H3C Technologies H3C SecPath F1000-E - Manual (Page 20)

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Figure 4 Multicast data transmission through a GRE tunnel
GRE tunnel
Unicast router
Unicast router
Unicast router
Unicast router
Multicast router Multicast router
Router A Source Receiver Router B
As shown in
Figure 4 , with a GRE tunnel established between Router A and Router B, Router A
encapsulates multicast data in unicast IP packets, which are then forwarded by unicast routers to Router
B along the GRE tunnel. Then, Router B strips off the unicast IP header and continues to forward the
multicast data down towards the receivers.
If unicast static routes are configured across the tunnel, any unicast packets can be transmitted through
the tunnel. If you want the tunnel to be dedicated to multicast traffic delivery, configure only a multicast
static route across the tunnel, so that only multicast packets are 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 is called the first-hop router.
3. Querier: A router that sends multicast traceroute requests is called a querier.
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 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.