From: Jason1 (jason1@xxxxxxxxxx)
Date: Fri May 04 2001 - 00:38:07 GMT-3
Could someone give me a real life example of when I should NOT use IP
Classless ? Thanks !! :-)
----- Original Message -----
From: "Dan" <dp595@optonline.net>
To: "David Goldsmith" <dgoldsmi@cisco.com>; "Peter Van Oene"
<pvo@usermail.com>
Cc: <ccielab@groupstudy.com>
Sent: Sunday, April 29, 2001 10:29 AM
Subject: Re: ip classless (A real world experience of a "no ip classless"
gotcha)
> 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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