Multicast Traceroute; Application Of Gre Tunnel In Multicast Forwarding - 3Com MSR 50 Series Configuration Manual

Multicast Traceroute

Application of GRE
Tunnel in Multicast
Forwarding
multicast information from Source travels from Router A to Router B and then to
Router C.
The multicast traceroute utility is used to trace the path that a multicast stream
passes from the multicast source to the last-hop router.
Concepts in multicast traceroute
1 Last-hop router: If a router has one of its interfaces connecting to the subnet the
given destination address is on, and if the router is able to forward multicast
streams from the given multicast source onto 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 packets
A multicast traceroute packet is a special IGMP packet, which differs from
common IGMP packets in that 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:
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 traceroute
1 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 by adding a response data block containing its interface addresses and
packet statistics to the end of the packet, and forwards the request packet via
unicast 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 data
block 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 type
to indicate a response packet, and then sends the completed packet via unicast to
the multicast traceroute querier.
There may be routers that do not support multicast protocols in a network. As
multicast traffic from a multicast source is forwarded hop by hop by multicast
routers along the forwarding tree, when the multicast traffic is forwarded to a
next 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 subnet
where the non-multicast-capable router resides by establishing a generic routing
encapsulation (GRE) tunnel between the routers at both ends of the unicast
subnet.
For details about GRE tunneling, refer to
Multicast Routing and Forwarding Overview
"GRE Configuration" on page
1101
1589.