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Dell PowerEdge FN I/O Module Configuration Guide 9.10(0.0)

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Enabling Fall Back to Trust Diffserve or dot1p

When using QoS service policies with multiple class maps, you can configure the Dell Networking OS to use the incoming DSCP or dot1p marking as a secondary option for packet queuing if no match occurs in the class maps.

When you use class-maps, traffic is matched against each class-map sequentially from first to last. The sequence is based on the priority of the rules, as follows:
  1. Rules with lowest priority, or in the absence of a priority configuration.
  2. Rules of the next numerically higher queue.

By default, if no match occurs, the packet is queued to the default queue, Queue 0.

In the following configuration, packets are classified to queues using three class maps:

Example of Viewing Packet Classes Based on DSCP Value 

!
                                 policy-map-input input-policy
                                 service-queue 1 class-map qos-BE1
                                 service-queue 3 class-map qos-AF3
                                 service-queue 4 class-map qos-AF4
                                 trust diffserv fallback
                                 !
                                 class-map match-any qos-AF3
                                 match ip dscp 24
                                 match ip access-group qos-AF3-ACL
                                 !
                                 class-map match-any qos-AF4
                                 match ip dscp 32
                                 match ip access-group qos-AF4-ACL
                                 !
                                 class-map match-all qos-BE1
                                 match ip dscp 0
                                 match ip access-group qos-BE1-ACL
The packet classification logic for the configuration shown is as follows:
  1. Match packets against match-any qos-AF4. If a match exists, queue the packet as AF4 in Queue 4, and if no match exists, go to the next class map.
  2. Match packets against match-any qos-AF3. If a match exists, queue the packet as AF3 in Queue 3, and if no match exists, go to the next class map.
  3. Match packets against match-all qos-BE1. If a match exists, queue the packet as BE1, and if no match exists, queue the packets to the default queue, Queue 0.
  4. You can optionally classify packets using their DSCP marking, instead of placing packets in Queue 0, if no match occurs. In the above configuration, if no match occurs against match-all qos-BE1 , the classification logic continues.
  5. Queue the packet according to the DSCP marking. The DSCP to Queue mapping is as noted in Honoring dot1p Values on Ingress Packets .

The behavior is similar for trust dot1p fallback in a Layer2 input policy map; the dot1p-to-queue mapping is noted in Honoring dot1p Values on Ingress Packets .

To enable fall back to trust diffserve or dot1p, use the following command.

Classify packets according to their DSCP value as a secondary option in case no match occurs against the configured class maps. POLICY-MAP-IN mode trust {diffserve | dot1p} fallback

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