Routing Protocols and Concepts: Chapter 8 doc

Chapter 8
The Routing Table: A Closer Look
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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
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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
 Steps in the Route Table

Lookup Process
 Longest Match: Level 1
Routes
 Longest Match: Level 2
Routes
 Routing Behavior
 Classful and Classless
Routing Behavior
 Classful Routing Behavior: no
ip classless
 Classless Routing Behavior: ip
classless
The Routing Table Structure
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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
10.0.0.0/16 is subnetted, 1 subnets
S 10.1.0.0 is directly connected, Serial0/0/1
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
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Topology: Interface Configurations for R1 and R3
R1(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)# ip address 172.16.2.1 255.255.255.0
R1(config-if)# clock rate 64000
R1(config-if)# no shutdown
R3(config)# interface FastEthernet0/0
R3(config-if)# ip address 172.16.4.1 255.255.255.0
R3(config-if)# no shutdown
R3(config-if)# interface Serial0/0/1
R3(config-if)# ip address 192.168.1.2 255.255.255.0
R3(config-if)# no shutdown
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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
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
10.0.0.0/16 is subnetted, 1 subnets
S 10.1.0.0 is directly connected, Serial0/0/1
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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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.
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
10.0.0.0/16 is subnetted, 1 subnets
S 10.1.0.0 is directly connected, Serial0/0/1
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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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)# ip address 192.168.1.1 255.255.255.0
R2(config-if)# clock rate 64000
R2(config-if)# no shutdown
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>
C 192.168.1.0/24 is directly connected, Serial0/0/1

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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>
C 192.168.1.0/24 is directly connected, Serial0/0/1
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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)# ip address 172.16.3.1 255.255.255.0
R2(config-if)# no shutdown
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
172.16.0.0/24 is subnetted, 1 subnets
C 172.16.3.0 is directly connected, FastEthernet0/0
C 192.168.1.0/24 is directly connected, Serial0/0/1
Level 1 Parent Route
Level 2
Child Route
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Parent and Child Routes: Classful Networks
 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
172.16.0.0/24 is subnetted, 1 subnets
C 172.16.3.0 is directly connected, FastEthernet0/0
C 192.168.1.0/24 is directly connected, Serial0/0/1
Level 1 Parent Route
Level 2
Child Route
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Parent and Child Routes: Classful Networks
 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.
Level 1 Parent Route
Level 2
Child 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)# ip address 172.16.2.2 255.255.255.0
R2(config-if)# no shutdown
R2(config-if)# end
R2# show ip route
172.16.0.0/24 is subnetted, 2 subnets
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
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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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Parent and Child Routes: Classless Networks
 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 172.16.1.4/30 is directly connected, Serial0/0/0
C 172.16.1.8/30 is directly connected, Serial0/0/1
C 172.16.3.0/24 is directly connected, FastEthernet0/0