Model of operation

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b) the registration services offered by GID (12.3, 12.3.2) to allow Group membership information and Group service requirement information declared in the Bridged LAN to control the frame filtering behavior of participating devices with respect to frames destined for group MAC addresses.

Any new information registered with (or de-registered from) a particular Bridge is propagated by GARP across the Bridged LAN to the other Bridges that may be present, in the manner described in 12.2. This information is used to update Group Registration Entries (7.9.3) in the Filtering Database(s) of participating end stations and Bridges.

10.2.1 Propagation of Group Membership information

The Forwarding Process uses the Group Registration Entries in the Filtering Databases to ensure that frames are transmitted only through those Bridge Ports that are necessary in order to reach LANs to which Group members are attached. Figure 10-1 illustrates the Group Registration Entries created by GMRP for a single Group.

By receiving frames from all Ports and forwarding only through Ports for which GMRP has created Group Registration Entries, Bridges facilitate the implementation of Group distribution mechanisms based on the concept of an Open Host Group. Any GMRP Participants (12.3) that wish to receive frames transmitted to a particular Group or Groups register their intention to do so by requesting membership of the Group(s) con-cerned. Any MAC Service user that wishes to send frames to a particular Group can do so from any point of attachment to the Bridged LAN. These frames can be received on all LAN segments to which registered GMRP Participants are attached, but the filtering applied by Bridges for ports which have not had Group

Figure 10-1—Example Directed Graph LAN A

Filtering Database

LAN D LAN C

M

M

M

M Applicant station that is a member of Group M Applicant station that is not a member of Group M Group Registration for Group M

Filtering M Database Filtering Database Filtering

Database Filtering

Database

Filtering Database

Filtering

Database Bridge LAN B

M

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nei-ther have registered GMRP participants attached nor are in the path through the active topology between the sources of the frames and the registered Group members. GMRP and the Group Registration Entries thus act to restrict the frames to pruned subsets of the overall loop free active topology defined by the Spanning Tree Protocol, where each pruned subset only includes LAN segments necessary to reach Group members from a given source.

NOTE—The term “Open Host Group” used here comes from the terminology introduced in the definition of the Internet Group Membership Protocol (IGMP) defined by the IETF.

MAC Service users that are sources of MAC frames destined for the Group do not have to register as mem-bers of the Group themselves unless they also wish to receive frames transmitted to the Group address by other sources.

10.2.2 Propagation of Group service requirement information

GMRP propagates Group service requirement information in the same manner as for Group Registration information. If any Port in a given Bridge has a registered Group service requirement of All Groups or All Unregistered Groups (expressed in terms of the control information in Static Filtering Entries and/or Dynamic Filtering Entries with a MAC address specification of All Groups or All Unregistered Groups), this fact is propagated on all other Ports of the Bridge, resulting in the registration of that information on Ports of adjacent Bridges. As a consequence of that registration, the default Group filtering behavior of those Ports may change in order to maintain compatibility with the service requirements expressed by the registered information, as defined in 7.9.4. This ensures that connectivity can be maintained in LANs where the service requirements of different regions of the Bridged LAN differ.

NOTE—In a Bridged LAN where the default Group filtering behavior is not the same for all “edge” Ports, service requirement propagation will tend to result in all “backbone” Ports switching to the highest precedence Group filtering behavior in use in the Bridged LAN. The precedence rules are defined in 7.9.4.

10.2.3 Source pruning

As described in 10.2.1, the operation of GMRP defines a sub-tree of the Spanning Tree as a result of the cre-ation of Group Registrcre-ation Entries in the Filtering Databases of the Bridges in the Bridged LAN. End sta-tions in the Bridged LAN are also able to make use of the Group Membership information registered via GMRP to allow them to keep track of the set of Groups for which active members currently exist, and the service requirements of upstream devices. This allows end stations that are sources of frames destined for a Group to suppress the transmission of such frames, if their registered Group membership and Group service requirement information indicates that there are no valid recipients of those frames reachable via the LAN segment to which they are attached.

NOTE—In effect, for the purposes of frame transmission, the end station can be viewed as if it operates as a single Port Bridge, with its own default Group filtering behavior and “Filtering Database” entries updated via GMRP that tell it whether or not multicast frames that it has generated should be forwarded onto the attached LAN. In order to achieve this, it is necessary for the end station to implement both the Registrar and the Applicant functionality of GARP, as described in 12.7.3, 12.8.1 and 12.8.2. The Applicant Only and Simple Applicant Participants described in 12.7.7 and 12.7.8 do not contain the Registrar functionality that would be required for Source Pruning.

This end system behavior is known as source pruning. Source pruning allows MAC Service users that are sources of MAC frames destined for a number of Groups, such as server stations or routers, to avoid unnec-essary flooding of traffic on their local LAN segments in circumstances where there are no current Group members in the Bridged LAN that wish to receive such traffic.

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10.2.4 Use of Group service requirement registration by end stations

The ability to propagate Group service requirement information is described in this standard primarily as a means of propagating the requirements of the Bridges themselves. However, this mechanism may also be used by end stations that have requirements involving some aspect of promiscuous reception, such as Rout-ers or network monitors. A GMRP-aware end station requiring to be able to receive all multicast traffic can achieve this end by declaring membership of All Groups on the LAN segment to which it is attached; simi-larly, an end station that wishes to receive unregistered multicast traffic can do so by declaring membership of All Unregistered Groups. The circumstances under which these facilities might be used are further dis-cussed in F.3.