Cisco Systems CCNA 2 manual Using the ip classless command, Common RIP configuration issues

7.2.3 Using the ip classless command

The ip classless command allows packets that are bound for an unknown subnet to be routed out the same interface as other known subnets in the same range of addresses. IP classless only affects the operation of the forwarding processes in IOS. It does not affect the way the routing table is built.

When the no ip classless command is used, a packet bound for an unknown subnet will be dropped even if a route to a subnet in the same address range exists. The basic principle of classful routing is that if one part of a major network is known, but the subnet toward which the packet is destined within that major network is unknown, the packet is dropped. One aspect of this rule that may confuse students is that the router will only use the default route if the destination major network does not exist in the routing table at all.

7.2.4 Common RIP configuration issues

RIP is a distance-vector routing protocol and like all distance-vector protocols they are slow to converge and have to deal with routing loops and counting to infinity. To reduce the routing loops and counting to infinity, RIP uses the following mechanisms:

•Split horizon

•Poison reverse

•Holddown counters

•Triggered updates

RIP permits a maximum hop count of 15 and any destination greater than 15 hops away is tagged as unreachable. This maximum hop count prevents counts to infinity and endless network routing loops. The split horizon rule prevents information about a route from being sent out the same interface from which it was originally received. Split horizon is used to avoid the creation of routing loops due to multiple routers that advertise routes to each other about the same network. The no ip split-horizoncommand can be used to disable split horizon.

Hold-down timers are used to define the amount of time that a possible down route will be held and routes with higher metrics to the same network will not be accepted. The default hold- down time is 180 seconds, which is 6 times the regular update period. When a route goes down, the hold-down timer is started. During this time period, a route with a higher metric than the original metric will not be accepted. If the original route comes back up or a route with a lower metric than the original metric is advertised, they will be accepted immediately. The hold-down timer will reduce routing loops but it may also slow convergence. The timers basic 30 90 180 540 router configuration command can be used to adjust the basic timers. The holddown is the third number.

RIP updates are broadcast by default every 30 seconds. This can be increased to reduce network congestion or decreased to improve convergence with the timers basic 30 90

180540 command. The update timer is the first number listed. In some instances, it may necessary to avoid the advertisement of routing updates out a specific interface. This can be accomplished with the passive-interfaceinterface router configuration command. For RIP to function in a non-broadcast environment, neighbor relationships must be configured. This can be accomplished with the neighbor ip address router configuration

83 - 238 CCNA 2: Routers and Routing Basics v3.1 Instructor Guide – Module 7 Copyright ♥ 2004, Cisco Systems, Inc.