25. Multicast Filtering
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RuggedBackbone™ RX1500
membership in multicast groups with other switches on a LAN, and for that information to be
disseminated to all switches in the LAN that support Extended Filtering Services.
GMRP is an industry-standard protocol first defined in IEEE 802.1D-1998 and extended in IEEE
802.1Q-2005. GARP was defined in IEEE 802.1D-1998 and updated in 802.1D-2004. Note that GMRP
provides similar functionality at Layer 2 to that which IGMP, described in the preceding sections,
provides at Layer 3.
Joining a Multicast Group)
In order to join a multicast group, an end station transmits a GMRP “join” message. The switch that
receives the “join” message adds the port through which the message was received to the multicast
group specified in the message. It then propagates the “join” message to all other hosts in the VLAN,
one of which is expected to be the multicast source.
When a switch transmits GMRP updates (from GMRP-enabled ports), all of the multicast groups known
to the switch, whether configured manually or learned dynamically through GMRP, are advertised to
the rest of network.
As long as one host on the Layer 2 network has registered for a given multicast group, traffic from the
corresponding multicast source will be carried on the network. Traffic multicast by the source is only
forwarded by each switch in the network to those ports from which it has received join messages for
the multicast group.
Leaving a Multicast Group
Periodically, the switch sends GMRP queries in the form of a “leave all” message. If a host (either a
switch or an end station) wishes to remain in a multicast group, it reasserts its group membership by
responding with an appropriate “join” request. Otherwise, it can either respond with a “leave” message
or simply not respond at all. If the switch receives a “leave” message or receives no response from the
host for a timeout period, the switch removes the host from the multicast group.
GMRP Protocol Notes
Since GMRP is an application of GARP, transactions take place using the GARP protocol. GMRP
defines the following two Attribute Types:
• The Group Attribute Type, used to identify the values of group MAC addresses
• The Service Requirement Attribute Type, used to identify service requirements for the group
Service Requirement Attributes are used to change the receiving port’s multicast filtering behavior to
one of the following:
• Forward All Multicast group traffic in the VLAN, or
• Forward All Unknown Traffic (Multicast Groups) for which there are no members registered in the
device in a VLAN
If GMRP is disabled on the RuggedBackbone™, then GMRP PDUs received by the RX1500 will be
forwarded like any other traffic; but if GMRP is enabled on at least one of the ports, then GMRP packets
will be processed by the RX1500, and not forwarded.
25.2.1. GMRP Example
In the example depicted in
Figure 25.3, “Example using GMRP”
, there are two multicast sources, S1
and S2, multicasting to Multicast Groups 1 and 2, respectively. A network of five switches, including
one core Switch, B, connects the sources to two hosts, H1 and H2, which receive the multicast streams
from S1 and S2, respectively.
