Hi Kim,

As a result of applying the "ip classless" command the routing process will change.

Let's say you have a routing table with a 10.1.1.0 /24 prefix, a default route specified and you have "no ip classless".

If the router receives a packet with destination address 10.2.1.1 then it will drop the packet, because your routing table has a subnet of the 10.0.0.0 classful network, but it did not find a match.

(So basically, the classful nature of the routing table assumes, that if you have a subnet of a classful network in the routing table, then you have the whole classful network under your router).

Now, if you type in the "ip classless" command and you receive the same packet with destination 10.2.1.1, your packet will be sent to the default route and will not be dropped.

(The router does not assume that it has the whole classful network, and intelligently forwards the packet to other possible destinations).

Your second question:

The downside of using OSPF is that OSPF requires more memory and cpu resources than RIP or EIGRP.

Cheers:

Istvan