Alcatel-Lucent 7750 SR OS - Figure 3: Multipoint Queue Behavior with Shared Queuing Enabled

Introduction
Page 32 7750 SR OS Services Guide
Figure 3: Multipoint Queue Behavior with Shared Queuing Enabled
Ingress Multipoint Shared Queuing
Ingress multipoint shared queuing is a variation to the unicast shared queuing defined in Ingress
Shared Queuing on page 30. Ingress unicast service queues are mapped one-for-one with hardware
queues and unicast packets traverse the ingress forwarding plane twice. In addition to the above,
the multipoint queues defined in the ingress SAP QoS policy are not created. Instead, multipoint
packets (broadcast, multicast and unknown unicast destined) are treated to the same dual pass
ingress forwarding plane processing as unicast packets. In the first pass, the forwarding plane uses
the unicast queue mappings for each forwarding plane. The second pass uses the multipoint shared
queues to forward the packet to the switch fabric for special replication to all egress forwarding
planes that need to process the packet.
The benefit of defining multipoint shared queuing is the savings of the multipoint queues per
service. By using the unicast queues in the first pass and then the aggregate shared queues in the
second pass, per service multipoint queues are not required. The predominate scenario where
multipoint shared queuing may be required is with subscriber managed QoS environments using a
subscriber per SAP model. Usually, ingress multipoint traffic is minimal per subscriber and the
extra multipoint queues for each subscriber reduces the overall subscriber density on the ingress
forwarding plane. Multipoint shared queuing eliminates the multipoint queues sparing hardware
queues for better subscriber density. Figure 2.3 demonstrates multipoint shared queuing.
One disadvantage of enabling multipoint shared queuing is that multipoint packets are no longer
managed per service (although the unicast forwarding queues may provide limited benefit in this
Second
Pass
Service
Queue
Hardware
Queue
Unicast Service Queue is Represented by a Single Hardware
Queue But Dual Pass through the Forwarding Plane
Hardware
Des t Queue
Hardware
Des t Queue
Hardware
Des t Queue
Hardware
Des t Queue
Unic a st
SF Egres s
MDA 1
SF Egres s
MDA 2
SF Egres s
MDA 3
SF Egres s
MDA n
Multicast
Queue
Multipoint Service Queue is Represented
by a Single Hardware Queue (Single Pass)
Hardware
Queue
Multicast
Switch Fabric