From: Richard Gallagher (rgallagh@cisco.com)
Date: Thu Jul 21 2005 - 01:12:29 GMT-3
BGP does not support tagging, the only thing close is to use
communities. Not sure this will help in this situation though.
Rich
Joe Smith wrote:
> How about tagging the BGP that is redistributed into EIGRP on R2?
>
> R2:
> redistribute bgp 2 route-map TEST
>
> route-map TEST permit 10
> set tag 50
>
>
> R3:
> router bgp 2
> network x.x.x.x route-map TEST
>
> route-map TEST deny 10
> match tag 50
> !
> route-map TEST permit 20
>
>
>> From: "Geert Nijs" <geert.nijs@simac.be>
>> Reply-To: "Geert Nijs" <geert.nijs@simac.be>
>> To: "Schulz, Dave" <DSchulz@dpsciences.com>
>> CC: <ccielab@groupstudy.com>
>> Subject: RE: Preventing routing loops in this BGP situation
>> Date: Wed, 20 Jul 2005 15:03:55 +0200
>>
>> Dave,
>>
>> No that doesnt work. have tried both.
>>
>> PS. It is the purpuse that network x.x.x.x comes from AS1. This should
>> be allowed. The only mistake in the
>> topology is that someone forgot to remove the network statement on R3,
>> where this network was previously situated.
>>
>> 1) setting the noexport community on R2 will not prevent R2 from putting
>> the route in EIGRP. Also R3 would still
>> advertise the network to R2.
>>
>> I have been thinking about some sort of filtering on R3 so that the
>> network x.x.x.x command will only
>> inject networks into BGP when a) they are in the routing table en b)
>> they are NOT tagged. I would tag
>> redistributed routes on R2...but i havent found a way to implement
>> this...
>>
>> Geert Nijs
>> CCIE #13729
>>
>> -----Original Message-----
>> From: Schulz, Dave [mailto:DSchulz@dpsciences.com]
>> Sent: woensdag 20 juli 2005 13:12
>> To: Geert Nijs
>> Cc: ccielab@groupstudy.com
>> Subject: RE: Preventing routing loops in this BGP situation
>>
>> Geert -
>>
>> I believe that if you were to use the community attribute in BGP and set
>> it to No Export this would break the loop. Or, we could also create an
>> expression with an as-path list to only allow networks that originated
>> from the advertising AS. Does this work for you?
>>
>>
>> Dave Schulz,
>> Email: dschulz@dpsciences.com
>>
>>
>> -----Original Message-----
>> From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
>> Geert Nijs
>> Sent: Wednesday, July 20, 2005 3:34 AM
>> To: lab
>> Subject: Preventing routing loops in this BGP situation
>>
>> Hi group,
>>
>> I wonder how service providers prevent routing loops in the following
>> situation:
>>
>> R1 is part of AS1
>> R2 and R3 are part of AS2
>> R2 and R3 are also running an IGP like ie EIGRP
>> R4 is part of AS4
>>
>> **** On router R2 and R3 redistribution of BGP into EIGRP is configured
>> !
>>
>> R1 (AS1) ---- EBGP ----- R2 (AS2) ----------- IBGP + EIGRP
>> ----------- R3 (AS2) --------- EBGP ---------- R4 (AS4)
>>
>> On router R3 someone forgot to remove a network statement, so router R3
>> contains the statement:
>> router bgp 2
>> network x.x.x.x
>>
>> but network x.x.x.x is not in the routing table so it is not advertised
>> (no problems yet).
>>
>> Now, the problems begin when AS1 (R1) starts advertising this network
>> x.x.x.x
>>
>> The network x.x.x.x enters AS2 through router R2, this router
>> redistributes the route into EIGRP, and now,
>> the network x.x.x.x is part of the routing table on router R3, so R3
>> suddenly starts advertising network x.x.x.x back to R2 over IBGP
>> R2 prefers this route (AS path is empty). R2 starts advertising network
>> x.x.x.x back out AS2 to R1 and stops redistributing x.x.x.x into EIGRP
>> (by default, no redistribution of IBGP routes). So the route is removed
>> from the routing table on R3, so that R3 stops advertising the route
>> again.
>> When R2 removes the route from BGP table, it prefers back the route
>> received from R1 and starts advertising the route again to R3 over
>> EIGRP....
>> and so we have a beautifull routing loop and lots of constant BGP
>> advertisements going on forever.....
>>
>> I am looking for a method to prevent this from happening.
>> Any ideas ?
>>
>>
>> Geert Nijs
>> CCIE #13729 <http://www.cisco.com/en/US/learning/le3/ccie/index.html>
>>
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