Concept Guide

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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 congured 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 120. Statically and Dynamically Assigned dot1p for VLAN Stacking
When conguring 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
congurations; 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 dierent CAM entries, each in a dierent 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 congured 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 EMC Networking OS Behavior: For Option A shown in the previous illustration, when there is a conict between the queue selected
by Dynamic Mode CoS (vlan-stack dot1p-mapping) and a QoS conguration, the queue selected by Dynamic Mode CoS takes precedence.
However, rate policing for the queue is determined by QoS conguration. For example, the following access-port conguration maps all
trac to Queue 0:
vlan-stack dot1p-mapping c-tag-dot1p 0-7 sp-tag-dot1p 1
However, if the following QoS conguration also exists on the interface, trac 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 trac. This is an expected behavior.
Examples of QoS Interface Conguration 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
Service Provider Bridging
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