Using IP Multicast Traceroute 445Using IP MulticastTracerouteYou can perform an IP multicast traceroute from a Layer 3 module. Theability to trace the path of a IP multicast group packet from a source to aparticular destination is desirable for troubleshooting purposes.Unlike unicast traceroute, IP multicast traceroute requires the ability forrouters to understand a special IGMP packet type and the relatedprocesses.Beginning a trace from an IP multicast source would be difficult because,at forks in the network paths, there is no way to determine whichdirection to take. You would have to flood the entire tree and wait forresponses (or the lack thereof) to find the path. Thus, a more efficientapproach is to start at the destination and travel backwards toward thesource, using the knowledge held by IP multicast routing protocols thatwork 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 groupaddress.2 The module sends a traceroute Query packet to the last-hop multicastrouter (the upstream router for this source-group pair).3 The last-hop router turns the Query packet into a Request packet byadding a response data block containing its interface addresses andpacket statistics. It then forwards the Request packet via unicast to therouter 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 Requestpacket, then forwards it via unicast to the next previous hop router.5 Finally, the first-hop router — that is, the router that believes that thesource-group packets originate on one of its directly-attachedsubnetworks — adds its data, changes the Request packet to a Responsepacket, and sends the completed response back to the destination nodethat issued the traceroute query.6 You see a display that shows IP addresses of the interfaces that span fromyour module back to the source that you specify. The display also showsthe number of hops back to those interfaces, the multicast routingprotocols used, and the amount of time it takes to reach each hop fromthe receiver.
