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.
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.
participants (2)
-
Danny McPherson -
Yu Ning