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 ?
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.
Hope this helps,