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@securian.com [mailto:Andrew.Claybaugh@securian.com] Sent: Friday, September 11, 2009 1:55 PM To: nanog@nanog.org Subject: Multi-homed implementation and BGP convergence time 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. 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? 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. Thank you. Andy Claybaugh