From: Dan (dp595@xxxxxxxxxxxxx)
Date: Sat Apr 28 2001 - 23:29:39 GMT-3
I had received an escalation from level 2 support regarding a T1 customer of
ours.
Apparently their DNS was broken. Upon inspecting the router config, I found
that the DNS servers "were" present in the config and were up and running,
but not reachable from this router only, which looked something like this:
ip name-server 129.250.35.250
ip name-server 129.250.35.251
!
!
Interface Serial0
Ip address 129.250.59.10 255.255.255.252
!
!
no ip classless
!
ip route 0.0.0.0 0.0.0.0 Serial0
!
!
I first pinged www.yahoo.com and got no response.
Then I tried to ping the DNS servers specified in the config (129.250.35.250
& 129.250.35.251), with no response.
I telnetted to the router on the other side of the Serial link
(129.250.59.9) and was able to ping by hostname and reach the DNS servers
from there, even when sourcing my ping with the 129.250.59.9 address.
I thought to myself, if I can reach these DNS servers from 129.250.59.9, why
can't I reach them from 129.250.59.10 which is on the same subnet "and" has
a default route pointing to 129.250.59.9 as its gateway?
I did a "show ip route" on the customer's router and that's when I realized
that it listed 129.250.0.0/16 as "directly connected"
and because the router was not configured for classless routing (note the
"no ip classless" in the config) the router believed that the DNS servers
were on the directly connected Serial link, and must have put out an ARP on
the Serial instead of routing it to the default route.
I put in "ip classless" and that solved the problem.
Apparently someone had renumbered the Serial link (which had previously been
using an entirely different classful network than that of our DNS resolvers,
therefore everything had worked fine) and did not notice or think to enable
"ip classless" and therefore the DNS was broken, until I entered the magic
command.
Hope this real-world example helps.
Dan Pontrelli
Customer Installation Engineer - Verio NYC
CCNP, MCSE, CNA
----- Original Message -----
From: "David Goldsmith" <dgoldsmi@cisco.com>
To: "Peter Van Oene" <pvo@usermail.com>
Cc: <ccielab@groupstudy.com>
Sent: Friday, April 27, 2001 11:57 AM
Subject: Re: ip classless
> Peter,
>
> I don't believe that's true, I have tried this in the lab with OSPF and it
does change the behavior of the router. If I get time I will verify with
ISIS. Ip classless is to
> have the router function classless or classful mode.
>
> Thanks,
> Dave G.
>
>
> Peter Van Oene wrote:
>
> > However, it has been verified that configuring IS-IS or OSPF on a router
will automatically place the router in classless mode regardless of the ip
classless nob's setting
> >
> > *********** REPLY SEPARATOR ***********
> >
> > On 4/27/2001 at 2:57 PM Atif Awan wrote:
> >
> > >This command changes the router's lookup algorithm. No doubt OSPF is a
> > >classless protocol but it only provides the router with the routes, it
> > >does
> > >not in any way affect the way a router determines how to forward
packets (
> > >that is using the classful or the classless algorithm ).
> > >
> > >>From: "radha rani" <radhaccie@hotmail.com>
> > >>Reply-To: "radha rani" <radhaccie@hotmail.com>
> > >>To: ccielab@groupstudy.com
> > >>Subject: ip classless
> > >>Date: Fri, 27 Apr 2001 10:00:06 -0400
> > >>
> > >>I have a question re: ip classless. My understanding is that if there
is
> > >>no
> > >>specific route for a subnet in the routing table, this command allows
the
> > >>router to forward packets to unknown destinations using the default
route.
> > >>This being the case when the destination is a subnet of a network
which is
> > >>known to the router.
> > >>
> > >>My question is : since OSPF is a classless protocol, why do I need
this
> > >>command to have the router forward packets using the default route. I
see
> > >>this all the time where the router will not utilize the default route
> > >until
> > >>this command is added.
> > >>
> > >>I can unserstand needing it with IGRP/RIP but why with OSPF?
> > >>
> > >>Can someone expalin. Thanks so much.
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