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Re: Community NO-EXPORT

  • From: Yu Ning
  • Date: Thu Aug 24 21:53:22 2000

Hi Haas,

Danny's reply is clear and right! I just want a little supplement.
While full mesh IBGP in the transit backbone eliminate the need for
synchronization, we usually flood all the IBGP/EBGP routers their
next-hop via IGP. So you needn't worry about synchronization of the 
next-hops, but their reachibility.


(Mr.) Yu(2) Ning(2)
Int'l/Domestic Routing/Resource Man.
ChinaNET(AS4134) Backbone Operation Center
Networking Dep.,Datacom Bureau, China Telecom.
----- Original Message ----- 
From: "Danny McPherson" <[email protected]>
To: <[email protected]>
Sent: Friday, August 25, 2000 5:55 AM
Subject: Re: Community NO-EXPORT 

> No, BGP synchronization does indeed refer to the 
> requirement that the destination network be available
> via the IGP.  
> If it were just the BGP NEXT_HOP value it wouldn't be 
> of much use, as intermediate nodes perform forwarding 
> based on the DA in the packet and [if not synchronized] 
> won't find a match.  As a result, the packet will be 
> discarded.
> Of course, most folks simply have full mesh IBGP 
> (perhaps via RR or confeds) and so there's no reason
> too enable [or not disable] BGP synchronization. 
> -danny
> > Color me confused, but isn't the synchronization waiting on the 
> > NEXT_HOPs showing up in your IGP, not the actual BGP route?
> > 
> > After all, the issue is this:
> > 
> > BR-A - (your internal network) - BR-B
> > 
> > A route shows up at BR-A with a nexthop of some interface on BR-A
> > (or the loopback interface of BR-A).  It is then propogated via
> > iBGP to BR-B.
> > 
> > It is only unsafe to install said route and propogate it BR-B's peers
> > if the route's nexthop is not reachable by BR-B.
> > 
> > This is a far cry from having to inject your BGP into your IGP.
> > 
> > I will note that this isn't how Cisco has it documented, and I don't know
> > how they actually treat the sync issue.  The documentation actually
> > says it does wait for the route to show up in the IGP.