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Quality of Service: Managing Bandwidth More Effectively
Globally-Configured QoS
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 where you want to honor (continue) a policy set on an edge switch.
The IP-diffserv classifier 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).
Figure 5-15. 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 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 (described later in this section), as long as the
DSCPs specified in the two options do not match.
LAN
A5
Edge
Switch
“A”
LAN
Interior
Switch
“B”
Work-
Group
Work-
Group
Marked Traffic from port A5 on Edge Switch “A”
Other Traffic
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