Classless to Classful Redistribution

Classless to Classful Redistribution
There are only two routing protocols we’ve covered for which we really need to worry about
classless to classful redistribution: RIPv1 and IGRP. The problem is these two routing protocols
are still used in networks today.
So what causes the problem with redistribution between classless and classful routing protocols?
The problem arises because classful routing protocols do not send a subnet mask with
a route. Because classless routing protocols do send the subnet mask with the route, you will
encounter situations where you need to redistribute routes into the classful routing protocol
that do not fall within the classful boundaries.
How do we overcome these instances? There are actually two ways you can overcome the
classless to classful redistribution limitation:
 Make sure you have summarized all networks to their classful boundaries that need to be
redistributed before they reach the redistributing router.
 On the redistributing router, create a static route with the classful variant of the classless route
and point it to interface null0. Redistribute the static route into the classful routing protocol.
Null0 is an interface that doesn’t exist. The null0 interface can be used with static routes to
route a packet into a black hole of sorts. In other words, if you have traffic that you do not want
to reach a particular destination, you can create the static route for that destination with the
next-hop interface pointing to null0. Any traffic that enters the router destined for the network
specified in the static route is sent to the null0 interface and dropped. In other words, the traffic
will not reach that destination.
So if static routes to null0 keep packets from reaching a destination, why would we ever want
to use that type of static route for classless to classful redistribution? The reason we would do
this is the route will be advertised into the classful routing protocol. All of the traffic destined
for that network is forwarded back to the router that originated the classful advertisement from
its static route. Because the originating router has more specific routes for the destination, it will
use those routes and will never use the route to null0, hence solving the redistribution limitation.
To better understand these two methods, we will walk through a couple of examples. We’ll
start with the summarization solution. Refer to Figure 10.11.