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Bronze

IP Phone QoS at L3

Do IP phones send out IPPrec 5 or DSCP 46/EF on L3 or both?

Thx

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Re: IP Phone QoS at L3

Actually, the phones do mark DSCP, as configured in Callmanager Service or Enterprise Parameter, depending on CCM version. They also mark 802.1p COS bits to match the corresponding IP Prec bits if the frame is a 802.1q trunk frame.

So the answer to the original post is: For 3.x, the default is af31 for sccp, and ef for rtp. For 4.1.x, it's cs3 for sccp, and ef for rtp. However, all are configurable. And the first three bits for EF are the same as IP Prec 5, so for an EF marked packet, a device that only is ip prec aware (looks only at the first three TOS bits) will see a 5.

While you can trust dscp or ip prec on a phone port, the advantage to trusting cos is that you can deny trust/special treatment to the connected PC's data traffic. If you trust dscp, and the PC marks DSCP along with the phone, then you trust PC traffic as well which is usually not desirable.

You will also want to trust dscp on ports used by Callmanger, Unity, VG248, voice gateways, WAN router ports, IPCC servers, and/or any servers that legitimately mark dscp for special treatment.

Callmanger 3.x CCM Service Parameter

IP Precedence BitMask :

This parameter specifies a 8 bit TOS (Type of service) bitmask value that some SCCP (Skinny Client Control Protocol) devices use when setting RTP streams. Although (RFC 2474 and 2475) can be supported, the current default value follows RFC 791. Do not change the default vaule unless the user is instructed to do so by a Cisco support engineer with reference of proper RFC.

This is a required field.

Default: 0xB8.

Minimum: 0x0.

Maximum: 0xFF.

Version 4.1.x Enterprise Parameter

DSCP for SCCP Phone Configuration :

This parameter specifies the Differentiated Service Code Point (DSCP) IP classification for any SCCP-based phone configuration, including any TFTP, DNS, or DHCP access that is necessary for phone configuration. Note: You must restart SCCP-based phones for this parameter change to take effect.

This is a required field.

Default: CS3(precedence 3) DSCP (011000).

Restart SCCP phones for the parameter change to take effect.

9 REPLIES
Community Member

Re: IP Phone QoS at L3

IP Prec for 5 for sure

Community Member

Re: IP Phone QoS at L3

Yep, the ip phones send out their packets with IP precedence of 5. The switch has a cos-dscp mapping that translates IP Precedence 5 to dscp ef by default, tho all of the mappings can be altered if need to.

Bronze

Re: IP Phone QoS at L3

K, that is what I orinally thought, but some of the newer documentation is implying otherwise.

http://www.cisco.com/en/US/products/hw/switches/ps646/products_configuration_guide_chapter09186a00802cb831.html#wp1152559

"For most Cisco IP Phone configurations, the traffic sent from the telephone to the switch should be trusted to ensure that voice traffic is properly prioritized over other types of traffic in the network. By using the mls qos trust cos interface configuration command, you configure the switch port to which the telephone is connected to trust the CoS labels of all traffic received on that port. 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."

If you look at Step 6 in this link, it implies DSCP is supported on IP Phones. I see the same blurb on other newer swich OS doc (2970, 3560, 3570)

I did not think IP Phones supported DSCP EF/46 just IPPrec. It would be great to trust DSCP so avoids additional configs such as mapping cos-dscp values.

Community Member

Re: IP Phone QoS at L3

The phones themselves don't use dscp, only ip prec. DSCP gives you 64 levels of qos treatment where IP prec fives you 8, although you really only use up to 5. 6-8 are supposed to be reserved for routing protocols and network glue.

Irregardless of dscp or ip prec, when the voice traffic hits a layer 3 switch or router it should be classified with nbar and access lists. Once it is classified you may reset the ip prec or institute dscp as you wish.

If you look at you above quote if you read carefully you will se that ip prec is applied to a switched interface, and dscp is applied to a routed interface.

the mls qos trust dscp command would be used on the switch to trust inbound packets that had already been classified by a router or other layer 3 device.

Re: IP Phone QoS at L3

Actually, the phones do mark DSCP, as configured in Callmanager Service or Enterprise Parameter, depending on CCM version. They also mark 802.1p COS bits to match the corresponding IP Prec bits if the frame is a 802.1q trunk frame.

So the answer to the original post is: For 3.x, the default is af31 for sccp, and ef for rtp. For 4.1.x, it's cs3 for sccp, and ef for rtp. However, all are configurable. And the first three bits for EF are the same as IP Prec 5, so for an EF marked packet, a device that only is ip prec aware (looks only at the first three TOS bits) will see a 5.

While you can trust dscp or ip prec on a phone port, the advantage to trusting cos is that you can deny trust/special treatment to the connected PC's data traffic. If you trust dscp, and the PC marks DSCP along with the phone, then you trust PC traffic as well which is usually not desirable.

You will also want to trust dscp on ports used by Callmanger, Unity, VG248, voice gateways, WAN router ports, IPCC servers, and/or any servers that legitimately mark dscp for special treatment.

Callmanger 3.x CCM Service Parameter

IP Precedence BitMask :

This parameter specifies a 8 bit TOS (Type of service) bitmask value that some SCCP (Skinny Client Control Protocol) devices use when setting RTP streams. Although (RFC 2474 and 2475) can be supported, the current default value follows RFC 791. Do not change the default vaule unless the user is instructed to do so by a Cisco support engineer with reference of proper RFC.

This is a required field.

Default: 0xB8.

Minimum: 0x0.

Maximum: 0xFF.

Version 4.1.x Enterprise Parameter

DSCP for SCCP Phone Configuration :

This parameter specifies the Differentiated Service Code Point (DSCP) IP classification for any SCCP-based phone configuration, including any TFTP, DNS, or DHCP access that is necessary for phone configuration. Note: You must restart SCCP-based phones for this parameter change to take effect.

This is a required field.

Default: CS3(precedence 3) DSCP (011000).

Restart SCCP phones for the parameter change to take effect.

Bronze

Re: IP Phone QoS at L3

As a followup comment/question, the lastest qos srnd states to either use auto-qos or not. Apparently if you setup the phone qos manually, the interface will have issues with "qos trust device cisco-phone"

This has change from the older SRND qos doc, which states to "trust cos"

I wonder if the "qos trust device cisco-phone" is trusting at the cos or dscp level or both?

TIA

Re: IP Phone QoS at L3

It trusts cos, again to distinguish phone traffice from attached pc traffic. The main difference is that it uses cdp to determine if the attached device is a phone. If the attached device is a phone, then the trust boundary is extended to the phone.

It is preceeded by a "cdp enable" and a "mls qos trust cos" in the interface config.

This helps prevent the switch from trusting cos from a directly attached pc.

Bronze

Re: IP Phone QoS at L3

Yes, your right. I double checked the SRND again and it does reference the cos

Thanks again for your help

Re: IP Phone QoS at L3

Thanks for the feedback - It's good to know I helped.

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