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T5 · T5-R01

DSCP and the trust boundary

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Learn it in 5 minutes

  • Voice RTP is commonly marked EF, DSCP 46.
  • RFC 3246 defines Expedited Forwarding.
  • RFC 4594 maps telephony to EF and points signaling at CS5.
  • Cisco campus designs often mark call signaling CS3.
  • Say which document you are following.
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Visual

DSCP and the trust boundary diagram
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What is going on

Phones can mark packets. Switches can trust that mark or rewrite it. If the access port does not trust the phone, the WAN queue never sees EF, and the priority class stays empty while voice is delayed in the default class.

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Worked example

A capture at the phone shows DSCP 46. A capture after the access switch shows DSCP 0. The trust boundary is the switchport. Fix the policy there before you resize the WAN queue.

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Quiz

1. EF and CS3 are the same mark.

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Ecosystem

Sister pieces sit on the ecosystem page: the glossary, the mind map, the integration map, and the sibling labs.

  • EF — Expedited Forwarding, DSCP 46.
  • Trust boundary — The place you decide to keep or rewrite a mark.

Config. Record the policy-map and the interface direction.

Troubleshooting. Look at the mark on the wire, not the mark configured on the phone alone.

Bridge. Webex Calling media on the internet is not your EF queue unless you have a path that still honors DSCP. Often it does not.

Mind map, sister technologies, and future hooks live on the ecosystem page.

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Sources

Product screens move. Confirm the guide for your release before you change a live system.

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Sources and go deeper

Opened 2026-10. A row marked confirm has no link, because that page was not verified.

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