ServerIron ADX Global Server Load Balancing Guide 151
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Round-trip times 1
If both the ICMP and DNS fast-aging commands are enabled on the Site ServerIron ADX, then
failure of either ICMP or DNS probes will quickly age out LDNS prefixes from the active RTT cache.
Typically you should enable only one of these commands. Follow the guidelines below to determine
which command to enable:
•If you want DNS RTT measurements to be the primary source of RTT and DNS prober is
enabled on the Site ServerIron ADX, then enable the gslb agent-fast-age-dns-fail command if
LDNS prefixes need to quickly age out if a DNS probe fails.
•If you want ICMP RTT measurements to be the only source of RTT and DNS prober is not
enabled on the Site ServerIron ADX, then enable the gslb agent-fast-age-icmp-fail command if
LDNS prefixes need to quickly age out if an ICMP probe fails.
•If you want the LDNS prefix to be aged out when either DNS or ICMP prober fails, then enable
both commands on the Site ServerIron ADX.
•If no fast aging is desired, then do not enter either of the commands.
Active RTT gathering and high availability support
Brocade GSLB with active RTT gathering will operate in all High Availability (HA) configurations
supported in earlier software releases.
To ensure that active RTT gathering is synchronized between devices that are in an HA
configuration, GSLB active RTT gathering does the following:
•If two GSLB ServerIron ADXs are in an HA configuration and both have active RTT gathering
enabled, then each active RTT gathering Site ServerIron ADX reports the actively gathered RTT
information to both the GSLB ServerIron ADXs. This ensures that the standby GSLB ServerIron
ADX receives the actively gathered RTT information and is able to immediately use this
information if the primary GSLB ServerIron ADX fails over.
•If two Site ServerIron ADXs are in an HA configuration and have active RTT gathering enabled,
then the GSLB ServerIron ADX sends the active RTT gathering request with the list of LDNS
hosts that are to be probed to both the Site ServerIron ADXs. Each of the Site ServerIron ADXs
in the HA configuration actively gathers RTT measurements from the LDNS hosts in the list and
reports information to the GSLB ServerIron ADX. This ensures that the GSLB ServerIron ADX
receives RTT information for both the Site ServerIron ADXs. If the primary Site ServerIron ADX
fails over and the standby Site ServerIron ADX takes over, the GSLB ServerIron ADX will already
have the RTT information for the standby Site ServerIron ADX. The GSLB ServerIron ADX will be
able to immediately select the best IP address based on the information it has.