Brocade Communications Systems ServerIron ADX 12.4.00 - Manual (Page 159)

ServerIron ADX Global Server Load Balancing Guide 147
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Round-trip times
1
Configuring the active RTT refresh interval
The Site ServerIron ADX maintains a timestamp for each of the LDNS prefixes in its active RTT
cache. The time stamp indicates the last time RTT was probed. If the time that has elapsed since
the last probe is greater than the RTT refresh interval on the Site ServerIron ADX, then the Site
ServerIron ADX initiates a new RTT measurement probe to the LDNS host for that prefix.
Periodically refreshing the RTT ensures that the values for a LDNS host reflect the RTT between the
LDNS host and the Site ServerIron ADX correctly.
To configure the active RTT refresh interval on the Site ServerIron ADX, enter the following
command.
SITE-ServerIronADX# configure terminal

SITE-ServerIronADX(config)# gslb agent-rtt-refresh-interval 40
Syntax: [no] gslb agent-rtt-refresh-interval
Enter a value from 30-1800 seconds for . The default is 600 seconds.
Setting the RTT algorithm modes
The GSLB ServerIron ADX on which active RTT gathering is enabled may be communicating with
Site ServerIron ADXs that support active RTT gathering and other Site ServerIron ADXs that support
only passive RTT gathering (for example, ServerIron ADX). Therefore, depending on the network
configuration and the topology, some of the RTT values that have been gathered for a prefix may be
passive RTT values while others may be active RTT values.
For example, GSLB ServerIron ADX has three Site ServerIron ADXs. Each of the Site ServerIron ADX
reported RTT values for prefix 201.53.x.x:
• Site ServerIron ADX-1 (1.1.1.102) supports active RTT gathering and reported an actively
probed RTT value of 7ms for prefix 201.53.x.x.
• Site ServerIron ADX-2 (1.1.1.117) supports passive RTT gathering and reported a passively
gathered RTT value of 12 ms for prefix 201.53.x.x.
• Site ServerIron ADX-3 (1.1.1.18) supports passive RTT gathering and reported a passively
gathered value of 18ms for 201.53.x.x.
The following will be recorded for prefix 201.53.x.x in the prefix cache of the GSLB ServerIron ADX.
Prefix: 201.53.0.0, prefix length = 16
ServerIron ADX= 1.1.1.102, rtt = 7ms, source = active (probe type = ICMP)
ServerIron ADX= 1.1.1.117, rtt = 12ms, source = passive (probe type = Not applicable)
ServerIron ADX= 1.1.1.118, rtt = 18ms, source = passive, (probe type = Not applicable)
With active RTT gathering implementation, you can create a GSLB policy and indicate which values
in the cache will be used when selecting the optimal IP address:
• RTT algorithm selection based only on passive RTT values (Mode 1): Only RTT values that were
gathered passively by Site ServerIron ADXs will be used in determining the optimal IP address.
• RTT algorithm selection based only on active RTT values (Mode 2): Only RTT values that were
gathered actively by Site ServerIron ADXs will be used in determining the optimal IP address.
• RTT algorithm selection based on both active and passive RTT values (Mode 3): All RTT values
will be used when determining the optimal IP address, whether they were gathered using
active or passive RTT measurements.