Ip Multicast Filtering - 3Com corebuilder 3500 Implementation Manual

336 C 13: IP M R
HAPTER ULTICAST 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 System
Your system uses 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 devices. Your 3Com options include the CoreBuilder 9400
®
switch, CoreBuilder 9000 Layer 2 modules, SuperStack II Switch 3900,
and SuperStack II Switch 9300.
Supporting Protocol in Your System
To track which ports in an interface require IP multicast group traffic, your
system supports the Internet Group Management Protocol (IGMP)
version 1 and version 2. IGMP covers two main functions: querying and
snooping. These are explained later in this chapter.