Question about BGP Update Messages

kalebksp

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!

cisco_trooper

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 The Update Message also contains a Withdrawn routes field which contains a similar list.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_trooper

4.3 UPDATE Message Format

kalebksp

cisco_trooper wrote: »

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_trooper

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_trooper

Here we go: 3.1 Routes: Advertisement and Storage

cisco_trooper

I think it may be time to sniff some BGP traffic....

kalebksp

cisco_trooper wrote: »

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.

kalebksp

cisco_trooper wrote: »

Here we go: 3.1 Routes: Advertisement and Storage

Nice find! So a route can have multiple prefixes associated with it.

APA

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

kalebksp

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.