IP failover/migration question.

Mon Jun 12 10:10:00 UTC 2006

> clear understanding as to what is involved in terms of moving the IPs,
> and how fast it can potentially be done.

I don't believe there is any way to get the IPs
moved in any kind of reasonable time frame for
an application that needs this level of failover
support.

If I were you I would focus my attention on
maintaining two live connections, one to each
data centre. If you can change the client software,
they they could simply open two sockets, one for
traffic and one for keepalives. If the traffic
destination datacentre fails, your backend magic
starts up the failover datacentre and the traffic
then flows over the keepalive socket.

And if you can't change the clients, you can do
much the same by using two tunnels of some sort,
MPLS LSPs, multicast dual-feed, GRE tunnels. 
The Chicago Mercantile Exchange has published
a network guide that covers similar use cases.
In the case of market data, they generally run
both links with duplicate data and the client
chooses whichever packets arrive first. Since
market data applications can win or lose millions
of dollars per hour, they are the most time-sensitive
applications on the planet.
http://www.cme.com/files/NetworkingGuide.pdf

> When I desire to migrate hosts to the failover site, B would send a
> BGP update advertizing  that the redundant link should become
> preferred,

There is your biggest timing problem which is 
also effectively out of your control. By maintaining
two live connections over two separate paths to
two separate data centers, you have more control
over when to switch and how quickly to switch.

--Michael Dillon