3com S7906E 설치 설명서

 

1-6 

Figure 1-4 Multicast data transmission through a GRE tunnel 

GRE tunnel

Unicast router

Unicast router

Unicast router

Unicast router

Router A

Router B

 

 

As shown in 

, 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 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.  

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. 

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.  

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:  

z 

Query, with the IGMP Type field set to 0x1F,  

z 

Request, with the IGMP Type field set to 0x1F, and  

z 

Response, with the IGMP Type field set to 0x1E.  

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