BGP Update Messages

Answered Question
Apr 19th, 2006
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Can a bgp UPDATE message carry more than a single prefix ? I have read a lot of text about this ( Halabi, Doyle, RFC 1771 ) but I think the wording that this literature is never clear to me about what they are exactly saying.


below is a quote from RFC 1771


" An UPDATE message can advertise at most one route, which may be

described by several path attributes. All path attributes contained

in a given UPDATE messages apply to the destinations carried in the

Network Layer Reachability Information field of the UPDATE message. "


In the wording of this RFC, is " route " same as prefix ?


Here is what Doyle says


" The Update message, whose format is shown in Figure 2-45, is used to advertise a single feasible route to a peer, or to withdraw multiple unfeasible routes, or both. "


And here is what Odom says in the Official CCIE R&S Guide


" The central concept in an individual Update message is the set of PAs. Then, all the prefixes (NLRIs) that share the exact same set of PAs and PA values are included at the end of the Update message. If a router needs to advertise a set of NLRIs, and each NLRI has a different setting for at least one PA, then separate Update messages will be required for each NLRI "


Anyone has the time to authoritatively explain this to me ?


Herbert



Correct Answer by Harold Ritter about 11 years 4 months ago

There can indeed be several prefixes per BGP update as long as they all share the same path attributes. The maximum BGP update length is 4096 bytes.


BTW: The new RFC for BGP4 is 4271 and obsoletes RFC1771.


http://www.ietf.org/rfc/rfc4271.txt?number=4271


Hope this helps,

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mheusinger Wed, 04/19/2006 - 07:02
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Hello Herbert,


the RFC is always right. ;-)

There is one prefix (network/mask) in a single BGP Update message. There can be, however, several BGP update messages in a single TCP segment. This is why one shoud always enable path MTU discovery on a BGP speaker ( ip tcp path-mtu-discovery).

While writing this I am looking at a sniffer trace from an MPLS PE router exchanging BGP updates. So I am pretty sure, that a Cisco PE (3640 with IOS 12.3T) will do exactly, what the RFC tells you.


Hope this helps! Please rate all posts.


Regards, Martin

maosa7578 Wed, 04/19/2006 - 08:08
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Hello Martin,


Again, thanks for your reply.


Below another quote from RFC 1771


" This variable length field contains a list of IP address prefixes. "


and here is another


" Reachability information is encoded as one or more 2-tuples of the form .....".


Dont these quotes suggest that the update message can advertise more than one prefix ?


Herbert.

Correct Answer
Harold Ritter Wed, 04/19/2006 - 10:52
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There can indeed be several prefixes per BGP update as long as they all share the same path attributes. The maximum BGP update length is 4096 bytes.


BTW: The new RFC for BGP4 is 4271 and obsoletes RFC1771.


http://www.ietf.org/rfc/rfc4271.txt?number=4271


Hope this helps,

maosa7578 Wed, 04/19/2006 - 13:07
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Hi Harold,


Thanks for your response. The wording in the new RFC is certainly different and a lot clearer. Thanks for the link.


This new RFC clearly states

" Routes are advertised between BGP speakers in UPDATE messages. Multiple routes that have the same path attributes can be advertised in a single UPDATE message by including multiple prefixes in the NLRI field of the UPDATE message. "


Thanks.

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