Alcatel-Lucent 7750 SR Manual page 749

Egress Multicast Group Commands
egress-multicast-group
Syntax egress-multicast-group egress-multicast-group-name
no egress-multicast-group group-name
Context config>service
Description This command creates an egress multicast group (EMG) context. An EMG is created as an object
used to group VPLS SAPs that are allowed to participate in efficient multicast replication (EMR).
EMR is a method to increase the performance of egress multipoint forwarding by sacrificing some
destination-based features. Eliminating the requirement to perform unique features for each
destination allows the egress forwarding plane to chain together multiple destinations into a batch
replication process. In order to perform this batch replication function, similar characteristics are
required on each SAP within the EMG.
Only SAPs defined on Ethernet access ports are allowed into an egress-multicast-group.
In order to understand the purpose of an egress-multicast-group, an understanding of the system's use
of flooding lists is required. A flooding list is maintained at the egress forwarding plane to define a set
of destinations to which a packet must be replicated. Multipoint services make use of flooding lists to
enable forwarding a single packet to many destinations. Examples of multipoint services that use
flooding lists are VPLS, IGMP snooping and IP multicast routing. Currently, the egress forwarding
plane will only use efficient multicast replication for VPLS and IGMP snooping flooding lists.
In VPLS services, a unique flooding list is created for each VPLS context. The flooding list is used
when a packet has a broadcast, multicast or unknown destination MAC address. From a system
perspective, proper VPLS handling requires that a broadcast, multicast or unknown destined packet
be sent to all destinations that are in the forwarding state. The ingress forwarding plane ensures the
packet gets to all egress forwarding planes that include a destination in the VPLS context. It is the
egress forwarding plane's job to replicate the packet to the subset of the destinations that are reached
through its interfaces and each of these destinations are included in the VPLS context's flooding list.
For IGMP snooping, a unique flooding list is created for each IP multicast (s,g) record. This (s,g)
record is associated with an ingress VPLS context and may be associated with VPLS destinations in
the source VPLS instance or other VPLS instances (in the case of MVR). Again, the ingress
forwarding plane ensures that an ingress IP multicast packet matching the (s,g) record gets to all
egress forwarding planes that have a VPLS destination associated with the (s,g) record. The egress
forwarding plane uses the flooding list owned by the (s,g) record to replicate the packet to all VPLS
destinations in the flooding list. The IGMP Snooping function identifies which VPLS destinations
should be associated with the (s,g) record.
With normal multicast replication, the egress forwarding plane examines which features are enabled
for each destination. This includes ACL filtering, mirroring, encapsulation and queuing. The
resources used to perform this per destination multicast processing are very expensive to the egress
forwarding plane when high replication bandwidth is required. If destinations with similar egress
functions can be grouped together, the egress forwarding plane can process them in a more efficient
manner and maximize replication bandwidth.
7750 SR OS Services Guide
Virtual Private LAN Services
Page 749