From: mark salmon (masalmon@xxxxxxxxx)
Date: Wed Sep 13 2000 - 20:15:22 GMT-3
COmments in line
Kinton Connelly wrote:
>
> I think you almost have it in your #3a (but not b) below. As far as I know,
> the best way to do this is to:
>
> 1) Have R2 send an "ip default-network" into IGRP pointing to another
> classful address that's already in your routing table (if it's not in your
> routing table already, a static route will be automatically generated and
> this is usually a bad thing in the lab).
I disagree. I have heard that static routes are not allowed in the lab
and although I have not yet attempted it, I disagree with that blanket
statement. Almost ALL classless routing protocols generate a static
route to null 0 when one configures summarization. The only way to
remove it is to disable summarization.
True IP default network is a good way to achieve reachability from IGRP
domains to OSPF (or other classless networks), however, it is mutually
exclusive with mutual redistribution. In other words, as the goal of
redistribution is reachability, if one uses ip default network, it makes
mutual redistribution moot and vice versa.
I have not yet seen an automatic generation of a static route (I am
assuming to null 0) by doing ip default network.
>
> 2) Do your redistribution between OSPF and IGRP on R2 with the proper
> route-maps/distribute-lists.
>
> This should do it for you. R3 won't really see all your redistributed OSPF
> routes because of the classful nature of IGRP - but you'll still be able to
> reach those networks because of the injected default-network - anything the
> router doesn't find in the routing table will be send that way (to R2).
>
> If you search the CCIELAB archives for "ospf;igrp" you'll find a number of
> different suggestions for how to do this. Some of those ways are pretty
> creative (complicated) but the one I listed above is the one that's worked
> for me.
>
I have not yet seen the need to use distribute list to prevent routing
loops (one major reason to use them) is there is only one path between
networks due to split horizon (one reason to keep split horizon on for
classfull protocols)
The reason why IGRP will not see all the routes is not because it is a
classful protocol but because of the difference in the subnet mask
length between OSPF and IGRP. Because IGRP does not report SMs in its
pdates, it assumes the sm on the input intgerface for routes it
receives in the network that it has interfaces already configured in.
If the OSPF and IGRP nets are using the same SM lengths, it will see all
the networks (except system routes)
> Kinton
>
> --
> Kinton Connelly
> CCIE #5867
> kinton@oldmedia.com
>
> At 9/13/00, you wrote:
> >Just the last few days of cramming here until my lab.
> >I have a question about OSPF and Classless routing protocols such as IGRP.
> >
> >Consider the scenario below.
> >
> > ---------------------\ /----------------
> > \ /
> > \ /
> >10.10.20.128/25 \ / 10.10.30.0/24
> > | )( |
> > |---------R1----------R2-----------R3--------|
> > | )( |
> > OSPF AREA 0 / \ IGRP 100
> > / \
> > / \
> > ---------------------/ \-----------------
> >
> >3 routers, R1, R2 and R3. R2 is performing mutual redistribution between
> >IGRP and OSPF.
> >We are using different masks, IGRP is a /24 and OSPF a /25.
> >My question is this, is there any way of getting routes from OSPF into IGRP?
> >As I understand,
> >
> >1. We must get the mask down to /24 for IGRP to even look at it as it won't
> >cope with VLSM.
> >2. Normally you could summarize into area 0 with the area range command but
> >as we are in area 0 with this route we are out of luck there.
> >3. The only way I know of doing this is
> > a. Use the ip default-network and point it to a glassful net
> > other than
> >your own. (still a default route)
> > b. Create a static route on R2 for 10.10.20.0/24 and redistribute
> > that
> >static into IGRP. (A static route)
> >
> >Is there any way other than what I have mentioned above of doing this? I
> >would really appreciate any cool walk arounds for this situation. The
> >easiest thing to do would be to change the area of this route so we could
> >summarize it down to /24, however I am looking for a solution that doesn't
> >involve changing the area. I'm not sure there is a solution to this
> >situation but there's a lot of smart people on this board so who knows.
> >
> >
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