question regarding multi-homing

Dylan Ebner dylan.ebner at
Wed Dec 30 11:38:28 CST 2009

   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:


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,


More information about the NANOG mailing list