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Chapter 6: Routing
Distance Vector Routing
CCNAs deal with routing problems on a daily basis; some of these problems are a result of the
logic behind distance vector routing protocols. To understand what distance vector routing
means is to understand how a routing protocol accomplishes the following goals:
·
Learning routing information
·
Noticing failed routes
·
Adding the current best route after one has failed
·
Preventing loops
The following list summarizes the behavior of a router that uses the RIP-1 or IGRP distance
vector routing protocols:
·
Directly connected subnets are already known by the router; these routes are advertised to
neighboring routers.
·
Routing updates are broadcast (or multicast, in many cases). This is so that all neighboring
routers can learn routes via the single broadcast or multicast update.
·
Routing updates are listened for so that this router can learn new routes.
·
A metric describes each route in the update. The metric describes how good the route is;
if multiple routes to the same subnet are learned, the lower metric route is used.
·
Topology information in routing updates includes, at a minimum, the subnet and metric
information.
Table 6-2
Interior IP Routing Protocols and Types
Routing Protocol
Type
Loop Prevention
Mechanisms
Mask Sent in
Updates?
RIP-1
Distance vector
Holddown timer, split
horizon
No
RIP-2
Distance vector
Holddown timer, split
horizon
Yes
IGRP
Distance vector
Holddown timer, split
horizon
No
EIGRP
Balanced hybrid
DUAL and feasible
successors
Yes
OSPF
Link-state
Dijkstra SPF algorithm
and full topology
knowledge
Yes
ch06.fm Page 362 Monday, March 20, 2000 5:11 PM