static route is classless..or..not

Unanswered Question
Feb 22nd, 2007

Can any body clarify static route is classless or not and i am using static route and can i disable classless routing behaviour in my router...

I have this problem too.
0 votes
  • 1
  • 2
  • 3
  • 4
  • 5
Overall Rating: 5 (1 ratings)
Loading.
ahmednaas Thu, 02/22/2007 - 02:18

Static routes can be either, depending on the larger routing context.

If you are using static routes only with classful masks, there is no harm in disabling classless routing.

Richard Burts Thu, 02/22/2007 - 04:54

A static route is really neither classful or classless. What is classful or classless is how you do the routing lookup. For example lets assume that we have 3 static routes:

ip route 10.0.0.0 255.0.0.0 4.5.6.7

ip route 172.16.2.0 255.255.255.0 4.5.6.7

ip route 0.0.0.0 0.0.0.0 1.2.3.4

So we have a route for a class A network, for a class B subnet, and a default route.

The static routes describe classful masks but really are neither classful or classless. That becomes an issue when we do a lookup in the routing table. If a packet comes into the router with destination address 172.16.4.5 what should the router do? It has a route for a subnet in class B 172.16.0.0 but does not have a route for this particular subnet. If ip classless is enabled then the router will forward the packet using the default route. If no ip classless is configured (classful behavior) then the router discards the packet as unroutable.

That is the difference between classful and classless. It is in the routing table lookup not in the static route (or dynamic route).

HTH

Rick

Actions

This Discussion