3Com 4007, Implementation Manual, Page: 428 / 684

428 C
HAPTER
18: IP M
ULTICAST
R
OUTING
A multicast router is required at each end of the tunnel. At each tunnel
entrance, the router encapsulates the IP multicast packets in standard IP
unicast packets — that is, it puts them in a format that the unicast routers
can understand. When these packets reach the end of the tunnel, the
router strips the encapsulation away and returns the packet to its native
IP multicast format.
Supporting Protocol in Your Module
Switch 4007 Multilayer Switching Modules use the Distance-Vector
Multicast Routing Protocol (DVMRP) to form IP multicast tunnels. Specific
aspects of tunnel configuration are described later in this chapter.
IP Multicast Filtering When a router discovers that at least one IP multicast group member
resides on a directly attached subnetwork, it forwards group traffic on
that interface until it determines that group members no longer require
the traffic. If multiple ports are configured in an interface, a router sends
copies of the group traffic to all ports, even if only one port of those ports
leads to group members. This is because the multicast routing protocol
does not track
exactly where group members reside on that interface.
The ability to filter IP multicast traffic on ports within a routing interface
that do not lead to group members is highly desirable (although it is not
required in the IP multicast standard) because it allows you to further
optimize the LAN environment. Through targeted filtering, a router can
conserve even more network bandwidth and minimize unnecessary
interruptions to endstation CPUs.
It is also important to have a similar IP multicast filtering capability in
Layer 2 switches. See Chapter 11
to learn about this capability in
Switch 4007 Layer 2 Switching Modules.
Supporting Protocol in Your Multilayer Switching Module
Your module supports the Internet Group Management Protocol (IGMP)
version 1 and version 2. to track exactly which ports in an interface
require IP multicast group traffic. IGMP covers two main functions:
querying and snooping. These are explained later in this chapter.