3Com 4007, Implementation Manual, Page: 445 / 684

Using IP Multicast Traceroute 445
Using IP Multicast
Traceroute
You can perform an IP multicast traceroute from a Layer 3 module. The
ability to trace the path of a IP multicast group packet from a source to a
particular destination is desirable for troubleshooting purposes.
Unlike unicast traceroute, IP multicast traceroute requires the ability for
routers to understand a special IGMP packet type and the related
processes.
Beginning a trace from an IP multicast source would be difficult because,
at forks in the network paths, there is no way to determine which
direction to take. You would have to flood the entire tree and wait for
responses (or the lack thereof) to find the path. Thus, a more efficient
approach is to start at the destination and travel backwards toward the
source, using the knowledge held by IP multicast routing protocols that
work by calculating previous hops back toward sources.
An IP multicast traceroute proceeds as follows:
1 At the destination node (your module), you specify a source and group
address.
2 The module sends a traceroute Query packet to the last-hop multicast
router (the upstream router for this source-group pair).
3 The last-hop router turns the Query packet into a Request packet by
adding a response data block containing its interface addresses and
packet statistics. It then forwards the Request packet via unicast to the
router that it believes is the previous hop for the given source-group pair.
4 Each previous hop router adds its response data to the end of the Request
packet, then forwards it via unicast to the next previous hop router.
5 Finally, the first-hop router — that is, the router that believes that the
source-group packets originate on one of its directly-attached
subnetworks — adds its data, changes the Request packet to a Response
packet, and sends the completed response back to the destination node
that issued the traceroute query.
6 You see a display that shows IP addresses of the interfaces that span from
your module back to the source that you specify. The display also shows
the number of hops back to those interfaces, the multicast routing
protocols used, and the amount of time it takes to reach each hop from
the receiver.