Multicast Routing and Forwarding Overview 1101multicast information from Source travels from Router A to Router B and then toRouter C.Multicast Traceroute The multicast traceroute utility is used to trace the path that a multicast streampasses from the multicast source to the last-hop router.Concepts in multicast traceroute1 Last-hop router: If a router has one of its interfaces connecting to the subnet thegiven destination address is on, and if the router is able to forward multicaststreams from the given multicast source onto that subnet, that router is calledlast-hop router.2 First-hop router: the router that directly connects to the multicast source.3 Querier: the router requesting the multicast traceroute.Introduction to multicast traceroute packetsA multicast traceroute packet is a special IGMP packet, which differs fromcommon IGMP packets in that its IGMP Type field is set to 0x1F or 0x1E and thatits destination IP address is a unicast address. There are three types of multicasttraceroute packets:■ Query, with the IGMP Type field set to 0x1F,■ Request, with the IGMP Type field set to 0x1F, and■ Response, with the IGMP Type field set to 0x1E.Process of multicast traceroute1 The querier sends a query to the last-hop router.2 Upon receiving the query, the last-hop router turns the query packet into a requestpacket by adding a response data block containing its interface addresses andpacket statistics to the end of the packet, and forwards the request packet viaunicast to the previous hop for the given multicast source and group.3 From the last-hop router to the multicast source, each hop adds a response datablock to the end of the request packet and unicasts it to the previous hop.4 When the first-hop router receives the request packet, it changes the packet typeto indicate a response packet, and then sends the completed packet via unicast tothe multicast traceroute querier.Application of GRETunnel in MulticastForwardingThere may be routers that do not support multicast protocols in a network. Asmulticast traffic from a multicast source is forwarded hop by hop by multicastrouters along the forwarding tree, when the multicast traffic is forwarded to anext hop router that does not support IP multicast, the forwarding path is blocked.In this case, you can enable multicast traffic forwarding across the unicast subnetwhere the non-multicast-capable router resides by establishing a generic routingencapsulation (GRE) tunnel between the routers at both ends of the unicastsubnet.For details about GRE tunneling, refer to “GRE Configuration” on page 1589.
