Subject: Re: [GROW] draft-scudder-bmp-01

From: "John G. Scudder" <[email-masked]>
Date: Wed, 1 Apr 2009 15:30:33 -0400

Hi Shane,

Very belated (sorry!) answers to your questions in-line below.

On Nov 19, 2008, at 12:09 AM, Shane Amante wrote:
> A few questions:
> 1)  The draft says the following in Section 2:
> ---snip---
> If the peer is a "Global Instance Peer", this field is zero filled.  
> If the peer is a "L3VPN Instance Peer", it is set to the route  
> distinguisher of the particular L3VPN instance that the peer belongs  
> to.
> ---snip---
> I'm a little confused by the second sentence.  Is that referring to  
> the case where a router is doing RFC 4364 "Option A" style eBGP with  
> a remote peer, or something else?

Basically, this means "anything that terminates on a VRF", which  
includes "Option A".

> 2)  In Section 3, the draft talks about sources of routing  
> information (Adj-RIB-In or Loc-RIB) sent in Route Monitoring  
> messages.  Would it be useful to have a bit in the BMP "Peer Flags"  
> that indicates if the path being sent in a RM message was retrieved  
> pre- (Adj-RIB-In) or post- (Loc-RIB) policies, so the receiver knows  
> which he/she is looking at?

The "L" flag will be returning in the forthcoming revision.

> 3)  How will BMP cope with BGP flap dampening being enabled on a BMP  
> source router?  In other words, a router (configured as a BMP  
> source) receives a series of WITHDRAW's & UPDATE's that it is  
> configured to apply flap dampening on, suppressing re-advertisement  
> of these updates further into an AS.  If the BMP source is  
> forwarding messages from Adj-RIB-In, would a BMP receiver see all  
> incoming WITHDRAW's & UPDATE's associated with a "flap" event, (even  
> though these updates would have been suppressed from further re- 
> advertisement after flap dampening is applied)?  How would, or  
> should, a BMP receiver know that flap dampening is enabled and/or  
> would be applied?

Yes, all the flaps would be seen.  (Modulo the possibility of state  
compression eliding some messages, but the point is dampening wouldn't  
be applied.)

In terms of the monitor knowing if dampening is enabled, conveying the  
configuration of the router is well beyond the scope of BMP.  I think  
the answer is "look at the router config".  In terms of the monitor  
knowing if dampening was applied to a particular route, one can think  
of heuristics to work this out by correlating BMP-learned data with  
data from a regular BGP session.

>    The reason I ask is perhaps this is more a question of the  
> 'authenticity' of "monitoring data" at a BMP receiver with respect  
> to formulating what did (or could) have happened with a series of  
> Updates being propagated further within an AS?  Or, perhaps there's  
> another answer?

The conceptual model is to come as close as feasible to having the  
monitor see what the router sees arriving from its peers.  That's the  
type of 'authenticity' we're going for.

HTH,

--John