When packets are processed through the device, there are several opportunities to influence the processing by configuration as described in the steps below. The processes performed to map packet priority to internal priority and drop precedence can be described as following:
- Collect priority and drop precedence information from various portions of the packet header:
- If a packet's EtherType matches 8100, decoding the
PCP value derives a priority value and drop precedence.
- For MPLS packets on supported SLX devices, decoding
the EXP bits derive priority value and drop precedence.
- For IPv4 or IPv6 packets, decoding the DSCP bits
derive priority value and drop precedence.
- For untagged Layer 2 packet, the port's default
value derives traffic class and drop precedence.
- The derived values for PCP, EXP and DSCP are mapped using either a default map or a configured ingress decode policy map.
- To assist the device in the decoding process described, decode map tables are defined.
- The priority and drop precedence values are obtained in descending order of priority, as follows:
- If tag exists and packet is switched, by decoding the PCP value from the tag.
- For IPv4 or IPv6 packets, and when the packet is routed, by decoding the DSCP field from the IP header.
- For MPLS packets on supported SLX devices, by
decoding the EXP value from MPLS header.
- Physical port default value.
