9.2.2.1. Ingress Shared Queuing

To avoid the hardware queue expansion issues associated with normal service-based queuing, the system allows an ingress logical service queue to map to a single hardware queue when shared queuing is enabled. Shared queuing uses two passes through the ingress forwarding plane to separate ingress per service queuing from the destination switch fabric queuing. In the case of shared queuing, ingress unicast service queues are created one-for-one relative to hardware queues. Each hardware queue representing a service queue is mapped to a special destination in the traffic manager that ‘forwards’ the packet back to the ingress forwarding plane, allowing a second pass through the traffic manager. In the second pass, the packet is placed into a ‘shared’ queue for the destination forwarding plane. The shared queues are used by all services configured for shared queuing.

When the first SAP or SLA profile instance is configured for shared queuing on an ingress forwarding plane, the system allocates eight hardware queues per available destination forwarding plane, one queue per forwarding class. Twenty-four hardware queues are also allocated for multipoint shared traffic, but that is discussed in the following section. The shared queue parameters that define the relative operation of the forwarding class queues are derived from the shared queue policy defined in the QoS CLI node. Figure 37 demonstrates shared unicast queuing. SAP or SLA profile instance multipoint queuing is not affected by enabling shared queuing.

Figure 37:  Unicast Service Queuing With Shared Queuing Enabled 

Multipoint queues are still created as defined in the ingress SAP QoS policy and ingress multipoint packets only traverse the ingress forwarding plane a single time, as shown in Figure 38.

Enabling shared queuing may affect ingress performance due to double packet processing through the service and shared queues.

Figure 38:  Multipoint Queue Behavior with Shared Queuing Enabled 

9.2.2.2. Ingress Multipoint Shared Queuing

Ingress multipoint shared queuing is a variation of the unicast shared queuing defined in Ingress Shared Queuing. With ingress multipoint shared queuing, 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, then the aggregate shared queues in the second pass, per service multipoint queues are not required. The predominant 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 reduce the overall subscriber density on the ingress forwarding plane. Multipoint shared queuing eliminates the multipoint queues, sparing hardware queues for better subscriber density. Figure 39 demonstrates multipoint shared queuing.

One caveat 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 area). Multipoint packets in a multipoint service (VPLS, IES, and VPRN) use significant resources in the system, consuming ingress forwarding plane multicast bandwidth and egress replication bandwidth. Usually, the per service unicast forwarding queues are not rate limited to a degree that allows adequate management of multipoint packets traversing them when multipoint shared queuing is enabled. It is possible to minimize the amount of aggregate multipoint bandwidth by setting restrictions on the multipoint queue parameters in the QoS nodes shared queue policy. Aggregate multipoint traffic can be managed per forwarding class for each of the three forwarding types (broadcast, multicast, or unknown unicast – broadcast and unknown unicast are only used by VPLS).

Another caveat for multipoint shared queuing is that multipoint traffic now consumes double the ingress forwarding plane bandwidth due to dual-pass ingress processing.

Figure 39:  Multipoint Shared Queuing Using First Pass Unicast Queues 

Multipoint shared queuing cannot be enabled on the following services:

For information about the tasks and commands necessary to access the CLI and to configure and maintain your router, refer to CLI Usage chapter in the Basic System Configuration Guide.

9.3.1.1. Applying Shared-Queue Policies

The default shared queue policy is applied at the SAP level just as sap-ingress and sap-egress QoS policies are specified. If the shared-queuing keyword is not specified in the qos policy-id command, then the SAP is assumed to use single-pass queuing.

Apply shared-queue policies to the following entities: