question regarding multi-homing

Simon Chen simonchennj at
Wed Dec 30 11:02:53 CST 2009

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,


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