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Re: Community NO-EXPORT

  • From: Danny McPherson
  • Date: Thu Aug 24 11:52:50 2000

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.