question regarding multi-homing

Steven Fischer sfischer1967 at
Wed Dec 30 12:08:27 CST 2009

If you are using Cisco...

On Wed, Dec 30, 2009 at 12:38 PM, Dylan Ebner <dylan.ebner at>wrote:

> Simon-
>   We do exactly what you are trying to accomplish. We have two routers and
> two providers. Provider A is our primary and we receive partial routes from
> them (no static route). Then Router B is connected to Provider B with no
> default route (basically it looks like we are not advertising to them). Our
> AS on router b is prepended several times. Router A and B are connected via
> iBGP to eachother. Then, using interface tracking (we are a cisco shop) we
> can fail to provider B. So, about the only failure we cannot automatically
> recover from is if we have our router A interface / layer1 to provider A
> start to fail and we get enough traffic through to keep BGP up, but errors
> make ip traffic fail.
> This failover has worked server times while in production. Mostly we see
> our BGP drop from provider A, but we have also seen link down from provider
> a. In testing we failed links and routers, which always recovered just fine.
> But we all know the lab can be completely different from the real world.
> If you want to see how this work for us, go to and enter the
> following:
> Network:
> Start: 26/12/2009 20:00:00
> End: 27/12/2009 07:00:00
> Pull out 19629 (ME)
> 209 (Qwest, provider A)
> 7263 (GoFast. Dba Sungard, provider B)
> At about 20:11 you see the routes start failing to AS7263 and then at about
> 6:23 the next day they start failing back.
> This example happened when Qwest lost an edge router in Minnesota. Link
> status was up, but BGP tables were lost, so we had no router out to qwest.
> Dylan Ebner, Network Engineer
> Consulting Radiologists, Ltd.
> 1221 Nicollet Mall, Minneapolis, MN 55403
> ph. 612.573.2236     fax. 612.573.2250
> dylan.ebner at
> -----Original Message-----
> From: Simon Chen [mailto:simonchennj at]
> Sent: Wednesday, December 30, 2009 11:03 AM
> To: nanog at
> Subject: question regarding multi-homing
> Hi all,
> Happy new year...
> I have a question regarding multi-homing, mostly from stub network's
> operational point of view. My big question is: what kind of failures
> do you usually see from your providers? Link down? Link up, but
> withdraw some routes? Link up, no route change, but blackholing
> partial or all traffic? Anything else?
> Let's say that I have two local routers (Ra and Rb) connecting to two
> providers, A and B. If router Ra sees provider A with problems of the
> first two cases (link down, link up but withdraw routes), the Rb can
> easily step up. My question is, if I am using provider A as the
> default, but provider A has the third problem (link up, no route
> change, but blackholing traffic), how can I detect it and switch
> provider automatically?
> To state this problem in detail: I use a static default route on Ra to
> forward traffic to provider A, or receive 0/0 from provider A via BGP.
> For some reason, provider A can no longer reach a /24. My network
> cannot be notified (unless, I receive a full internet routing table).
> In this case, all I know is that my traffic to /24 is blackholed
> through provider A. In this case, is there an automatic way for my
> stub network to switch over to provider B? Do I have to do the
> detection and switch over manually? I don't think VRRP can help here,
> right?
> Thanks.
> -Simon

To him who is able to keep you from falling and to present you before his
glorious presence without fault and with great joy

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