Network tracking

The threat posed by any strict priority-scheduling algorithm is that it could starve lower-priority traffic. To prevent this, the LLQ mechanism has a built-in policer. This policer (like the queuing algorithm itself) engages only when the interface is experiencing congestion. Therefore, it is important to provision the priority classes properly. If the total amount of priority class bandwidth provisioned is lower than the total amount of voice traffic offered to the PQ (including the Layer 2 overhead), the excess voice traffic might be dropped (by the LLQ policer). As with any other policer, LLQ's policer, when engaged, drops traffic indiscriminately and affects the quality of all voice calls active on the interface (not just the last one to go through). In earlier network tracking Releases, the PQ of LLQ policed strictly to the bandwidth assigned, regardless of traffic present or absent in other classes. In later releases, this operation changed to police strictly only if the interface was congestedthat is if other classes were underutilized, PQ traffic would be allowed to exceed its bandwidth without dropping packets

Another benefit provided by the implicit policer within the LLQ mechanism is the capability to use TDM for the single strict-priority queue. TDM abstracts the fact that only a single strict-priority queue exists and allows for the configuration and servicing of "multiple" low-latency queues.