KB.15.15

Figure 55 IPv4 ToS/IPv6 traffic class byte with DSCP codepoint and precedence bits

Assigning an 802.1p priority for a global IP-diffserv classifier

One of the best uses for this global QoS packet-marking option is on an interior switch to honor

(continue) a policy set on an edge switch. The IP-diffserv classifier enables selecting incoming

packets having a specific DSCP and forwards these packets with the desired 802.1p priority. For

example, if an edge switch A marks all packets received on port A5 with a particular DSCP, you

can configure a downstream (interior) switch B to handle such packets with the desired priority

(regardless of whether 802.1Q-tagged VLANs are in use).

Figure 56 Interior switch B honors the policy established in edge switch A

To do so, assign the desired 802.1p priority to the same codepoint that the upstream or edge

switch assigns to the selected packets. When the downstream switch receives an IP packet carrying

one of these codepoints, it assigns the configured priority to the packet and sends it out the

appropriate outbound port queue. (The packet retains the codepoint it received from the upstream

or edge switch). You can use this option concurrently with the diffserv DSCP Policy option, as long

as the DSCPs specified in the two options do not match.

252 Quality of Service: Managing bandwidth effectively