From: Fred Ingham (fningham@xxxxxxxxxxxxxxxx)
Date: Wed Dec 06 2000 - 22:44:00 GMT-3
The ip classless command is used so that the routing protocol will not
assume it knows all subnets of a classful network if one subnet is in
the routing table.
For instance if you are using the 172.16.0.0 network and 172.16.31.0/24
is in the IGRP
routing table, classful behavior would be that all subnets of
172.16.0.0 are known. Therefore if a ping to another subnet, say,
172.16.25.2, on r2 is tried, it will fail if 172.16.25.0 is not in the
routing table.
Classless behavior would be to search the routing table for the subnet
and, if it is not present, use the default gateway. IP classless is the
default in recent IOS versions so it usually isn't an issue but it's
always nice to know why.
Enabling split horizon will prevent the routes redistributed
from OSPF from being re-advertised from r4 back to r3. This typically
happens when the link between r3 and r4 is a frame relay link with a
physical interface on r4. The default for a frame physical interface
has split horizon disabled.
Comments? Fred.
Kevin Baumgartner wrote:
>
> Yes you are right that igrp to OSPF redistribution has to take place on r3.
> So why the need for ip classless and split horizon enabled on r4?
>
> Kevin
>
> At 05:05 PM 12/6/00 -0500, Fred Ingham wrote:
> >You also would have to redistribute IGRP into OSPF on r3 to ping the R4
> >addresses from r2. Best to use route maps on both OSPF and IGRP
> >redistributions to prevent routing loops. Default network is the way to
> >get a default into IGRP. The default network should be in r3's routing
> >table and not the same classful address configured on r4. r4 should
> >have the ip classless command and check that the r4 interface to r3 has
> >split horizon enabled.
> >
> >Fred.
> >
> >Robert DeVito wrote:
> > >
> > > R2
> > > |
> > > | Area 0
> > > |
> > > R3-------------------R4
> > >
> > > OSPF is configured between R2 and R3 (working good). I am trying to
> > > figure out what they mean below and how would you do that? Would you
> > > configure IGRP between R3 and R4 and then redistribute OSPF into IGRP
> > > and use the " ip default-network" in R3?
> > >
> > > Configure IGRP between R3 and R4. Ensure that all routers in the
> > > network can access all interfaces on all the routers. Configuring
> > > ONLY R3, send a default route to R4 without using static routes, route
> > > maps, or summary addresses.
> > >
> > > Thank you,
> > > Robert DeVito
> > >
> > > ----------------------------------------------------------------------
> > > Get more from the Web. FREE MSN Explorer download :
> > > http://explorer.msn.com
> > >
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