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

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Dynamic Mode CoS for VLAN Stacking

One of the ways to ensure quality of service for customer VLAN-tagged frames is to use the 802.1p priority bits in the tag to indicate the level of QoS desired.

When an S-Tag is added to incoming customer frames, the 802.1p bits on the S-Tag may be configured statically for each customer or derived from the C-Tag using Dynamic Mode CoS. Dynamic Mode CoS maps the C-Tag 802.1p value to a S-Tag 802.1p value.
Figure 1. Statically and Dynamically Assigned dot1p for VLAN Stacking Illustration of statically and dynamically assigned dot1p for VLAN stacking.
When configuring Dynamic Mode CoS, you have two options:
  • Mark the S-Tag dot1p and queue the frame according to the original C-Tag dot1p. In this case, you must have other dot1p QoS configurations; this option is classic dot1p marking.
  • Mark the S-Tag dot1p and queue the frame according to the S-Tag dot1p. For example, if frames with C-Tag dot1p values 0, 6, and 7 are mapped to an S-Tag dot1p value 0, all such frames are sent to the queue associated with the S-Tag 802.1p value 0. This option requires two different CAM entries, each in a different Layer 2 ACL FP block.
  • NOTE: The ability to map incoming C-Tag dot1p to any S-Tag dot1p requires installing up to eight entries in the Layer 2 QoS and Layer 2 ACL table for each configured customer VLAN. The scalability of this feature is limited by the impact of the 1:8 expansion in these content addressable memory (CAM) tables.
Dell Networking OS Behavior: For Option A shown in the previous illustration, when there is a conflict between the queue selected by Dynamic Mode CoS (vlan-stack dot1p-mapping) and a QoS configuration, the queue selected by Dynamic Mode CoS takes precedence. However, rate policing for the queue is determined by QoS configuration. For example, the following access-port configuration maps all traffic to Queue 0: 
vlan-stack dot1p-mapping c-tag-dot1p 0-7 sp-tag-dot1p 1

However, if the following QoS configuration also exists on the interface, traffic is queued to Queue 0 but is policed at 40Mbps ( qos-policy-input for queue 3) because class-map "a" of Queue 3 also matches the traffic. This is an expected behavior.

Examples of QoS Interface Configuration and Rate Policing 

policy-map-input in layer2
                                 service-queue 3 class-map a qos-policy 3
                                 !
                                 class-map match-any a layer2
                                 match mac access-group a
                                 !
                                 mac access-list standard a
                                 seq 5 permit any
                                 !
                                 qos-policy-input 3 layer2
                                 rate-police 40

Likewise, in the following configuration, packets with dot1p priority 0–3 are marked as dot1p 7 in the outer tag and queued to Queue 3. Rate policing is according to qos-policy-input 3. All other packets will have outer dot1p 0 and hence are queued to Queue 1. They are therefore policed according to qos-policy-input 1. 

policy-map-input in layer2
                                 service-queue 1 qos-policy 1
                                 service-queue 3 qos-policy 3
                                 !
                                 qos-policy-input 1 layer2
                                 rate-police 10
                                 !
                                 qos-policy-input 3 layer2
                                 rate-police 30
                                 !
                                 interface GigabitEthernet 0/21
                                 no ip address
                                 switchport
                                 vlan-stack access
                                 vlan-stack dot1p-mapping c-tag-dot1p 0-3 sp-tag-dot1p 7
                                 service-policy input in layer2
                                 no shutdown

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