CoS (Class of Service) marks frames at Layer in the Q tag. The -bit PCP field provides priority levels.
CoS values:
- : Voice (EF equivalent)
- : Video conferencing
- : Call signaling
- : High priority data
- : Best effort (default)
CoS limitations: Only exists in VLAN-tagged frames. Lost when routed because Layer headers are rewritten. Works only within a Layer domain.
CoS to DSCP mapping: Switches map CoS to DSCP at Layer boundaries, preserving priority when traffic crosses subnets.