[AusNOG] Small Pipe BNE/Agile issue

Andy Davidson andy at nosignal.org
Fri Jun 27 08:35:33 EST 2008

On 15 Jun 2008, at 01:37, James Spenceley wrote:
> We are about to bring up the Any2 MLPA in San Jose and they (from  
> first glance) don't actually send the route-server AS in the path,  
> which is really cool.
> I'm not sure how they do it, but this is from their technical FAQ.
> "no bgp enforce-first-as, This is due to the fact that a route  
> server doesn't send itself (its AS and IP you peer with in the next  
> hop - it sends you the AS and IP of the peer's next hop)."

Hi, James

This reply is pretty late, but I scanned the rest of the thread and  
didn't see the answer, so I hope I am not too late to offer a  
meaningful insight.

The route server is likely to be a pc running Quagga rather than a  
traditional router as you may expect.  Quagga has a command in the  
config which causes the behaviour you describe -- it readvertises the  
prefix with an unmodified next hop, preserves any meds, and crucially  
retains the as-path.  This behaviour is invoked with 'neigh x.x.x.x  
route-server-client' in the bgp config.

This isn't the most compelling reason to use Quagga as an ixp MLP  
route server.

The extremely neat feature of Quagga is that one should enable the  
'bgp multiple-instance' feature when deploying a route server.  This  
causes a per-neighbour view of the RIB to be generated, so if route- 
server peers are sending communities that would cause prefixes not to  
be sent to *some* other peers, then will not conflict with each  
other.    Example :

  prefix originated by as2.  as2 is a customer of as3.
  as2, as3 and as1 are members of an MLP.  The MLP sees  
through two paths   3  2   2

The 'best' route is the shortest as-path of '2', compared with '3 2',  
so the Quagga software selects the latter as the best path.  As 2 set  
a community that prevents their announcements being reflected to as1  
via the ixp route server.  as3 have an open peering policy, and are  
happy for as1 to see the route.  However, with a single view, this  
route wont be sent to as1, because it is discarded in the single RIB,  
in favour of the shorter route.

Of course, the per neighbor RIB leads to a greater hunger for system  
resources on the route-server itself.

Best wishes

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