Cross-colo fail-over design, DNS level fail-over?

Question

I\'m interested in cross-colo fail-over strategies for web applications, such that if the main site fails users seamlessly land at the fail-over site in another colo.

Th

Answer 1

DNS based mechanisms are troublesome, even if you put low TTLs in your zone files.

The reason for this is that many applications (e.g. MSIE) maintain their own caches which ignore the TTL. Other software will do a single gethostbyname() or equivalent call and store the result until the program is restarted.

Worse still, many ISPs' recursive DNS servers are known to ignore TTLs below their own preferred minimum and impose their own higher TTLs.

Ultimately if the site is to run from both data centers without changing its IP address then you need to look at arrangements for "Multihoming" via global BGP4 route announcements.

With multihoming you need to get at least a /24 netblock of "provider independent" (aka "PI") IP address space, and then have that only be announced to the global routing table from the backup site if the main site goes offline.