From: Rohan Grover \(rohang\) (rohang@cisco.com)
Date: Tue Aug 09 2005 - 10:22:05 GMT-3
Hi Gustavo,
I think I got most of what you explained. So just to clarify further, in which situation would I then need to use the tagging method?
Thanks
Rohan
-----Original Message-----
From: Gustavo Novais [mailto:gustavo.novais@novabase.pt]
Sent: Tuesday, August 09, 2005 4:09 PM
To: Rohan Grover (rohang); ccielab@groupstudy.com
Subject: RE: Some questions...
Hi,
Let me try to take a shot at that...
-R2-
R1--| |--R4
-R3-
On R2 and R3, O -> E, you are setting tag 110 and denying tag 90. Without denying tag 90, eigrp internal (90) looped routes through OSPF would appear as external (170) so they should never be installed on RT of R4.
Unless you have D EX coming from R4 and you are setting a lower metric to the routes coming from R4 when redistributing ospf to eigrp, than their original routes coming from R4. (its confusing but it is supposed to be a loop - Confusing!)
By denying tag 90 - note that by redistributing you are allowing routes already ON routing table pass to the other routing protocol, you are not avoiding that they be installed on the RT of the redistributing router - In this case I'd probably use distribute lists to really avoid that R4 external routes appear as ospf routes on R2/R3.
Probably what you see is that on R2 you see them as EIGRP EX from R4 and on R3 you see them as OSPF E1/2, or vice versa. Your guarantee with tagging is that those OSPF-that-in-fact-are-EIGRP routes will not be passed back to R4.
Unfortunately this "unbalanced" routing sometimes may be wanted if you wish full redundancy. (imagine that R2 - R4 link went down? R2 would be completely isolated, if it didn't know routes coming from R3)
On R2 and R3, E -> O, you set tag 90 and deny tag 110. As EIGRP sets external routes as 170, when you try to redistribute Eigrp to OSPF, the EIGRP-routes-that-once-were-OSPF are seen on EIGRP topology on the redistribution router, but they are not installed on RT, because they are already there as 110, (OSPF). I think the deny tag 110 could be not there.
Redistribution is still a confusing topic. Particularly if you wish optimized routing, I'd go for distribute lists between both routers inside ospf denying EIGRP routes from one side to another. If you wish full redundancy, I'd also use tagging.
One other choice could be setting ospf external distance to 180 for example. But that's another story...
Please correct me if I'm wrong in any aspect, I'm always learning!
Hope this helps
Gustavo
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of Rohan Grover (rohang)
Sent: terga-feira, 9 de Agosto de 2005 10:34
To: Group Study
Subject: Some questions...
Hi,
Some questions regarding the lab
1. Since RIP1 is not on the lab, would it be fair to say that if the RIP section does not specify anything particular it is ok to configure 'version 2' and 'no auto-summary'?
2. In ISIS, when it is mentioned, configure R1 in net 69 and R2 in net 25, why is it that these nets are interpreted as areas in the answers?
what I mean is, why can I not configure R1 net as '69.1111.1111.1111.00'
instead of '47.0069.1111.1111.1111.00'?
3. For ATM questions when not specified in the question as to where a pvc/svc is to be configured, can I configure them on the main interface or do I need to create multipoint subinterface?
4. Lastly, I am pretty comfortable using distance prevent routing loops etc but I'm a little confused as to how route tagging will achieve the same thing
ex
R1---------R2-------------R4
| |
|-----------R3--------------|
R1,R2, R3 are on the same ethernet LAN and running OSPF.
R2,R3,R4 are on a different network running EIGRP.
Now, we need to do something to prevent route feedback while doing mutual redistribution on R2 and R3. Controlling this with distance is fine, but I was also under the impression that using the following route tags on R2 and R3 will also prevent route feedback.
route-map o2e deny 10
match tag 110
route-map o2e permit 20
set tag 90
route-map e2o deny 10
match tag 90
route-map e2o permit 20
set tag 110
This does not work :-(. In fact after having applied both these route-maps on R2 and R3, I see routing table flap...EIGRP route appear disappear.
Any help would really be appreciated.
Thanks
Rohan
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