Site serveriron adx’s administrative preference, The least response selection, Round robin selection – Brocade Communications Systems ServerIron ADX 12.4.00 User Manual
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ServerIron ADX Global Server Load Balancing Guide
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Global Server Load Balancing overview
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Site ServerIron ADX’s administrative preference
The administrative preference is an optional metric. This metric is a numeric preference value from
0-255 that you assign to each site ServerIron ADX, to select that ServerIron ADX if the previous
metrics do not result in selection of a best site. The GSLB policy prefers the site ServerIron ADX with
the highest administrative preference.
The administrative preference allows you to do the following:
•
You can temporarily change the preference of a site to accommodate changing network
conditions. For example, if sites are offering proxy content service, the link between a site proxy
server farm and the content origin may be highly congested, making that site less desirable.
This factor is not visible to the ServerIron ADXs and thus cannot be reflected in the other GSLB
metrics.
•
You can temporarily disqualify a site ServerIron ADX from being selected, without otherwise
changing the site’s configuration or the GSLB ServerIron ADX’s configuration. For example, you
can perform maintenance on the site ServerIron ADX without making network changes. In this
case, set the administrative preference to 0.
•
You can bias a GSLB ServerIron ADX that is also configured as a site ServerIron ADX (for locally
configured VIPs) to always favor itself as the best site. In this case, assign an administrative
preference of 255 to the site for the GSLB ServerIron ADX itself, and assign a lower
administrative distance to the other site ServerIron ADXs, or use the default (128) for those
sites.
The administrative preference is disabled by default, which means it is not included as one of the
GSLB metrics. When you enable this metric, the default administrative preference for sites is 128.
You can change the preference on an individual site basis. To change a site’s preference, refer to
The least response selection
If multiple sites still compare equally based on all the metrics above, the GSLB ServerIron ADX
selects the site that it has selected least often before. For example, if the GSLB ServerIron ADX has
selected Site 1 and placed its IP address on top in 40% of the DNS replies, but has selected Site 2
60% of the time, then in this instance the GSLB ServerIron ADX selects Site 1. To display the
response selection percentages for the sites you have configured, use the show gslb dns zone
command. Refer to
“Displaying DNS zone and hosts”
This metric is a tie-breaker in case multiple addresses pass through all the above comparisons
without one address emerging as the best choice. If this occurs, the address of the site that has
been selected least often in previous DNS responses is selected.
Least response selection is enabled by default. You can disable the metric only by enabling the
round robin selection metric to act as the tie breaker instead. See the following section.
Round robin selection
The round robin selection metric is an alternative to the least response selection metric as the final
tie breaker. When you enable round robin selection, the GSLB ServerIron ADX automatically
disables the least response selection metric, and instead uses the round robin algorithm to select
a site. round robin selection chooses the first IP address in the DNS response for the first client
request, then selects the next address for the next client request, and so on.