Multi-homed implementation and BGP convergence time

Andy Davidson andy at
Mon Sep 21 12:26:50 UTC 2009

On 11 Sep 2009, at 21:54, 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.

Does this mean that right now, you BGP peer with your ISP on a private  
ASN which they have given you ?

I also assume that you have your own PI, and that you are not  
deaggregating some of your providers' addressing ....

> 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 will be hours if you don't plan the work in advance, but if you  
partner with someone who rolls this stuff out all of the time to plan  
and execute the work, then there will be a short amount of downtime.

If your kit supports local-as, then I would roll this out in a few  

  - Migrate to your new ASN for ibgp, use local-as to announce via the  
old asn on your ebgp session with ISP1.  This is the bit where the  
service disruption will be.  By keeping the scope of this window  
small, you increase the chances of this disruptive maintenance working  
  - Turn up isp2.  Test, thoroughly.
  - Migrate isp1 from the private asn to your new public asn.  All  
traffic should pass through isp2, so disruption should be limited.  
Test, thoroughly.

> 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?

Downtime is less the more competent your ISP. :-)  "Tierness" is not a  
measure of this.

Sorry for the late reply, if this still needs to be rolled out, then  
we can help.

Best wishes

Regards, Andy Davidson       +44 (0)20 7993 1700
NetSumo  Specialist networks consultancy for ISPs, Whitelabel 24/7 NOC
/* Opinions are mine & do not constitute policy of those I work for */

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