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A GSLB Reality Check

Multiple data centers lessen that risk. A secondary data center can serve as a backup or hot standby site, or you can use it to share the load of client requests. If one data centers fails, you can redirect clients to another that works. One way to do this is to modify your authoritative DNS server so that it monitors the health of each data center. There are freely available scripts for BIND and other types of DNS servers, but GSLB products from Cisco Systems, F5 Networks, Foundry Networks, NetScaler, Nortel Networks, Radware and others provide the same benefit. Such GSLB devices act as authoritative name servers.

Say is hosted at data centers in Los Angeles and London, with IP addresses and, respectively. A GSLB device monitors the health and load of each data center using a simple ping test, periodic HTTP get requests, or a more advanced interrogation based on SNMP or a proprietary protocol between the GSLB device and the equipment at the data center. When a client attempts to resolve the FQDN (Fully Qualified Domain Name), that request eventually arrives at the GSLB device. (For an overview of DNS resolution in GSLB, see "Step by Step," at far right.)

The GSLB device decides whether to direct the client to or to If it determines that the London data center has failed, it will direct all clients to the Los Angeles IP address until London is back up.

The Catch to Caching

DNS servers and GSLB devices can limit the life of an answer using the TTL (time to live) response parameter, which GSLB device vendors usually recommend setting to a low (or zero) value. Unfortunately, most browsers and some popular proxy servers ignore the TTL value, caching DNS answers for 15 minutes to six hours. This browser-based DNS caching hurts business continuity.

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