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418 Chapter 6: Routing
Most of the issues with distance vector routing protocols arise when working with networks
with multiple paths. Two of these issues are not obvious. Table 6-21 summarizes these issues,
and each is explained in succession.
Table 6-22 outlines the features of RIP and IGRP.
Table 6-21
Issues Relating to Distance Vector Routing Protocols in Networks with (Multiple Paths)
Issue
Solution
Multiple routes to same subnet, with
equal metric
Implementation options include either using the first route
learned or putting multiple routes to the same subnet in the
routing table.
Routing loops occurring due to updates
passing each other over a single link
Split horizon--The routing protocol advertises routes out
an interface only if they were not learned from updates
entering that interface.
Split horizon with poison reverse--The routing protocol
advertises all routes out an interface, but those learned from
earlier updates coming in that interface are marked with
infinite distance metrics.
Routing loops occurring due to updates
passing each other over alternate paths
Route poisoning--When a route to a subnet fails, the
subnet is advertised with an infinite distance metric.
Counting to infinity
Holddown timer--After knowing that a route to a subnet
has failed, a router waits a certain period of time before
believing any other routing information about that subnet.
Triggered updates--The process of immediately sending
an update rather than waiting on the update timer to expire
when a route has failed. Used in conjunction with route
poisoning, this ensures that all routers know of failed routes
before any holddown timers can expire.
Table 6-22
RIP and IGRP Feature Comparison
Feature
RIP (Defaults)
IGRP (Defaults)
Update timer
30 seconds
90 seconds
Metric
Hop count
Function of bandwidth and
delay (default); can include
reliability, load, and MTU
Holddown timer
180
280
Flash (triggered) updates
Yes
Yes
Mask sent in update
No for RIP-1, yes for RIP-2
No
Infinity metric value
16
4,294,967,295
ch06.fm Page 418 Monday, March 20, 2000 5:11 PM