Hi John,
I would like to add another point to your observation. With 'no ip
classless' , EIGRP also behaves the same way as OSPF when you have a
supernet for the specific major net.
> Now for the really interesting part (if you've read this far and are still
> awake!) I set a static 0.0.0.0/0 route on Router B but then also
advertised
> 10.1.0.0/16 from router A. Now Router B behaved classlessly but only for
> subnets of 10.1.0.0/16! If I tried to ping 10.2.1.1, for instance, it was
> unroutable, but any subnet of 10.1.0.0/16--even the unknown ones--would be
> routed based on the OSPF-installed supernet route. I then added
10.2.0.0/16
> to the advertisement and saw what I expected: packets destined for either
> of those two subnets would be routed, all others failed.
It works the same way if you repeat the above with EIGRP.
But, the default route or GOLR is not considered under this situation if it
is installed by EIGRP, whereas it is looked up when we use OSPF. Looks like
'ip classless' command is closely tied with the default route, rather than
generically relating to a supernet.
Regards,
Srikanth.
----- Original Message -----
From: John Neiberger <[EMAIL PROTECTED]>
To: <[EMAIL PROTECTED]>
Sent: Friday, March 30, 2001 5:09 AM
Subject: The Finale: OSPF and IP Classless
> If you thought this behavior was odd before, this will really bake your
> noodle. I did some more experiments as we discussed in the thread earlier
> today. Here's another short recap to catch everyone up.
>
> I have two routers, A and B, running OSPF. The link between them is
> 10.1.1.0/24, and A is originating a default into B. Router B has 'no ip
> classless' configured. This means that by Cisco's explanations, if I were
> to ping any unknown subnet of 10.0.0.0/8 it would fail and debugging would
> show that it was unroutable. However, that wasn't happening. If I used
> OSPF to originate a 0.0.0.0/0 default route, it would be installed as GOLR
> and router B would behave classlessly.
>
> I tried this using 0.0.0.0/0, 10.0.0.0/8, and 8.0.0.0/5. In all cases,
when
> using OSPF to originate the route, router B would behave classlessly.
This
> behavior would not occur when I used RIP v1 or v2, IGRP, or EIGRP. (If I
> understood IS-IS, I'd try that too.)
>
> Tonight I changed tactics and tried some new things. First, I ran two
> routing protocols, OSPF and RIP, but I let RIP advertise the default
> 0.0.0.0/0 to B. As expected, B behaved classfully and would not use the
> supernet route. This shows us that it's not merely the presence of OSPF
on
> a router that can cause it to override 'no ip classless'.
>
> Next, I configured a manual static default 0.0.0.0/0 route on B while
Router
> A was also advertising the same route. Of course the OSPF route would not
> be installed into the table because of the higher AD, but I wanted to
verify
> Router B's behavior. In this case, it was classfull.
>
> Next, I set the AD of the static route to 120, higher than the 110 AD of
the
> OSPF route. This means that the new GOLR, even thought it looks *exactly*
> the same in the routing table, was installed by OSPF. Guess what? Yep,
> classless behavior!
>
> Now for the really interesting part (if you've read this far and are still
> awake!) I set a static 0.0.0.0/0 route on Router B but then also
advertised
> 10.1.0.0/16 from router A. Now Router B behaved classlessly but only for
> subnets of 10.1.0.0/16! If I tried to ping 10.2.1.1, for instance, it was
> unroutable, but any subnet of 10.1.0.0/16--even the unknown ones--would be
> routed based on the OSPF-installed supernet route. I then added
10.2.0.0/16
> to the advertisement and saw what I expected: packets destined for either
> of those two subnets would be routed, all others failed.
>
> This means that the router behaves classlessly if there is a supernet
route
> that was installed by OSPF...but only up to that point! In the situation
I
> just mentioned, remember that there was also a static default route that
was
> being ignored!
>
> So, the new rule is this: a router with 'no ip classless' configured will
> not forward traffic to unknown subnets of known major networks UNLESS
THERE
> IS A VALID SUPERNET ROUTE INSTALLED BY OSPF. (sorry for the caps. <g>)
>
> Yikes, can this thread die now? :-) I know, I keep it going, but I
wanted
> to really chase this down. I think I chased it down, kicked it, hit it
with
> a stick, and now it's gone belly up not unlike the Norwegian Blue. As for
> me, I think I'm through with my 'no ip classless' experiments. Now maybe
I
> can finally get to those NAT labs I've been trying to get to for a week!
>
> Regards,
> John
>
>
>
>
>
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