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Re: Community NO-EXPORT

  • From: Jeff Haas
  • Date: Thu Aug 24 11:20:34 2000

On Wed, Aug 23, 2000 at 09:55:59AM -0700, Bradley Dunn wrote:
> On Wed, Aug 23, 2000 at 10:03:21AM -0400, Kai Schlichting wrote:
> > Correct me if I'm wrong, but isn't there a synchronization issue here
> > as well? E.g.: AS2 is transit provider, probably has bgp synchronization
> > on (default), and will only propagate routes to other AS's that have
> > made it into their IGP. The question then is: have all 3 routes made
> > it into AS2's IGP successfully? Only if the answer is yes, will it
> > actually propagate that /16.
> Synchronization is almost universally disabled in the real world.

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.

> None of the IGPs in use today would cope well with a full BGP
> table redistributed into them. Redistribution of BGP->IGP is
> rarely needed or advisable.

However, its a wonderful way to see the failure states of your router's
IGPs. :-)

Jeffrey Haas - Merit RSng project - [email protected]