Failover solution using BGP

Malte von dem Hagen mvh at
Wed Dec 31 01:49:04 UTC 2008


Am 31.12.2008 01:19 Uhr, Braun, Mike schrieb:
> Why not just AS prepend your secondary site if the services to the
> Internet are the same at both sites and tied to the same IP addresses?

because that simply does not work (reliably). It would depend on
AS-paths of the same length from every possible source.

Simple, reliable and quite stylish is another way:

Choose primary and secondary location by announcing more specifics at
Sacramento, e.g. all networks as /20 subnets. As "longest match always
wins", any source seeing both routes for an IP address will choose

The only way traffic could reach LA would be a missing route to
Sacramento. In any other case, Sacramento is chosen. Thus, if Sacramento
(manually or automatically) stops announcing the /20s, LA's /18 and /19
will be chosen.

CAVE:  This is no failover solution for single services, just for whole
       subnets depending on the announcement at Sacramento.

CAVE2: My suggestion creates inconsistent announcements for the source
       AS. That may or may not be a problem.

Kind regards,

Malte v. dem Hagen

Abteilung Technik - Network Operations Centre
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GF: Uwe Braun - Alex Collins - Mark Joseph - Patrick Pulvermüller

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