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3.1 | ![]() |
Routing | |
3.1.3 | ![]() |
Configuring dynamic routing |
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
![]() This module focuses on
IGPs. A comprehensive discussion of EGPs, particularly BGP4, will be
presented in Module 8. These protocols can be further categorized as
either distance vector or link-state routing protocols, depending on
their method of operation.
Routing protocols for IPX and AppleTalk IP routing protocols and the routing table Figure 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. Hop count is the only factor that IP RIP uses to determine the
metric.
In the sample entry shown in Figure
Each routing protocol calculates its metrics differently. EIGRP uses a complex combination of factors that include bandwidth and reliability to calculate its metric. Using the default settings, the EIGRP 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, the router will use the administrative distance metric to compare the two. This process is similar to comparing three apples to over three million oranges. When a router receives updates from different routing protocols about the same network, it cannot use dissimilar metrics to evaluate a route. The router will use administrative distance as the deciding metric for route validation. Cisco IOS assigns a default administrative distance to every routing protocol. A lower value signifies a more trustworthy routing protocol. A complete list of administrative distances can be found in Module 8, Route Optimization.
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