Dynamic routing of TCP/IP can be implemented using
one or more protocols. These protocols are often grouped according to where they
are used. Routing protocols designed to work inside an autonomous system are
categorized as interior gateway protocols (IGPs), and protocols that work
between autonomous systems are classified as exterior gateway protocols (EGPs).
Figure lists widely
supported EGPs and IGPs for TCP/IP routing.
A comprehensive discussion of EGPs, in particular
BGP4, can be found in Chapter 8, BGP. Chapter 3 focuses on IGPs. As shown in
Figure , you can
further categorize these protocols as either distance-vector routing protocols
or link-state routing protocols, depending on their method of operation.
Routing protocols for IPX and AppleTalk
Despite the dominance of IP, a significant number of organizations continue to
support legacy protocols, such as Novell's IPX and Apple's AppleTalk. A legacy
technology is one that is supported because of a significant past investment or
deployment. Many organizations continue to support IPX and AppleTalk to leverage
a past investment in protocol-specific printers, software, and servers. Although
Cisco's EIGRP offers comprehensive support for both IPX and AppleTalk, it is
important to be familiar with the names of the following proprietary routing
protocols: IPX RIP (or Novell RIP), NetWare Link Services Protocol (NLSP), and
AppleTalk's Routing Table Maintenance Protocol (RTMP). 
Implementing these Apple and Novell proprietary
routing protocols is beyond the scope of this curriculum.
IP routing protocols and the routing table
The Cisco IOS commands to enable dynamic routing vary depending on the routing
protocol used. Figure
displays the routing table of a router configured to use four IP routing
protocols: RIP, IGRP, EIGRP, and OSPF. Note that most organizations would not
normally use more than one or two routing protocols. This example is provided to
show different types of routing table entries.
Figure dissects the
specific table entry for 192.168.1.0/24. Routes in the routing table that are not
directly connected include two numbers offset by brackets, in the form [administrative
distance/metric]. Therefore, [120/3] means that the
administrative distance is 120 and the metric is 3. Routers base their
evaluations of routes on these two numbers. Since this is a RIP route, the
metric represents hop count.
Routers use metrics to evaluate, or measure,
routes. When multiple routes to the same network exist and the routes are from
the same routing protocol, the route with the lowest metric is considered the
best. IP RIP uses only one factor to determine the metric: hop count. In the
sample entry shown in Figure ,
the number 3 indicates that the destination network is three hops away, which is
a better metric than four hops.
Each routing protocol calculates its metrics
differently. For example, EIGRP uses a sophisticated combination of factors that
typically includes bandwidth and reliability to calculate a metric. With default
settings, EIGRP's metric for the same route to 192.168.1.0 is 3,219,456!
If RTA receives a RIP update and an EIGRP update for this same network, how can
the router compare what is, in effect, three apples against more than 3 million
oranges? That is where administrative distance comes in.
When a router receives updates from different
routing protocols about the same network, it can not use dissimilar metrics to
evaluate a route. It uses administrative distance to decide which protocol to
believe. The Cisco IOS assigns a default administrative distance to every
routing protocol; the lower the value, the more trustworthy the routing
protocol. A complete list of administrative distances can be found in Chapter 7,
Route Optimization.
|