Lucent Technologies P550 - Manual (Page 249)

Managing Intelligent Multicasting (Layer 2 & Layer 3)
Cajun P550/P220 Switch Operation Guide 15 -3
LGMP Server
Because Layer 2 cannot snoop IGMP messages, it requires Layer 3 to provide the
necessary information through a Lucent proprietary protocol, Lucent Group
Membership Protocol (LGMP).
Essentially, LGMP works by having the Layer 3 switch re-transmit received IGMP
messages to the Layer 2 switches by sending them to a well known multicast MAC
address. The Layer 2 switch then creates multicast sessions, adds clients to those sessions,
and removes sessions dynamically, as if it were directly receiving the IGMP messages.
If more than one LGMP server is on a VLAN, one server will be elected as the distributor.
Only the distributor disseminates intelligent multicasting information using LGMP
packets.
Note: IGMP snooping must be enabled for LGMP server availability. An LGMP
server is active only for VLANs that have an IGMP interface enabled.
Layer 2 Dynamic Intelligent Multicasting
Layer 2 does not support IGMP snooping to dynamically configure intelligent multicast
sessions. Therefore, LGMP client or CGMP snooping is used to achieve dynamic
intelligent multicasting.
LGMP Client
An LGMP server disseminates multicast session information using LGMP. An LGMP
client creates multicast sessions, adds clients to those sessions, and removes sessions
dynamically, using the information sent from the LGMP server.
CGMP Snooping
Cisco routers disseminate multicast session information via the CGMP protocol. The
switch has the ability to listen to these messages and dynamically create multicast
sessions, add client ports to those sessions, and remove sessions.
Pruning Dynamic Sessions, Client Ports, and Router Ports
Normally, dynamically created multicast sessions are removed by the application (for
example, IGMP Snooping, LGMP, or CGMP Snooping) that created the session when the
session is no longer active. You can also enable Automatic Session Pruning so that if a
session has not been active for Session Pruning Time, it is automatically removed.