Using QoS Classifiers to Configure Quality of Service for Outbound Traffic
Assigning an 802.1p Priority to IPv4 Packets on the
Basis of Incoming DSCP
One of the best uses for this option is on an interior switch where you want
to honor (continue) a policy set on an edge switch. That is, it enables you to
select incoming packets having a specific DSCP and forward 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).
Marked Traffic from port A5 on Edge Switch "A"
Figure 5-4. 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 down-
stream switch receives an IPv4 packet carrying one of these codepoints, it
assigns the configured priority to the packet and sends it out the appropriate
priority 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 (described later in this section), as long as the
DSCPs specified in the two options do not match.
Quality of Service (QoS): Managing Bandwidth More Effectively