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Chapter 14 Load Balancing in the P333R-LB
38 Avaya P333R-LB User’s Guide
with a Real IP address. These metrics tell the router which server should receive
each session.
SLB uses the following metrics:
• Round Robin.
•Hash.
• MinMiss Hash.
The default metric for SLB is Round Robin.
Each load balancing metric can be performed on one of the following metric
parameters:
•SRC IP (source IP address).
• DST IP (destination IP address).
• SRC and DST IP (a combination of source and destination IP addresses).
In addition, each of the metrics may be weighted. For full information on weighted
Real Servers, see Weighted Real Servers on page 40.
Round Robin
Round Robin is the default metric for SLB.
Using the Round Robin metric, new sessions are issued to each server in turn based
on the Real Server weight. The first Real Server in the group receives the first ‘n’
sessions, where ‘n’ is the Real Server weight. The second Real Server receives the
next ‘n’ sessions, and so on. When all the servers receive at least one session, the
issuing process starts over with the first Real Server.
Hash
Using the Hash metric, sessions are distributed to Real Servers using a predefined
mathematical hash function. This hash function is created by a hash value,
performed on a specific parameter. The source and destination IP addresses are
used as the hash function input.
P333R-LB creates a list of all the currently available servers. The result of the hash
function is used to select a server from the list. Specific addresses always produce
the same hash function, providing natural persistency.
If a server is added or removed to the group, the persistency will be broken. This
occurs, since the order of the servers in the list changes, but the hash still points to
the same list entries. The following figure illustrates how a loss of persistency occurs
when a server becomes non-operational.