CoS Priority and ToS Rewrite

The two parameters configurable for CoS priority are 802.1p and Type of Service (ToS). Each CoS can be mapped to an 802.1p priority and a ToS rewrite value. The 802.1p parameter is:

• A subset of ToS with values 0–7 (upper 3 bits of the 8 bit ToS field)
• Supported in both layer 2 and layer 3

The ToS parameter is:

• An 8-bit field with values 0–255
• Supported in layer 3 only
• Also referred to as the Differentiated Services Code Point (DSCP) when limited to the lower 5 bits of the field

Assigning and Marking Traffic with a Priority displays the relationship between your application, priority level, 802.1p, and ToS assignments (shown here using DSCP terminology).

QoS priority/ToS configuration:

• Derives its characteristic requirements from the end-system application
• Is configured on the edge device the application is connected to
• Is propagated through the network in the protocol packet header
  

  Assigning and Marking Traffic with a Priority

  

The ICMP protocol, used for error messaging, has a low bandwidth requirement, with a high tolerance for delay and jitter, and is appropriate for a low priority setting. HTTP and FTP protocols, used respectively for browser-generated and file transfer traffic, have a medium to high bandwidth requirement, with a medium to high tolerance for delay and jitter, and are appropriate for a medium priority level. Voice (VoIP), used for voice calls, has a low bandwidth requirement, but is very sensitive to delay and jitter and is appropriate for a high priority level.

See RFC 1349 for further details on ToS. See RFCs 2474 and 2475 for further details on DSCP.