Cisco QOS trust boundaries

Vocab:

QOS - Quality of Service - Prioritizing packets so that time or delay sensitive applications move through the network first.
COS - Class of Service - Layer 2 QOS, also referred to as IP Precedence, what a switch uses to determin priorities (0-7)
DSCP - differentiated services code point - Layer 3 QOS, what routers use to determine qos priority. Six bits give you 63 levels, but I believe only 56 are usually used.
-Devices keep a map to translate COS and DSCP values, switching between the two as needed, but every device can have a different map, so a COS value of 5 is not always translated to DSCP 46.


Ok, since I got qos trust questions both times I took the BCMSN, I thought I had better cover it. You should be able to look at a diagram and write/match a config or vice versa.

Since VoIP phones have a jack for a PC in the back, the question of trusting QOS values comes into play. The options are to re-write all the QOS values on the switch, therby not trusting the phone or the PC. You can trust the phone's default QOS tag, or tell it what QOS tag to use, therby trusting the phone to act correctly. Last, if you have a PC plugged in the back of the phone that needs special QOS for an application, you can instruct the phone to pass on the PC's QOS tags without changing them, and trust everything.

Case one is to simply re-write all QOS values at the switch port. You can do this by choosing a cos value followed by the command instructing the switch to override all qos tags with the value you just chose.

mls qos cos [#]
mls qos cos override

Case two is the default. You can simply let the switch do it's thing, and it will trust the qos values of the phone, while the phone rewrites any qos values the PC may attempt to use. The other option that results in the same trust boundary is to instruct the phone to use a particular COS value. Even though the switch is instructing the phone to use a particular COS value, you are still trusting the phone to listen and act as instructed, hence the trust boundary after the phone.

mls qos trust cos
switchport priority extend cos [#]

Case three is not as common. This instructs the phone to not re-write any qos values on the packets coming in from the PC, and to pass them on as is so that the switch uses those qos values. Only extend the trust to the PC if the computer is under your control/management and has a specific need for an application to have qos priority, such as a soft phone.

switchport priority extend trust