Black Box LR1114A-T1/E1 manual Multipath Multicast Configurations, 11.1Multipath Multicast

11

MULTIPATH MULTICAST

CONFIGURATIONS

11.1Multipath Multicast

The multicast multipath feature allows load balancing on multicast traffic across equal cost paths. Equal cost multipath routing is useful when multiple equal cost routes to the same destination exist. These routes can be discovered and be used to provide load balancing among redundant paths. Commonly used methods for multipath forwarding are Round-Robin and Random. While these methods do provide a form of load balancing, but variable path MTUs, variable latencies, and debugging can limit the effectiveness of these methods.

The following methods have been developed to deal with the load balancing limitations of the Round-Robin and Ran- dom methods:

„Modulo-N Hash —To select a next-hop from the list of N next-hops, the router performs a modulo-N hash over the packet header fields that identify a flow.”

„Hash-Threshold—The router first selects a key by performing a hash over the packet header fields that identify the flow. The N next-hops have been assigned unique regions in the hash functions output space. By comparing the hash value against region boundaries the router can determine which region the hash value belongs to and thus which next-hop to use.

„Highest Random Weight (HRW)—The router computes a key for each next-hop by performing a hash over the packet header fields that identify the flow, as well as over the address of the next-hop. The router then chooses the next-hop with the highest resulting key value.

The Round-Robin and Random methods are disruptive by design (that is, if there is no change to the set of next-hops, the path a flow takes changes every time). Modulo-N, Hash Threshold, and HRW are not disruptive.

RFC 2991 recommends to use HRW method to select the next-hop for multicast packet forwarding. or this reason, Black Box-only scenarios apply the HRW method as the default. This is similar to the Cisco Systems IPv6 multicast multipath implementation.