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. regards, ------------------------------------------------------ (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" <danny@tcb.net> To: <nanog@merit.edu> 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.