Foundry Networks AR3202-CL, AR3201-CL, AR1204, AR1216 manual Multicast Multipath, Multipath Commands

Multicasting

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:

TABLE 7 MULTIPATH COMMANDS

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 <addr#
	<addr# - source or RP address

June 2004 © 2004 Foundry Networks, Inc. 14 - 7