Comparing Routing Protocols 25
Comparing Routing Protocols
The majority of this book is devoted to understanding how routing protocols work and how they
are optimized. Before delving into the details, though, it’s worth thinking about the characteristics
of routing protocols, how the protocols differ, and how those differences impact design. This
section discusses RIP (versions 1 and 2), OSPF, EIGRP, IS-IS, and BGP.
Distance Vector and Link State Routing Protocols
Routing protocols are built to employ one of two basic strategies to communicate routing
information. Distance vector routing protocols work by passing copies of their routing table to
their neighbors (this is also known as “routing by rumor” because neighbors talk to neighbors
and not the source of the route). Link state routing protocols work by advertising a list of
their neighbors and the networks attached to their neighbors until all routers have a copy of
all lists. The routers then run the Shortest Path First algorithm to analyze all paths and determine
best paths.
Distance vector routing is less processor- and memory-intensive than link state routing, but can
have loops because decisions are made on incomplete information (solely the portion of the
routing table sent by a neighbor). Link state routing is loop-proof because routers know all
possible routes, but link state routing requires more CPU time and memory.
Table 1-4 shows the various routing protocols and the technique they employ.
NOTE This book assumes that you have completed CCNA or have equivalent experience.
Basic knowledge and techniques used with RIP, EIGRP, and OSPF will be found with the
CCNA material.
Table 1-4 Distance Vector and Link State Protocols
Protocol Technique
RIP Distance Vector
RIPv2 Distance Vector
EIGRP Distance Vector
OSPF Link State
IS-IS Link State
BGP Path Vector
26 Chapter 1: Network Design
Classless and Classful Routing
Another characteristic of routing protocols is the manner in which they advertise routes. Older
routing protocols pass just the prefix, such as “192.168.1.0.” Given that example, there is no way
for a router to understand if the network advertised uses a 24-bit mask or a 27-bit mask.
Older routing protocols, such as RIP and IGRP, assume the subnet mask is the same as the one on
the receiving interface or that it is the default mask. The default mask for Class A networks is /8,
for Class B it is /16, and for Class C it is /24. This behavior is called classful, because the
assumption is based on the class of the IP address.
Example 1-1 shows an advertisement from a Routing Information Protocol (RIP) router. Notice
that no subnet mask is advertised. For instance, the first route is 10.0.0.0 with no indication of the
appropriate subnet mask. This shows that RIP is a classful routing protocol.
Modern routing protocols (OSPF, IS-IS, and EIGRP) explicitly advertise the mask. There is no
assumption involved, the mask is clearly indicated. This behavior is referred to as classless.
Variable Length Subnet Masks (VLSM) refers to the property of a network that allows different
subnet masks to be mixed throughout the network. For instance, office networks might each
use /24 while point-to-point lines use /30. Classless Interdomain Routing (CIDR) is a property of
a network that allows classful networks to be aggregated—for example, combining 192.168.0.0/
24 and 192.168.1.0/24 into a “supernet” that includes 512 addresses. Classless routing protocols
support VLSM and CIDR. In fact, the three terms are so closely linked that they are sometimes
used synonymously.
Example 1-2 shows RIP version 2 (RIPv2) enabled on Router1. Notice that the subnet mask is now
advertised. RIPv2 is a classless routing protocol.
Example 1-1 Classful RIP Advertisements
Router1#dd
dd
ee
ee
bb
bb
uu
uu
gg
gg
ii
ii
pp
pp
rr
rr
ii
ii
pp
pp
RIP protocol debugging is on
00:03:40: RIP: received v1 update from 172.16.2.200 on Serial1/0
00:03:40: 10.0.0.0 in 1 hops
00:03:40: 172.16.4.0 in 1 hops
00:03:40: 172.16.6.0 in 1 hops
00:03:40: 172.16.44.0 in 2 hops
00:03:40: 172.16.66.0 in 2 hops
Example 1-2 Classless RIPv2 Advertisements
Router1#cc
cc
oo
oo
nn
nn
ff
ff
ii
ii
gg
gg
uu
uu
rr
rr
ee
ee
tt
tt
ee
ee
rr
rr
mm
mm
ii
ii
nn
nn
aa
aa
ll
ll
Enter configuration commands, one per line. End with CNTL/Z.
Router1(config)#
rr
rr
oo
oo
uu
uu
tt
tt
ee
ee
rr
rr
rr
rr
ii
ii
pp
pp
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