Okay, I just tried this with RIP advertising the default route and I'm even
more confused! Now, it behaves as I would expect. With no ip classless,
pings to unknown 10.x.x.x subnets are unroutable even though there is a
default route in the routing table.
With no ip classless, why does my router take the default route when it was
installed by OSPF but not when it was installed by RIP? I would expect it
to never take the default route for 10.x.x.x addresses with no ip classless.
This really concerns me because I was taking a practice CCIE written exam a
few days ago and ran across a question like this and I answered the question
assuming normal behavior of no ip classless and got it right. Now I'm
thinking there are some more twists to its behavior that i'm not aware of.
John
> Sure, I'll try that but I don't see why it should matter. As I
understand
> it, ip classless affects routing table lookups only and it doesn't care
how
> those routes were installed into the table.
>
> Although, given this behavior, my assumption might be wrong.
>
> Thanks,
> John
>
> > John,
> > Interesting. I think this is due to OSPF, not redistribution problem.
> Can you try running RIP instead of OSPF ?
> >
> > Cheers,
> > YY
> >
> >
> >
> > -----Original Message-----
> > From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]]On Behalf Of
> > John Neiberger
> > Sent: Sunday, March 25, 2001 5:28 AM
> > To: [EMAIL PROTECTED]
> > Subject: IP Classless Revisited (this is just odd...)
> >
> >
> > Ok, just when you thought it was safe to go back in the water.... Or
> should
> > I say, just when I thought I understood the behavior of 'ip classess'
and
> > 'no ip classless'.... Let me summarize my lab setup.
> >
> > RouterA-----RouterB------RouterC
> >
> > Pretty simple. AtoB is 10.1.1.0/24, BtoA is 10.1.2.0/24. OSPF is
> running
> > on both links. 'ip classless' is on A and C, but not B initially. On
B
> I
> > see these routes:
> >
> > 10.0.0.0/24 is subnetted, 2 subnets
> > C 10.1.2.0 is directly connected, Serial1
> > C 10.1.1.0 is directly connected, Serial0
> >
> > That's what I expect to see. Then I add a default route on B, 'ip
route
> > 0.0.0.0 0.0.0.0 10.1.1.2'. With no ip classless configured, any
packets
> to
> > unknown subnets of 10.0.0.0/8 should be dropped. I tested it and that
is
> > the case. With 'ip classless' configured, and unknown packets
regardless
> of
> > major network get routed to 10.1.1.2.
> >
> > Now here is what I don't understand. Let's turn off ip classless on B
> > again, then go to Router C and add a default route to null0 and
> > default-information originate to the ospf process. I now see this in
> router
> > B:
> >
> > 10.0.0.0/24 is subnetted, 2 subnets
> > C 10.1.2.0 is directly connected, Serial1
> > C 10.1.1.0 is directly connected, Serial0
> > O*E2 0.0.0.0/0 [110/1] via 10.1.2.2, 00:06:38, Serial1
> >
> > There is indeed a default route. With no ip classless configured, I
> would
> > expect the same behavior as before. If I were to ping 10.5.5.5 the
> packets
> > should be unroutable, but they're not! They get routed to the default
> route
> > whether or not ip classless is configured.
> >
> > Why is a default route learned through a routing protocol treated
> > differently than a manually configured default route? I went through
> this
> > entire process twice and I just don't understand the behavior.
> >
> > What am I missing? I know it's going to be something obvious, but I
> don't
> > see it yet.
> >
> > Ok, I just now tried this: with the ospf external default route still
in
> > the routing table, I pinged 10.5.5.5 and it took the default route.
Then
> I
> > manually added a default static route and the destination became
> unroutable
> > due to 'no ip classless' being configured. Removing the static
default
> it
> > becomes routable again.
> >
> > Weird. What's going on?
> >
> > Thanks,
> > John
> >
> >
> >
> >
> >
> > _______________________________________________________
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> >
> >
> > _________________________________
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>
>
>
>
>
> _______________________________________________________
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