The Rlb Round Robin Algorithm - D-Link NetDefendOS User Manual

processing steps is as follows:
1. Route lookup is done in the routing table and a list of all matching routes is assembled. The
routes in the list must cover the exact same IP address range (further explanation of this
requirement can be found below).
2. If the route lookup finds only one matching route then that route is used and balancing
does not take place.
3. If more than one matching route is found then RLB is used to choose which one to use. This
is done according to which algorithm is selected in the table's RLB Instance object:
• Round Robin
Successive routes are chosen from the matching routes in a "round robin" fashion
provided that the metric of the routes is the same. This results in route lookups being
spread evenly across matching routes with same metric. If the matching routes have
unequal metrics then routes with lower metrics are selected more often and in
proportion to the relative values of all metrics (this is explained further below).
• Destination
This is similar to Round Robin but provides "stickiness" so that unique destination IP
addresses always get the same route from a lookup. The importance of this is that it
means that a particular destination application can see all traffic coming from the same
source IP address.
• Spillover
Spillover is not similar to the previous algorithms. With spillover, the first matching
route's interface is repeatedly used until the Spillover Limits of that route's interface are
continuously exceeded for the Hold Timer number of seconds.
Once this happens, the next matching route is then chosen. The Spillover Limits for an
interface are set in the RLB Algorithm Settings along with the Hold Timer number of
seconds (the default is 30 seconds) for the interface.
When the traffic passing through the original route's interface falls below the Spillover
Limits continuously for the Hold Timer number of seconds, route lookups will then revert
back to the original route and its associated interface.
Figure 4.5. The RLB Round Robin Algorithm
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Chapter 4: Routing