From: David Buechner (dbuechn@attglobal.net)
Date: Wed Oct 20 2004 - 19:15:17 GMT-3
Jim,
There are some additional issues which you need to address before there's a
reasonable answer to your question. The biggest is - are you doing this
redistribution on one router or are there multiple routers involved?
If you're redistributing among 3 routing protocols on one router and there
are not other routers redistributing between any pair of these same
protocols than you don't need any tags at all. If you're working within
one router you don't have to worry about duplicating routes. Why you
ask? Because of the way redistribution really happens. We talk about
redistributing from one routing protocol to another, but a careful read of
the Cisco Doc will get you to statements about redistributing "protocol x
DERIVED routes" (DERIVED capitalized for emphasis). What this means is
that routes are redistributed from the main routing table if they were put
there by the "source" protocol of our redistribution.
For example, say I'm learning routes from EIGRP and OSPF and I want to
redistribute between them. The OSPF process will essentially do a "show ip
route eigrp" command to get the routes out of the main routing table that
it should redistribute. EIGRP will do a "show ip route ospf" command for
the same purpose. (Note that I doubt the code actually executes these
commands, but it using the same search functionality.)
Also, the redistribution you laid out below doesn't talk about
redistributing between EIGRP and RIP. If you're doing all this on one
router the end result would be that RIP would have OSPF-derived routes, but
not EIGRP-derived routes; EIGRP will have OSPF-derived routes, but not
RIP-derived routes; and OSPF will have both EIGRP and RIP-derived
routes. Make sense?
If you're not doing all this in the same router than "do you need tags?"
still gets an "it depends."
For example say you're doing something like:
R1 - EIGRP - R2 - OSPF - R3 - RIP - R4
Here you'd be redistributing between EIGRP and OSPF on R2 and between OSPF
and RIP on R3. There is no possibility for loops here, so you don't need
to do tags (or any other kind of filtering).
Many other possible scenarios exist based on your question - too many to go
into in this note. If you can draw us a diagram I could give you a better
answer.
Hope this helps!
David Buechner, CCIE #13539
At 01:34 PM 10/20/2004, Balogh, Jim wrote:
>I was hoping someone could help clarify my use of tagging during
>redistribution.....
>
>Lets say we are running 3 routing protocols: RIP, EIGRP and OSPF (OSPF
>in the core). I want to do mutual redistribution. The steps I have taken
>(which seem to work) are as follows:
>
>
>* redistribute OSPF into RIP and set the tag on all routes to 77
>* redistribute RIP into OSPF, but use a route-map to ONLY allow
>routes into OSPF with tag=0
>
>* redistribute OSPF into EIGRP and set the tag on all routes to 77
>* redistribute EIGRP into OSPF, but use a route-map to ONLY allow
>routes into OSPF with tag=0
>
>Does this sound like the proper way to accomplish mutual-redistribution
>without duplicating routes?
>
>Thx.
>
>Jim
>
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