Multicast Traceroute - 3Com S7906E Configuration Manual

Figure 1-4 Multicast data transmission through a GRE tunnel
Multicast router
Source Router A
As shown in Figure
encapsulates multicast data in unicast IP packets, which are then forwarded by unicast routers to
Router B across the GRE tunnel. Then, Router B strips off the unicast IP header and continues
forwarding the multicast data down towards the receivers.
If unicast static routes are configured across the tunnel, any unicast packet can be transmitted through
the tunnel. If you wish the tunnel to be dedicated to multicast traffic delivery, you can configure only a
multicast static route across the tunnel, so that unicast packets cannot be transmitted through this
tunnel.

Multicast Traceroute

The multicast traceroute utility is used to trace the path that a multicast stream flows down from the
first-hop router 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
Unicast router
Unicast router
GRE tunnel
Unicast router
Unicast router
1-4, with a GRE tunnel established between Router A and Router B, Router A
Multicast router
Router B
1-6
Receiver