Multi-homed implementation and BGP convergence time

Holmes,David A dholmes at
Fri Sep 11 22:25:48 UTC 2009

The time should be measured in seconds for your BGP advertised prefixes
to propagate to most of the Internet. It may take longer for some
isolated ISP's to receive the routes. If you use the longest prefix
method to advertise to your preferred ISP, a convergence to the backup
ISP (where shorter prefixes are advertised) may take 30 seconds or so
max. Converging back to the preferred ISP should take a few seconds max.

-----Original Message-----
From: Andrew.Claybaugh at
[mailto:Andrew.Claybaugh at] 
Sent: Friday, September 11, 2009 1:55 PM
To: nanog at
Subject: Multi-homed implementation and BGP convergence time

Hello - my company currently has two connections with a single tier 1
We are using the AS from our ISP at this time.  In the next month we
be implementing a third connection with a second tier 1 ISP, so we will
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
own AS number, how much downtime will there be?  So far the second ISP
only said that it could be hours for BGP to fully converge.  We are
for more detail about how long the outage will be and how widespread.

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

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

Thank you.

Andy Claybaugh

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