Brocade Communications Systems ServerIron ADX 12.4.00 - Manual (Page 155)

ServerIron ADX Global Server Load Balancing Guide 143
53-1002437-01
Round-trip times
1
Active RTT is always measured between the Site ServerIron ADX and the client LDNS. This method
of measuring RTT enables the GSLB ServerIron ADX to use this actively gathered RTT even if the
client and its LDNS do not share the same network prefix.
FIGURE 10 Active RTT gathering
Client
1. Client sends a DNS query
for www.brocade.com to
LDNS.
LDNS
Site ServerIron
2. LDNS sends the query to
the GSLB ServerIron. The
GSLB ServerIron replies with
the optimal IP address for the
client.
GSLB ServerIron
4. Site ServerIron sends
RTT probes to the LDNS
to actively gather RTT.
3. GSLB ServerIron
requests the Site
ServerIron to gather
RTT actively for the
LDNS server.
5. Site ServerIron
reports gathered RTT
for the LDNS server to
GSLB ServerIron
using the GSLB
protocol.
By default, the Site ServerIron ADXs actively gather RTT measurement by sending ICMP probes. In
addition to ICMP probes, Site ServerIron ADXs can be configured to send DNS probes to gather RTT.
Each Site ServerIron ADX maintains an active RTT cache. This cache contains the LDNS host
prefixes and LDNS host IP addresses that the Site ServerIron ADX received from the GSLB
ServerIron ADX. When a Site ServerIron ADX receives an active RTT request from the GSLB
ServerIron ADX, along with a list of LDNS host IP addresses, it compares the LDNS host IP address
in the list to the LDNS prefixes in its active RTT cache. If the host prefix already exists in its active
RTT cache, the Site ServerIron ADX refreshes that prefix entry. If it does not exist, then the Site
ServerIron ADX creates a new entry for the LDNS prefix and LDNS host IP address. It also initiates
RTT measurement for the prefix by sending a probe to that LDNS host.
The Site ServerIron ADX maintains a timestamp for each LDNS prefix in the active RTT cache. The
timestamp indicates the last time the prefix was probed for RTT. If the time that has elapsed since
the last probe is greater than the active RTT refresh interval on the Site ServerIron ADX, then the
Site ServerIron ADX initiates a new RTT probe to the LDNS host for that prefix. Periodically
refreshing the RTT ensures that the values accurately reflect the RTT between the LDNS host and
the Site ServerIron ADX and takes into account any changes in the network conditions.
Support for both active and passive RTT
Brocade GSLB supports both active and passive RTT gathering.
Active RTT gathering is available on ServerIron ADXs running GSLB protocol version 3 or later. Use
the show gslb site command to determine what protocol version a ServerIron ADX is running.
GSLB ServerIron ADXs periodically exchange version information with the Site ServerIron ADXs
using the GSLB protocol. A protocol version number greater than or equal to 3 indicates that active
RTT gathering is available in the software running on the ServerIron ADXs. ServerIron ADXs with a
version less than 3 supports passive RTT gathering only.