Question about BGP Update Messages
kalebksp
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:
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!
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
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cisco_trooper
Member Posts: 1,441 ■■■■□□□□□□
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 The Update Message also contains a Withdrawn routes field which contains a similar list.
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cisco_trooper
Member Posts: 1,441 ■■■■□□□□□□
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kalebksp
Member Posts: 1,033 ■■■■■□□□□□
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
Member Posts: 1,441 ■■■■□□□□□□
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
Member Posts: 1,441 ■■■■□□□□□□
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kalebksp
Member Posts: 1,033 ■■■■■□□□□□
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
Member Posts: 1,033 ■■■■■□□□□□
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
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
CCNA | CCNA:Security | CCNP | CCIP
JNCIA:JUNOS | JNCIA:EX | JNCIS:ENT | JNCIS:SEC
JNCIS:SP | JNCIP:SP -
kalebksp
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.
