QoS Trust Boundaries
- CoS is 3 bits, or 7 values
- DSCP is 6 bits, or 64 values
Config
Trust if its a phone
!
! if a phone detected with CDP
! then trust dscp
!
mls qos trust device cisco-phone
mls qos trust dscp
Absolute Trust
mls qos trust dscp
DSCP transparency
Is off by default.
This means the CoS bits take priority and re-write the DSCP portion of the L3 header.
To turn this feature on, (to let DSCP move transparently through the switches) use no mls qos rewrite ip dscp
IOS-XE C9300 trust and queuing behavior
An example for one platform to demonstrate how complex QoS trust can become.
| Incoming Packet | Outgoing Packet | Trust Behavior | Queuing Behavior |
|---|---|---|---|
| Layer 3 | Layer 3 | Preserve DSCP/Precedence | Based on DSCP |
| Layer 2 | Layer 2 | Not applicable | Based on CoS |
| Tagged | Tagged | Preserve DSCP and CoS | Based on DSCP (trust DSCP takes precedence) |
| Layer 3 | Tagged | Preserve DSCP, CoS is set to 0 | Based on DSCP |
Maps
| CoS Value | DSCP Value |
|---|---|
| 0 | 0 |
| 1 | 8 |
| 2 | 16 |
| 3 | 24 |
| 4 | 32 |
| 5 | 40 |
| 6 | 48 |
| 7 | 56 |
| DSCP Value | CoS Value |
|---|---|
| 0–7 | 0 |
| 8–15 | 1 |
| 16–23 | 2 |
| 24–31 | 3 |
| 32–39 | 4 |
| 40–47 | 5 |
| 48–55 | 6 |
| 56–63 | 7 |
References
Cisco - QoS Configuration Guide, Cisco IOS-XE - 26.x.x - C9300