Route Reflector full mesh
From what I understand, this kind of topology is _not_ recommended (I refer to http://www.cisco.com/univercd/cc/td/doc/cisintwk/ics/icsbgp4.htm#wp7158). But I really need to prove it(to show it to my client). I've checked
Hi list, Is it possible to prove that if we have 3 RRs with each has different cluster ID and each pointing to each other may create loop ? What I've been doing is to simulate (not simulate by doing it on simulator, but by drawing it on visio) BGP update mesg if the update comes from eBGP then it propagates throughout the network. Please refer this simplified topology below. +--------+ +-------+ | | | | | RR1 | --------- | RR2 | | | client | | +-------+ +--------+ \ / client \ ASX / client \ / +-------+ | | | RR3 | | | +-------+ Note : 1. RR1 is RR on the cluster ID 1 2. RR2 is RR on the cluster ID 2 3. RR3 is RR on the cluster ID 3 4. each RR is pointing to each other as a RR client (not iBGP peer) 5. on each cluster there are more than one RR client the specs (rfc4456) and try to follow the route reflection behavior based on the specs. It leaves me with more headache. So please bear with me with another questions as follow : 1. If the update is propagated from RR1 to RR2 then RR3, will the ORIGINATOR_ID on the update mesg still RR1 ? 2. and will the CLUSTER_LIST being used ? the cisco specs only said that the CLUSTER_LIST being used if the update mesg is reflected from clients to non clients. how about from clients to other client (which this client also a RR for another cluster) ? I've assumed that both ORIGINATOR_ID and CLUSTER_LIST is always used when route is being reflected and come to conclusion that the loops will not occur. Is it correct ? Any comments will be appreciated. thanks. regards, -- -adhy
On Feb 5, 2008, at 11:56 PM, Adhy wrote:
1. If the update is propagated from RR1 to RR2 then RR3, will the ORIGINATOR_ID on the update mesg still RR1 ?
Yes, the originator ID is preserved, while the cluster list would be additive.
2. and will the CLUSTER_LIST being used ? the cisco specs only said that the CLUSTER_LIST being used if the update mesg is reflected from clients to non clients. how about from clients to other client (which this client also a RR for another cluster) ?
There are typically two models for deployment within a cluster. Either fully-meshed clients with no client-to-client reflection, or clients that only peer with the RRs, with client-to-client reflection. If client-to-client reflection is employed then the RR will attach the RR attributes, including the originator ID set to the router ID of the client, and the local cluster ID value will be added to the cluster list. We did see some routing information loops on early deployments where client-to-client reflection is employed AND the RR would reflect a route back to a client that it learned from the client. The issue here is that the client is now required to know it's a client and poison updates it receives if the originator ID value equals the local router ID. The strict "don't reflect a route learned from a client back to that client because if you do a client would need to know about RR and know it's a client" behavior changed from RFC 1996 to RFC 2796 (perhaps so that some implementations could optimize message copy across client groups).
I've assumed that both ORIGINATOR_ID and CLUSTER_LIST is always used when route is being reflected and come to conclusion that the loops will not occur. Is it correct ?
Yes, when acting on behalf of a client, or if received and already attached to a prefix, unless sent to an eBGP peer. The originator ID should be preserved if it already exists. The real offshoot with RRs employing unique cluster IDs sharing common client sets is that you trigger unnecessary replication of updates, additional churn, require additional BGP RIB space in RRs, and require that routing information originated by a client be poisoned by that client potentially multiple times, rather than avoided through routing topology. Unique cluster IDs within topological clusters can be especially problematic hierarchical route reflection. Specifically to your question, see this text in Section 8 of RFC 4456: "A BGP speaker SHOULD NOT create an ORIGINATOR_ID attribute if one already exists". I'd consider modification/replacement of an existing originator ID value brokenness, and the topology you illustrate broken anyway. -danny
participants (2)
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Adhy -
Danny McPherson