Multi-POP design check/help question

Truman Boyes truman at
Mon Sep 21 04:34:33 UTC 2009

On 21/09/2009, at 7:37 AM, Fouant, Stefan wrote:

> 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

I agree with Stefan. You are better off extending your IGP across PoPs  
as it will give you more flexibility in the long run. If you ever want  
to go down the path of traffic engineering / MPLS / etc, you will find  
it much easier as this will allow for CSPF and multi-topology routing  


> ----- 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?

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