Foundry Networks AR3202-CL - Manual (Page 224)

Multicasting
June 2004 © 2004 Foundry Networks, Inc. 14 - 7
Multicast traceroute uses any information available to it in the router to try to determine a previous hop to forward
the trace towards. Multicast routing protocols vary in the type and amount of state they keep; multicast traceroute
tries to work with all of them by using whatever is available. For example, if a DVMRP router has no active state
for a particular source but does have a DVMRP route, it chooses the parent of the DVMRP route as the previous
hop. If a PIM-SM router is on the (*,G) tree, it chooses the parent towards the RP as the previous hop. In these
cases, no source/group-specific state is available, but the path may still be traced.
Foundry supports the following PIM related feature—a “traceroute” facility for IP multicast, as defined in draft-ietf-
idmr-traceroute-ipm-05.
The mtrace command for multicast traffic is similar to the traceroute command used for unicast traffic. Unlike
traceroute, however, mtrace traces traffic backwards, from the receiver to the source. mtrace uses other unicast
routing tables for RPF. For these, mtrace relies on Foundry’ implementation of the mtrace protocol is manageable
through the CLI and can be executed from any command sub-tree of the Foundry CLI.
Multicast Multipath
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
Random 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,
Foundry-only scenarios apply the HRW method as the default. This is similar to the Cisco Systems IPv6 multicast
multipath implementation.
Multipath Commands
The following table lists the multipath commands:
T ABLE 7 M ULTIPATH C OMMANDS
Enabling HRW method Foundry/configure/ip/multicast# multipath
Enabling Cisco method Foundry/configure/ip/multicast# multipath cisco
Disabling Multipath Foundry/configure/ip/multicast# no multipath
Foundry/configure/ip/multicast# no multipath cisco
Display RPF selection Foundry#show ip rpf