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206 C
HAPTER
10: C
LASS OF
S
ERVICE
(C
O
S)
Handling Tagged and
Untagged Packets
Consider the following points about how a Layer 2 module processes
tagged and untagged packets with respect to CoS information:
■
If CoS is enabled and an untagged packet enters a port, the packet is
always processed through the low priority queue.
■
As described earlier, the CoS priority level is carried in the IEEE 802.1Q
tag. After the packet enters the module, its format is subject to
change according to VLAN configurations and ingress and egress
rules.
■
If an untagged packet enters the module and the VLAN settings
modify the packet to become a tagged packet, the module can insert
VLAN information but cannot set a priority level other than 0 (whether
CoS is enabled or not).
■
If a tagged packet enters the module and the tag is retained upon
forwarding the packet, the module leaves the priority level as-is
(whether CoS is enabled or not).
■
If a tagged packet enters the module and VLAN rules cause the tag to
be stripped prior to forwarding, the CoS priority information is lost
thereafter unless the packet is later processed by a device that can
insert tags and priority levels other than 0.
Standards,
Protocols, and
Related Reading
The following standards provide more information about Class of Service:
■
IEEE 802.1p Traffic Class Expediting and Dynamic Multicast Filtering
A supplement to the IEEE 802.1D MAC Bridges base standard that
addresses traffic prioritization in local area networks.
■
IEEE 802.1Q Virtual Bridged LANs
A base standard that specifies requirements for virtual LANs and a
packet format that includes VLAN and CoS priority information.
To obtain copies of these standards, register for an on-line subscription
with the Institute of Electrical and Electronics Engineers (IEEE) Web site:
http://www.ieee.org