Multi-homed implementation and BGP convergence time

Jay Hennigan jay at
Fri Sep 11 21:17:07 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.

It should not take several hours.  Typically less than 15 minutes.

I would suggest that you first ensure that your networks and ASN are in 
the routing registries.  Then schedule a downtime with your present ISP 
and begin advertising using your ASN.

If you're not presently speaking BGP with your existing ISP, set that up 
first advertising your network(s) with your own ASN.

> 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?

There may be a short downtime when you switch to originating from your 
own ASN.  With sufficient clue on your part and that of your current 
ISP, and assuming that either of the two connections can handle all of 
your traffic, you may be able to eliminate most or all of it.  Adding 
the second ISP won't result in significant downtime especially if you're 
just taking default routes and your routers don't need to build large 
BGP tables.

"Tier 1", "tier 2" etc. are terms used primarily by salespeople, and 
don't have a lot to do with technical matters.

> 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.

Jay Hennigan - CCIE #7880 - Network Engineering - jay at
Impulse Internet Service  -
Your local telephone and internet company - 805 884-6323 - WB6RDV

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