Trust Boundaries Mark Where

Personal Computer Frames are typically unmarked (CoS=0) unless NIC is 802.1P- or 802.1Q- capable If marked, IP Phone can (and by default does) reclassify CoS but not DSCP

IP Phone Marks voice as Layer 2 CoS (default) or Layer 3 ToS or DSCP Reclassifies incoming PC data frames

Access Layer Based on switch capabilities Accept or remap here

Distribution Layer Example; Catalyst 6000

Mark traffic Accept CoS/ToS Remap CoS to ToS or DSCP

Distribution Layer Example; Catalyst 6000

Mark traffic Accept CoS/ToS Remap CoS to ToS or DSCP

Typically: Voice: CoS=5

PC: reclassify CoS=0

For scalability, marking should be done as close to the source as possible.

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Classification should take place at the network edge, typically in the wiring closet or within endpoints (servers, hosts, video endpoints, or IP telephony devices).

For example, consider the campus network containing IP telephony and host endpoints. Frames can be marked as important by using link-layer CoS settings, or the IP precedence or DSCP bits in the ToS and DiffServ field in the IPv4 header. Cisco IP Phones can mark voice packets as high priority using CoS as well as ToS. By default, the IP Phone sends 802.1P-tagged packets with the CoS and ToS set to a value of 5 for its voice packets. Because most PCs do not have an 802.1Q-capable network interface card (NIC), they send packets untagged. This means that the frames do not have an 802.1P field. Also, unless the applications running on the PC send packets with a specific CoS value, this field is zero.

Note A special case exists where the TCP/IP stack in the PC has been modified to send all packets with a ToS value other than zero. Typically this does not happen, and the ToS value is zero.

Even if the PC is sending tagged frames with a specific CoS value, Cisco IP Phones can zero out this value before sending the frames to the switch. This is the default behavior. Voice frames coming from the IP Phone have a CoS of 5 and data frames coming from the PC have a CoS of 0. If the DSCP is set, then the IP Phone cannot re-mark the DSCP.

If the end device is not a trusted device, the reclassification function (setting or zeroing the bits in the CoS and ToS fields) can be performed by the access layer switch, if that device is capable of doing so. If the device is not capable, then the reclassification task falls to the distribution layer device. If reclassification cannot be performed at one of these two layers (the access layer or the distribution layer), a hardware or Cisco IOS software upgrade or both may be necessary.

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