ServerIron ADX Global Server Load Balancing Guide 53
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Configuring GSLB protocol parameters 1
Modifying round-trip time values
The Round-trip time (RTT) is the amount of time that passes between when the remote site
receives a TCP connection (sends a TCP SYN) from the client and when the remote site receives the
client’s acknowledgment of the connection request (sends a TCP ACK). A site ServerIron ADX sends
RTT data to the GSLB ServerIron ADX every five seconds.
You can modify RTT parameters to change processing of the RTT information reported by the GSLB
and remote site ServerIron ADXs. You can change the following parameters, on an individual basis:
•RTT cache interval: The site ServerIron ADXs use the GSLB protocol to send RTT informat ion to
the GSLB ServerIron ADX. The GSLB ServerIron ADX stores this information in a cache. The
GSLB ServerIron ADX uses the entries in the cache when using the RTT metric to evaluate IP
addresses in a DNS reply. Entries in the cache age out if they remain unused. The default aging
interval for RTT cache entries is 120 seconds. You can change the interval to a value from
10-1,000,000 seconds (about 11-1/2 days).
•RTT cache prefix: The entries in the RTT cache include IP address information for the clients. To
avoid overflowing the cache, cache entries are aggregated based on the IP information. For
example, if the GSLB ServerIron ADX receives RTT information for clients at 192.21.4.69 and
192.21.4.18, and the cache prefix is 31 bits, both addresses go in as separate entries.
However, if the prefix is 16 bits, the GSLB ServerIron ADX aggregates the addresses. In this
case, only one entry, 192.21.x.x goes in the cache. The default number of bits in the prefix is
20. You can specify a value from 1-31.
•RTT tolerance: When the GSLB ServerIron ADX compares two site IP addresses based on RTT,
the GSLB ServerIron ADX favors one site over the other only if the difference between the RTT
values is greater than the specified percentage. This percentage is the RTT tolerance. You can
set the RTT tolerance to a value from
0-100. The default is 10%.
•RTT explore percentage: Site ServerIron ADXs send RTT information only for the sessions that
clients open with them. These are clients referred to the site ServerIron ADX by the GSLB
ServerIron ADX. If the metrics that come before this one (based on the GSLB policy order) do
not select a “best” site, the ServerIron ADX selects a site based on RTT.
Since the only RTT information received by the GSLB ServerIron ADX comes from the site
ServerIron ADXs to which the GSLB ServerIron ADX has referred clients, it is possible for the
GSLB ServerIron ADX to continually bias its selection toward the first site ServerIron ADX that
sent RTT information. To prevent this from occurring, the GSLB ServerIron ADX intentionally
ignores the RTT metric for a specified percentage of the requests from a given client network.
You can specify an RTT explore percentage from 0-100. The default is 5. By default, the GSLB
ServerIron ADX ignores the RTT for 5% of the client requests from a given network.
You also can add static RTT prefix cache entries.
Changing the RTT cache interval
You can change the round trip time (RTT) cache interval, which specifies how often the site
ServerIron ADXs use the GSLB protocol to send RTT information to the GSLB ServerIron ADX. The
GSLB ServerIron ADX stores this information in a cache. The GSLB ServerIron ADX uses the entries
in the cache when using the RTT metric to evaluate IP addresses in a DNS reply.
To change the RTT cache interval from 10 seconds to 30 seconds, enter commands such as the
following:
ServerIronADX(config)# gslb policy
ServerIronADX(config-gslb-policy)# round-trip-time cache-interval 30