Alcatel-Lucent 7750 SR OS Service Manual page 349

Moving a SAP from a chain to a stand alone condition or from a stand alone condition to a chain
may cause a momentary glitch in the forwarding plane for the time that the SAP is being moved.
Care is taken to prevent or minimize the possibility of duplicate packets being replicated to a
destination while the chains and flooding lists are being manipulated.
Chain Optimization
Chains are only dynamically managed during SAP addition and removal events. The system does
not attempt to automatically optimize existing chains. It is possible that excessive SAP removal
may cause multiple chains to exist with lengths less than the maximum chain length. For example,
if four chains exist with eight SAPs each, it is possible that seven of the SAPs from each chain are
removed. The result would be four chains of one SAP each effectively removing any benefit of
egress SAP replication chaining.
While it may appear that optimization would be beneficial each time a SAP is removed, this is not
the case. Rearranging the chains each time a SAP is removed may cause either packet duplication
or omitting replication to a destination SAP. Also, it could be argued that if the loop back
replication load is acceptable before the SAP is removed, continuing with the same loop back
replication load once the SAP is removed is also acceptable. It is important to note that the overall
replication load is lessened with each SAP removal from a chain.
While dynamic optimization is not supported, a manual optimization command is supported in
each egress multicast group context. When executed the system will remove and add each SAP,
rebuilding the replication chains.
When the dest-chain-limit is modified for an egress multicast group, the system will reorganize
the replication chains that contain SAPs from that group according to the new maximum chain
size.
IOM Mode B Capability
Efficient multicast replication uses an egress forwarding plane that supports chassis mode b due to
the expanded memory requirements to store the replication chain information. The system does
not need to be placed into mode b for efficient multicast replication to be performed. Any IOM
that is capable of mode "b" operation automatically performs efficient multicast replication when
a flooding list contains SAPs that are members of an egress multicast group.
7750 SR OS Services Guide
Virtual Private LAN Services
Page 349