The MBS determines the maximum queue depth after which no additional packets are accepted into the queue. Additional queue drop tails are available for the different packet profiles to allow preferential access to the queue's buffers which allows higher priority packets to be accepted into a queue when there is congestion for lower priority packets.
At ingress there is a low drop tail in addition to the MBS. High enqueuing priority packets (for ingress SAP priority mode queues) and in-profile packets (for ingress SAP profile mode queues, network, and shared queues) are allowed to fill up the queue up to the MBS, however, low enqueuing priority packets (for ingress SAP priority mode queues) and out-of-profile packets (for ingress SAP profile mode queues, network, and shared queues) can only fill the queue up to the queue's low drop tail setting.
At egress there are four drop tails in addition to the MBS, one for each profile state:
an exceed drop tail for exceed-profile packets
a low drop tail for out-of-profile packets
a high drop tail for in-profile packets
a highplus drop tail for highplus-profile packets
Each profile type can only fill the queue up to its corresponding drop tail. Figure: Ingress and egress queue drop tails shows the ingress and egress queue drop tails.
At both ingress and egress, the drop tails are configured as a percentage reduction from the MBS (specifying 10% places the drop tail at 90% of the MBS) and consequently all are limited by the queue's MBS.
The default percentage for the low drop tail for ingress SAP, queue group, and shared queues is a reduction from the MBS of 10% (low)
The default percentages for the drop tails for egress SAP, queue group, and network queues is a percentage reduction from the MBS of:
exceed = 20%
low = 10%
high = 0%
highplus = 0%
The exceed, high, and highplus drop tails are not configurable for network queues, however the exceed drop tail is set to a value of 10% in addition to low drop tail and capped by the MBS.
The four drop tails can be configured in any order within egress SAP and queue group queues, however it is logical to order them (from shortest to longest) as exceed, then low, then high, then highplus.
The low drop tail configuration can be overridden for ingress and egress SAP and queue group queues, and for network egress queues. It is also possible to override the low drop tail for subscriber queues within an SLA profile using the keyword high-prio-only.
When there is congestion the drop tail ordering gives preferential access to the queue's buffers. For example, if the drop tails on an egress SAP queue are configured as exceed = 20%, low = 10%, high = 5% and highplus = 0%, when the queue depth is below 80% all profile packet types are accepted into the queue. If the depth increases above 80%, then exceed profile packets are not accepted and are therefore dropped, while the out-of-profile, in-profile, and inplus profile packets are still accepted into the queue (giving them preference over the exceed profile packets). If the queue depth goes beyond 90% the out-of-profile packets are also dropped. Similarly, if the queue depth goes beyond 95% the in-profile packets are dropped. It is only when the MBS has been reached that the inplus profile packets are dropped. This example assumes that the pool in which the queue exists is not congested.