Cisco Phones

mattsthe2

Do Cisco Phones mark Cos or DSCP?

Find more posts tagged with

Free for TechExams community: Cybersecurity salary guide

Compare cert salaries and plan your next career move

Button

Comments

ciscog33k

It marks both, normally. Though you can tell the phone not to mark cos, it always marks at L3. After that, it's up tot he switch whether to trust the values or not.

mattsthe2

Thought so, so if it does both them surely most people would trust dscp instead of cos ? Then you wouldn't have to worry about the cos to dscp mapping (because in most networks your going to hit a layer 3 device and cos won't be. Carried through.) ?

Forsaken_GA

mattsthe2 wrote: »

Thought so, so if it does both them surely most people would trust dscp instead of cos ? Then you wouldn't have to worry about the cos to dscp mapping (because in most networks your going to hit a layer 3 device and cos won't be. Carried through.) ?

If I had a PC attached to the phone, I'd probably trust CoS instead of DSCP. CoS can only exist in a 802.1q frame, and the trunk is between the phone and the switch, not the pc and the switch

mattsthe2

Forsaken_GA wrote: »

If I had a PC attached to the phone, I'd probably trust CoS instead of DSCP. CoS can only exist in a 802.1q frame, and the trunk is between the phone and the switch, not the pc and the switch

Catalyst 3560 Switch Software Configuration Guide, Rel. 12.2(25)SED - Configuring QoS [Cisco Catalyst 3560 Series Switches] - Cisco Systems

So i found this (which looks like you use trust dscp for *routed* ports)

"Use the mls qos trust dscp interface configuration
command to configure a routed port to which the telephone is connected to
trust the DSCP labels of all traffic received on that port."

Something else i found interesting that i didnt know is you should use:
mls trust cos along with mls trust device cisco-phone together.

APA

mattsthe2 wrote: »

Catalyst 3560 Switch Software Configuration Guide, Rel. 12.2(25)SED - Configuring QoS [Cisco Catalyst 3560 Series Switches] - Cisco Systems

So i found this (which looks like you use trust dscp for *routed* ports)

"Use the mls qos trust dscp interface configuration
command to configure a routed port to which the telephone is connected to
trust the DSCP labels of all traffic received on that port."

Something else i found interesting that i didnt know is you should use:
mls trust cos along with mls trust device cisco-phone together.

This is assuming you have the Phone hooked up to a 3560 or linecard that can look at the IP header..... some switches have the smarts to look further into the frame from a end device and classify\trust based on the DiffServ marking in the IP header.

I'm pretty sure the 2960s have this capability as well(even though its only a L2 switch...)

Some of the older switches do not have this capability so you have to trust the cos marking in the dot1q tag's priority bits as all they see is frames and can't look past the ethernet header.

Yep spot on regarding the mls qos trust device cisco-phone.... but it really just means only trust the markings if the end device is a Cisco IP phone which the switch will learn if it is via CDP.

mattsthe2

Why does by default DSCP map to 40 for IP-Prec 5, isnt it more common to map to 46, like cos-to-dscp

3560#sh mls qos map ip-prec-dscp
IpPrecedence-dscp map:
ipprec: 0 1 2 3 4 5 6 7

dscp: 0 8 16 24 32 40 48 56

ciscog33k

mattsthe2 wrote: »

Why does by default DSCP map to 40 for IP-Prec 5, isnt it more common to map to 46, like cos-to-dscp

3560#sh mls qos map ip-prec-dscp
IpPrecedence-dscp map:
ipprec: 0 1 2 3 4 5 6 7
dscp: 0 8 16 24 32 40 48 56

It probably has to do with changing standards since they came up with their defaults but I don't know why cisco never changed it with later ios versions. You have to change the mapping manually if you want consistency with the modern standards. AutoQOS would also change it for you.