WAN links present special challenges. Lower bandwidth and provider involvement complicate design.
Bandwidth constraints: A Mbps LAN feeding Mbps WAN creates bottleneck. Apply QoS on WAN edge.
Serialization delay: Time to put packet on wire. A byte packet takes ms on Mbps. Large packets delay small ones.
Link fragmentation: Fragment large packets on slow links. Interleave voice between fragments. Required under Kbps.
Provider coordination:
- Provider must honor your DSCP
- You shape to provider CIR
- Avoid their policing
For MPLS, map DSCP to EXP bits consistently at both edges.