Question about BGP Update Messages

kalebkspkalebksp Member Posts: 1,033 ■■■■■□□□□□
I'm reading through RFC 1771 and I'm not sure I understand one section.

Here's the applicable section 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.

Am I correct in assuming that a single route (Update Message) can encompass multiple prefixes (which is NLRI, if I'm understanding correctly), provided they all have the exact same path attributes?

Thanks in advance!

Comments

  • cisco_troopercisco_trooper Too many Member Posts: 1,443 ■■■■□□□□□□
    The NLRI Field in the Update Message is a list of Prefixes for the advertised routes. The Update Message also contains a Withdrawn routes field which contains a similar list.
  • cisco_troopercisco_trooper Too many Member Posts: 1,443 ■■■■□□□□□□
  • kalebkspkalebksp Member Posts: 1,033 ■■■■■□□□□□
    The NLRI Field in the Update Message is a list of Prefixes for the advertised routes.

    Right, but the RFC says that an update message can only advertise one route, so I'm wondering what their definition of a route is.
  • cisco_troopercisco_trooper Too many Member Posts: 1,443 ■■■■□□□□□□
    kalebksp wrote: »
    Right, but the RFC says that an update message can only advertise one route, so I'm wondering what their definition of a route is.


    I see what you are saying...that's a good question. I would ASSUME a "route" to be defined as the next hop address for destination NLRI...if that makes sense. More to follow. Now I need closure.
  • cisco_troopercisco_trooper Too many Member Posts: 1,443 ■■■■□□□□□□
  • cisco_troopercisco_trooper Too many Member Posts: 1,443 ■■■■□□□□□□
    I think it may be time to sniff some BGP traffic....
  • kalebkspkalebksp Member Posts: 1,033 ■■■■■□□□□□
    I see what you are saying...that's a good question. I would ASSUME a "route" to be defined as the next hop address for destination NLRI...if that makes sense. More to follow. Now I need closure.

    That's sort of what I was thinking too, although the NLRI would also have to have the same path attributes to be contained in the same update message.
  • kalebkspkalebksp Member Posts: 1,033 ■■■■■□□□□□

    Nice find! So a route can have multiple prefixes associated with it.
  • APAAPA Member Posts: 959
    The way I understand it is....

    A update message can 'WITHDRAW' multiple prefixes.... but can only 'ADVERTISE' one route\prefix via the NLRI portion of the message.

    CT - I to believe it may be time to sniff some BGP pkts.... I setup a quick BGP lab last night with dynamips so I should be able to sniff this tonight... I'm at work now though icon_sad.gif

    CCNA | CCNA:Security | CCNP | CCIP
    JNCIA:JUNOS | JNCIA:EX | JNCIS:ENT | JNCIS:SEC
    JNCIS:SP | JNCIP:SP
  • kalebkspkalebksp Member Posts: 1,033 ■■■■■□□□□□
    Well I feel silly, 1771 is the original RFC, the current RFC is 4271.

    From RFC 4271:
    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.

    That seems pretty clear.

    EDIT: That's in section 3.1, if anyone is interested.
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