Multi-POP design check/help question

Fouant, Stefan Stefan.Fouant at
Sun Sep 20 21:37:06 UTC 2009

I don't know if you want to arbitrarily use local-pref and AS-Path prepend in a one-size-fits-all approach, as under certain scenarios it might be more beneficial to route traffic between POPs to take advantage of routes via shortest AS Path or other constraints.  Why not just extend your IGP across all POPs and set inter-POP links to a higher metric?  In this scenario, if a given route is received via muliple POPs and all things being equal (AS Path, etc.), you'll prefer to route traffic out the shortest-cost path to a POP exit point.

Sorry for the top post, I'm on my BB.

Stefan Fouant 
Neustar, Inc. / Principal Engineer
46000 Center Oak Plaza Sterling, VA 20166
Office: +1.571.434.5656 ▫ Mobile: + ▫ GPG ID: 0xB5E3803D ▫ stefan.fouant at

----- Original Message -----
From: Rick Ernst <nanog at>
To: nanog at <nanog at>
Sent: Sun Sep 20 16:17:41 2009
Subject: Multi-POP design check/help question

Cross-posted from cisco-nsp.  We are a (mostly) Cisco shop, but I'm looking
more for BCP and overall design, not provisioning specifics.


My Cisco bookshelf isn't helping me much with this...

We currently have a single POP with border/core/aggregation topology.
Upstreams each come in on their own border router and the core is used as a
route-reflector for border and aggregation. The internal network uses OSPF
as an IGP and each device is dual-connected for redundancy on independent
layer-2 networks.  OSPF load-shares with loopback IPs and IBGP uses the
loopback addresses for peering.

We are looking at turning up two additional POPs in the metro area, each
connected by redundant GigE loops to the original POP.  Each POP may have
zero or more direct upstream connections.  I'd like local traffic at each
POP to prefer both in and outbound traffic via the local upstream, but still
be able to failover to upstreams at other POPs if needed.

My initial thoughts are to BGP peer between POPs with a higher local-pref
for the local outbound traffic and to prepend between the POPs so inbound
traffic is more likely to take the shortest path inbound.

Is this too simplistic? Prone to trouble? What gotchas should I be looking
at, or other designs should I be considering?

More information about the NANOG mailing list