© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
ITE PC v4.0
Chapter 1 1
The Routing Table: A
Closer Look
Routing Protocols and
Concepts – Chapter 8
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Chapter 1 2© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
Objectives
 Describe the various route types found in the routing
table structure
 Describe the routing table lookup process.
 Describe routing behavior in routed networks.
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Introduction
 Chapter Focus
-Structure of the routing table
-Lookup process of the routing table
-Classless and classful routing behaviors
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Routing Table Structure
 Lab Topology
 3 router setup
-R1 and R2 share a common 172.16.0.0/16 network with
172.16.0.0/24 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.
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Routing Table Structure
 Routing table entries come from the following sources
-Directly connected networks
-Static routes
-Dynamic routing protocols
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Routing Table Structure
 Level 1 Routes
 As soon as the no shutdown command is issued the
route is added to routing table
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Chapter 1 7© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
Routing Table Structure
 Cisco IP routing table
is a hierarchical
structure
-The reason for this is
to speed up lookup
process
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Chapter 1 8© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
Routing Table Structure
 Level 1 Routes
-Have a subnet mask equal to or less than the
classful mask of the network address.
 Level 1 route can function as
-Default route
-Supernet route
-Network route
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Routing Table Structure
 Level 1 Routes
-Ultimate Route
Includes either:
-A next-hop address
OR
-An exit interface
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Chapter 1 10© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
 Parent and Child
Routes
-A parent route is a
level 1 route
-A parent route does
not contain any nexthop
IP address or exit
interface information
Routing Table Structure
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Routing Table Structure
 Automatic creation of parent
routes
-Occurs any time a
subnet is added to the
routing table
 Child routes
-Child routes are level
2 routes
-Child routes are a
subnet of a classful
network address
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Routing Table Structure
 Level 2 child routes contain route source & the network
address of the route
 Level 2 child routes are also considered ultimate
routes
Reason: they contain the next hop address
&/or exit interface
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Routing Table Structure
 Both child routes have the same subnet mask
-This means the parent route maintains the /24 mask
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Routing Table Structure
 Diagram illustrates 2 child networks belonging to the
parent route 172.16.0.0 / 24
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Routing Table Structure
 In classless networks, child routes do not have to share
the same subnet mask
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Chapter 1 16© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
Routing Table Structure
Network
Type
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
 Parent & Child Routes: Classless Networks
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Routing Table Structure
 Parent & Child Routes: Classless Networks
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Routing Table Lookup Process
 The Route Lookup Process
 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
 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
then Packet is forwarded. If not packet is dropped
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Chapter 1 19© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
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.
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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
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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.
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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
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Chapter 1 23© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
Routing Table Lookup Process
 Level 1 Parent & Level 2 Child Routes
 Before level 2 child routes are examined
-There must be a match between classful level one
parent route and destination IP address.
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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
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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
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Chapter 1 26© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
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
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Chapter 1 27© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
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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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
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Routing Behavior
 Classful Routing Behavior – Search Process
 An example of when classful routing behavior is in effect and
why the router drops the Packet
-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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Chapter 1 30© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
Routing Behavior
 Classful Routing Behavior – Search Process
 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 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
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Chapter 1 31© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
Routing Behavior
 ip Classless
 Beginning with IOS 11.3, ip classless was configured
by default
 Classless routing behavior works for
-Discontiguous networks
And
-CIDR supernets
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Routing Behavior
 Classless Routing Behavior: ip classless
 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
Packet is dropped
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Chapter 1 33© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
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
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Routing Behavior
 Classless Routing Behavior – Search Process
 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 bits in the match
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Chapter 1 35© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
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
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Chapter 1 36© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
Summary
Content/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
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Chapter 1 37© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
Summary
Routing 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 examined
Level 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 then
Router searches Level 1 supernet & default routes for a
match
 If a match is found then Packet if forwarded -Else
 Packet is dropped
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Chapter 1 38© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
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
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Chapter 1 39© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public