Multi-homed implementation and BGP convergence time

Seth Mattinen sethm at
Fri Sep 11 22:54:37 UTC 2009

Andrew.Claybaugh at wrote:
> Hello - my company currently has two connections with a single tier 1 ISP.
> We are using the AS from our ISP at this time.  In the next month we will
> be implementing a third connection with a second tier 1 ISP, so we will now
> be using our own AS number on all three routers.  My question is when we
> implement the new connection and update our existing connections to use are
> own AS number, how much downtime will there be?  So far the second ISP has
> only said that it could be hours for BGP to fully converge.  We are looking
> for more detail about how long the outage will be and how widespread.

Hours? No way. It's more like minutes.

> Will it be relatively short to our customers that are on one of the ISPs we
> are directly connected to?  Is downtime less for customers on other tier 1
> ISPs versus tier 2, etc. ISPs?

Doesn't matter.

> We will only be receiving a default route on each of the three connections.
> Our routers will be advertising a small number of routes - 6 to 8.

I strongly encourage you to reconsider and take more than a default if
you're multihoming and your routers have enough memory. Remember to
create a full mesh on your BGP routers.

And as already said, if you're totally new to BGP and multihoming, hire
someone with experience in such matters to set it up.


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