WCCP load distribution

The WCCP protocol provides the assignment method for dynamic symmetric and asymmetric load distribution in a cluster. WCCP detects node failures and performs redistribution based on the configuration communicated to it by Content Gateway.

Load distribution is configured in Content Gateway Manager and is pushed to the WCCP devices.

Load distribution is configured per service group.

For each service group:

Participating cluster members must be registered to the service group. (The WCCP device makes no decisions about load balancing.)

The HASH or MASK assignment method is selected. HASH is typically used with the GRE forward/return method, and MASK with the L2 forward/return method.


	Important MASK was developed specifically for the Cisco Catalyst series switches, and is one of the key characteristics that enable WCCP interception to be performed completely in hardware on these platforms. It should be used only with devices for which there is documented support.

One or more distribution attributes are selected. Typically the destination IP address is used.

If load is to be distributed to different cluster members in different proportions, a weight value is set on each cluster member. These values determine the proportion of requests each will receive relative to other members of the cluster. This option is only useful if the Synchronize in the Cluster option is disabled. See Configuring service groups in the Content Gateway manager.

Asymmetric load distribution using the weight value is helpful when:

There are multiple Content Gateway servers with different performance capabilities, for example a V-Series V10000 G2 and a V10000 G3.

The Internet traffic profile doesn't lend itself to even distribution due to preferences for specific origin servers (and therefore destination IP addresses).

How dynamic redistribution works:

Dynamic redistribution is accomplished when the WCCP device detects that a cluster member is offline. It then automatically redistributes the load to the remaining cluster members based on the load distribution configuration. When a cluster member returns to service and is detected by the WCCP device, load distribution is, again, automatically adjusted based on the configuration.

For configuration steps, see Configuring service groups in the Content Gateway manager.

How the weight value supports asymmetric load distribution:


	Important Weight is only useful if the Synchronize in the Cluster option is disabled. See Configuring service groups in the Content Gateway manager.

The weight value is unique to each service group and node. The weight value does not propagate around the cluster and must be set individually on every node in the cluster.

The value of weight, relative to the settings on other cluster members, determines the proportion of traffic that WCCP directs to the node.

By default, weight is set to 0, which results in equal distribution to all cluster members.

To achieve asymmetric distribution, weight is set relative to other members of the cluster. For example, assume a cluster of 3 nodes:


Node	Weight value	Load distribution
Node1	50	50%
Node2	25	25%
Node3	25	25%

If Node1 goes offline, Node2 and Node3 will get an equal amount of traffic. If Node3 goes offline, Node1 will get two thirds of the traffic and Node2 will get one third of the traffic.

Because the weight value is relative to the settings on other cluster nodes, the same distribution as above can be achieved with weight values of 10, 5, 5. (The valid range of weight is 0-255.)

If weight is changed from its default value of 0, it should be configured on all nodes in the cluster.