Brocade Communications Systems ServerIron ADX 12.4.00 - Manual (Page 43)

ServerIron ADX Global Server Load Balancing Guide 31
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Configuring GSLB protocol parameters
1
Removing IP addresses for sites that fail a health check
By default, the ServerIron ADX does not remove an IP address from a DNS reply even if the address
fails a health check.
You can configure the ServerIron ADX to remove IP addresses from DNS replies when those
addresses fail a health check. The ServerIron ADX removes the addresses that fail the check so
long as the DNS query still contains at least one address that passes the health check.
A site must pass all applicable health checks (Layer 4 and Layer 7) to avoid being removed.
NOTE
If all the sites fail their health checks, resulting in all the sites being rejected by the GSLB ServerIron
ADX, the ServerIron ADX sends the DNS reply unchanged to the client.
When DNS active policy is enabled, there is a case where a client will still get an IP that failed a
health check. Therefore, when an IP list for a zone is configured, you need to also configure DNS
override on the GSLB policy.
The GSLB default behavior is as follows:
• In DNS proxy, the entire list of IP addresses is sent back to the client with the best IP
address selected by the controller at the top of the list. This best IP is selected in
accordance with the GSLB policy. An administrator typically configures active only,
because the LDNS may cache this response for TTL time and may round robin the IPs in
this list in some cases.
• Health check in the GSLB policy is disabled. Typically administrators will not disable health
check if they are using active only.
• Active only applies only to the remaining IP addresses in the list, not the best one. An
administrator should enable health check for best IP selection to ensure that best IP is
healthy.
To configure the ServerIron ADX to remove IP addresses from DNS replies when those addresses
fail a health check, enter the following commands.
ServerIronADX(config)# gslb policy

ServerIronADX(config-gslb-policy)# dns active-only
Syntax: [no] dns active-only
Removing all addresses except the best address
By default, the GSLB ServerIron ADX retains the same number of IP addresses in the DNS replies
from the DNS server. The GSLB policy swaps the IP address on the top of the list with the “best”
address, selected by the GSLB policy. You can configure the ServerIron ADX to remove all
addresses except the one the GSLB policy selects as the best address.
NOTE
If the GSLB policy does not result in the selection of a “best” address, the DNS reply can still contain
multiple addresses.
To configure the GSLB ServerIron ADX to remove all addresses except the best address from the
DNS replies, enter the following commands.
ServerIronADX(config)# gslb policy

ServerIronADX(config-gslb-policy)# dns best-only
Syntax: [no] dns best-only