Cisco Systems CCNA 2 7.2.3 Using the ip classless command , 7.2.4 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-horizon command 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

180 540 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-interface interface 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