1-6Figure 1-4 Multicast data transmission through a GRE tunnelGRE tunnelUnicast router Unicast routerUnicast router Unicast routerMulticast router Multicast routerRouter ASource ReceiverRouter BAs shown in Figure 1-4, with a GRE tunnel established between Router A and Router B, Router Aencapsulates multicast data in unicast IP packets, which are then forwarded by unicast routers toRouter B across the GRE tunnel. Then, Router B strips off the unicast IP header and continuesforwarding the multicast data down towards the receivers.If unicast static routes are configured across the tunnel, any unicast packet can be transmitted throughthe tunnel. If you wish the tunnel to be dedicated to multicast traffic delivery, you can configure only amulticast static route across the tunnel, so that unicast packets cannot be transmitted through thistunnel.Multicast TracerouteThe multicast traceroute utility is used to trace the path that a multicast stream flows down from thefirst-hop router to the last-hop router.Concepts in multicast traceroute1) Last-hop router: If a router has one of its interfaces connecting to the subnet the given destinationaddress is on, and if the router is able to forward multicast streams from the given multicast sourceonto that subnet, that router is called last-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 from common IGMP packets inthat its IGMP Type field is set to 0x1F or 0x1E and that its destination IP address is a unicast address.There are three types of multicast traceroute packets:z Query, with the IGMP Type field set to 0x1F,z Request, with the IGMP Type field set to 0x1F, andz 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 request packet byadding a response data block containing its interface addresses and packet statistics to the end of
