parent and Level 2 Child Routes


Level 1 Parent & Level 2 Child Routes

Before level 2 child routes are examined, there must be amatch between classful level one parent route and

destination IP address.


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After the match with parent route has been made Level 2 childroutes 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 route chosen is the one with the longest match

Destination of172.16.3.10 10101100 00010000 00000011 00001010


Matches Parent Route23rd bit does not match. First 24 bit

much match next route entry

24rd bit does not match. First 24 bit

much match next route entryFirst 24 bits match

IP Packet

Level 1 ParentRoute

172.16.0.0/16 10101100 00010000 00000000 00000000

Level 2 ChildRoute

172.16.1.0/24 10101100 00010000 00000001 00000000

Level 2 Child

Route 172.16.2.0/24 10101100 00010000 00000010 00000000Level 2 Child

Route172.16.3.0/24 10101100 00010000 00000011 00000000


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Example: Route LookupProcess with VLSM

The use of VLSM does notchange the lookupprocess


If there is a matchbetween destination IP

address and the level 1parent route then

Level 2 child routes will besearched


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Routing Behavior



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Classful and Classless Routing Behavior


Classful & classless routingprotocols

Influence how routing table is

populated Classful & classless routing

behaviors

Determines how routing table is

searched after it is filled


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Classful and Classless Routing Behavior



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Classful Routing Behavior: no ip classless

What happens if thereis not a match with anylevel 2 child routes of

the parent? Router must determine

if the routing behavioris classless or classful


If router is utilizingclassful routingbehavior then lookupprocess is terminatedand packet is dropped

The commands no ip classlessand ipclasslessare global configuration

commands and can be viewed bytyping show running-config.

In IOS versions 11.3 and later, thecommand ip classlessis the default,implementing a classless route lookupprocess.


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Classful Routing Behavior: no ip classless



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Classful Routing Behavior Search Process

An example of when classful routing behavior is ineffect and why the router drops the Packet

The destinations 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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First 24 bits must match


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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 beused if the router does not have a better route. In our example,R2's default route is not examined nor used, although it is amatch. This is often a very surprising result when a networkadministrator does not understand the difference between classful


an c ass ess rou ng e av or.

The reason why the router will not search beyond the child routes Originally networks were all classful

This meant an organization could subnet a major networkaddress and enlighten all the organizations routers about

the subnetting

Therefore, if the subnet was not in the routing table, thesubnet did not exist and packet was dropped


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Classless Routing Behavior: ip classless

Commands: Router(config)#ip classless Beginning with IOS 11.3, ip classless was configured by default Classless routing behavior works for:

Discontiguous networks CIDR supernets


Cl l R ti B h i i l l


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Route lookup process when ip classless is in use If classless routing behavior in effect then

Search level 1 routes Supernet routes Checked first

If a match exists then forward packet


Default routes Checked second

If there is no match or no default route then the Packetis dropped

Cl l R ti B h i i l l


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Classless Routing Behavior: Search Process


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IP Packet 172.16.4.10 10101100 00010000 00000100 00001010

Longest-bit-match! (11 bits match, 8 bits minimum)

Supernet 172.0.0.0/8 10101100 00000000 00000000 00000000

Supernet 160.0.0.0/4 10100000 00000000 00000000 00000000


RouterB#show ip route

172.16.0.0/24 is subnetted, 3 subnets

R 172.16.1.0 [120/1] via 172.16.2.1, 00:00:20, Serial0

C 172.16.2.0 is directly connected, Serial0

C 172.16.3.0 is directly connected, FastEthernet0C 192.168.1.0/24 is directly connected, Serial1

S 172.0.0.0/8 is directly connected, Serial1

S 160.0.0.0/4 is directly connected, Serial1

S* 0.0.0.0/0 is directly connected, Serial1

RouterB#

e au . . .



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ass u vs. ass ess ou ng e av or

It is recommended to use classless routing behavior so supernetand default routes can be used whenever needed

Remember that classful and classless routing behaviors areindependent 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 alsobe configured with classless routing behavior (ip classless) and aclassful routing protocol, such as RIPv1.

Summary


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Summary



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CA Ex S2M8 the Routing Table

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