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Understanding Host Inbound Traffic Classification

The destination address of traffic that enters the switch can be an external device such as another switch, a router, or a server, or the destination can be the host (the switch Routing Engine or CPU). When the destination is an external device, the DSCP and IEEE 802.1p code-point bits of incoming traffic are preserved as the traffic travels through the switch to the egress port. At the egress port, the code-point bits are either preserved when the packets are sent to the next hop or they are rewritten according to the rewrite rule attached to the egress interface.

When the destination of incoming traffic is the host, DSCP bits are preserved. However, IEEE 802.1p bits are not preserved. The IEEE 802.1p bits of traffic destined for the host are set to zero (0). This does not affect system behavior because the switch prioritizes traffic destined for the host based on the protocol type. For example, the switch gives a higher priority to BPDU traffic than to ping traffic.