Black Box LR1102A-T1/E1 - Manual (Page 77)

11
M
ULTIPATH
M
ULTICAST
C
ONFIGURATIONS
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.