switchport priority extend cos 0

liven · December 2008

what difference does the number at the end of cos have?

switchport priority extend cos 0
switchport priority extend cos 2
switchport priority extend cos 3
switchport priority extend cos 4

kryolla · December 2008

Class of service the higher the number the more priority it gets for trunk links

This is taken from the command reference guide which you should be familair with

switchport priority extend
Use the switchport priority extend interface configuration command to set a port priority for the incoming untagged frames or the priority of frames received by the IP phone connected to the specified port. Use the no form of this command to return to the default setting.

Set the IP phone port to override the priority received from the PC or the attached device.

The class of service (CoS) value is a number from 0 to 7. Seven is the highest priority. The default is 0.

liven · December 2008

I guess I poorly worded the question.

From cisco's site:

When the voice VLAN feature is enabled, all untagged traffic is sent according to the default CoS priority of the port.

The default CoS value is 0 for incoming traffic.

So how does one determine the CoS value of traffic coming from the PC (and thus know to use a CoS value of 2,3,4 etc...).

And what is the CoS value for voice traffic?

Thanks

kryolla · December 2008

voice call set up I think is 3 and RTP is 5. You can trust the PC cos or remark it to 0 or whatever you want. Remeber the phone has a built in 3 port switch so you are telling the phon switch how to handle the COS value of the PC

dtlokee · December 2008

I think you are getting confused with the term of "untagged", this is in reference to an 802.1q tag. You can't put a CoS into a frame that is untagged (either 802.1q or 802.1p tag is required.) A host connected to the phone can be configured to insert a tag into the frame (802.1q or 802.1p).

kpjungle · December 2008

But using voice vlans, the phones traffic will come into the switch on this voice vlan, with the priority set to 5 (most likely).

Traffic from the pc will be untagged, and be on the vlan set on the switch with the switchport mode access <vlan> command. The cos set in the switchport priority extend cos <number> will be lost then since its not tagged ?

makes me a bit confused, since the issue we are trying to resolve is what classification/marking we wish to give to data comming from the pc, through the switch and further down the network so we can do QoS on it.

liven · December 2008

Exactly.

But I think the light is going on for me.

I am pretty sure that the default CoS for the phone is 5, so making the data from the PC lower than that should be good to go.

I know the highest we can set it to is 7, so setting it to 6 or 7 would make pc data trump phone data correct?

cisco_trooper · December 2008

There is a number of different ways to configure this. I wouldn't set anything to CoS 6 or 7. These are reserved for internal processes such as routing protocols, etc.

CoS 5 is typically referred to as Expedited Forwarding and is obviously used for real-time traffic such as voice, which you will want to have mapped to your QoS priority queue. CoS 3 is typically used for Call Signalling and is typically mapped to whichever queue you are giving 2nd highest priority. Using Weighted Round Robin your priority queue will be queue 4. Using Shaped Round Robin I BELIEVE it is queue 1.

Also, if you have your phone plugged into (or patched through to ) your switch and your PC plugged in to your phone you may use two different VLANs on that switchport.

You can use switchport voice vlan XXX to identify the VLAN your voice traffic will use.
You can use the usual switchport access vlan YYY to identify your data VLAN.

If your IP phone does not support CDP you will not be able to utilize the switchport voice vlan XXX command. You will either have to have your voice and data ride the same VLAN, or you MAY be able to set the switchport to trunk mode and allow only the voice and data VLANs, manually configure the VLAN id on your phone, etc...(haven't actually had to do that..??)

Before we go any further can you give us some more information. Is this a 2950, 2960, Standard, Enhanced..etc????

Does your IP phone support CDP, is it a Cisco phone?

How are you marking the voice data? This can be done directly on a lot of phones......

dtlokee · December 2008

CoS 5 is not "expidited forwarding" that is a DSCP value. In most cases CoS 5 will get mapped to EF by the ingress CoS to DSCP map but the phone should have already set this value in the IP header.

Keep in mind all the markings are meaningless if there is nothing acting on those markings. This means that CoS 5 can have a lower priority than CoS 0 if you configure it that way. The default configurations don't behave like this but to make a blanket statement that CoS 5 is better than CoS 0 is not accurate.

cisco_trooper · December 2008

dtlokee wrote:

CoS 5 is not "expidited forwarding" that is a DSCP value.

Keep in mind all the markings are meaningless if there is nothing acting on those markings. This means that CoS 5 can have a lower priority than CoS 0 if you configure it that way.

Agreed....

Netwurk · December 2008

liven wrote:

what difference does the number at the end of cos have?

switchport priority extend cos 0
switchport priority extend cos 2
switchport priority extend cos 3
switchport priority extend cos 4

That's a command for 802.1p

And you need to make sure you plug a PC into your Cisco IP Phone for it to have any effect.

A lot of places (like where I work) plug the phones into their own switch ports and give the PCs their own ports on a different VLAN. But if you're labbing it up for the CCNP, do it the Cisco way and connect the phone to a switch port and then plug the PC into your phone. I would also enable CDP (which you would most likely disable where you work) globally and on the ports.

The following is something I saved from an old Cisco web page. It may be slightly out of date, but I like the way they explain things:

The 802.1Q/p standard defines the use of the three-bit Class of Service (CoS) field in the 802.1Q tag to prioritize frames with eight classes (priorities) of traffic. The Cisco InterSwitch Link (ISL) trunk mode, which is also similar, provides the CoS field (least three significant bits in a four-bit user field). The Cisco IP phones, such as the Cisco 7960, tag the voice packets with a CoS value of five. These tagged packets are used by the switches to prioritize the voice traffic by queuing them in the priority queue in the egress port. This guarantees top priority to the time critical voice packets. Cisco switches do not provide any priority scheduling in the input side. However, they do provide priority scheduling in the output/egress port. The switches honor the incoming Class of Service (CoS) value in the dot1q tag. All the ports are considered trusted ports by default. Since dot1q does not tag the native VLAN traffic, issue the switchport priority default <0-7> interface level configuration command to classify the incoming untagged packet as desired. The switchport priority default <0-7> command is issued to provide CoS equivalent egress scheduling. If the egress port is a trunk port, the ingress CoS or port default priority configuration is marked in the outgoing frames as CoS values for the far-end device to treat them with the desired higher priority.

I'm not sure if you would get tested on ISL anymore, but the 802.1Q material should still be good.

malcybood · December 2008

cisco_trooper wrote:

If your IP phone does not support CDP you will not be able to utilize the switchport voice vlan XXX command. You will either have to have your voice and data ride the same VLAN, or you MAY be able to set the switchport to trunk mode and allow only the voice and data VLANs, manually configure the VLAN id on your phone, etc...(haven't actually had to do that..??)

For non-Cisco phones in a Cisco switch, you do need to trunk the switch port, however you do not need to manually configure the IP phone.

You can pass the gatekeeper/signalling server details and which VLAN the phone to become a member of via DHCP options then as long as you set the IP phone to full DHCP the DHCP server will pass the sig server IP addresses / VLAN with the DHCP request and allow the phone to register on the network on the correct VLAN.

The Cisco switch config we use for a Nortel phone is as follows and we use DHCP options 128 & 144 with scripts supplied by Nortel;

switchport trunk encapsulation dot1q
switchport trunk native vlan 2
switchport mode trunk
switchport voice vlan 701
switchport priority extend cos 0
spanning-tree portfast

I think Avaya use DHCP option 176 but not familiar with them as much as Cisco / Nortel.

kpjungle · December 2008

malcybood wrote:

cisco_trooper wrote:

If your IP phone does not support CDP you will not be able to utilize the switchport voice vlan XXX command. You will either have to have your voice and data ride the same VLAN, or you MAY be able to set the switchport to trunk mode and allow only the voice and data VLANs, manually configure the VLAN id on your phone, etc...(haven't actually had to do that..??)

For non-Cisco phones in a Cisco switch, you do need to trunk the switch port, however you do not need to manually configure the IP phone.

You can pass the gatekeeper/signalling server details and which VLAN the phone to become a member of via DHCP options then as long as you set the IP phone to full DHCP the DHCP server will pass the sig server IP addresses / VLAN with the DHCP request and allow the phone to register on the network on the correct VLAN.

The Cisco switch config we use for a Nortel phone is as follows and we use DHCP options 128 & 144 with scripts supplied by Nortel;

switchport trunk encapsulation dot1q
switchport trunk native vlan 2
switchport mode trunk
switchport voice vlan 701
switchport priority extend cos 0
spanning-tree portfast

I think Avaya use DHCP option 176 but not familiar with them as much as Cisco / Nortel.

Hmm. I thought the switchport voice vlan <vlan-id> was a "special" type of 802.1q trunk, so I didnt think that you could use regular trunking as well.
Also, for data from the PC (connected to the ip-phone), i thought the switchport access vlan <vlan-id> was used, but i guess that since its a regular trunk, everything not-tagged will go into vlan 2?

malcybood · December 2008

kpjungle wrote:

malcybood wrote:

cisco_trooper wrote:

If your IP phone does not support CDP you will not be able to utilize the switchport voice vlan XXX command. You will either have to have your voice and data ride the same VLAN, or you MAY be able to set the switchport to trunk mode and allow only the voice and data VLANs, manually configure the VLAN id on your phone, etc...(haven't actually had to do that..??)

For non-Cisco phones in a Cisco switch, you do need to trunk the switch port, however you do not need to manually configure the IP phone.

You can pass the gatekeeper/signalling server details and which VLAN the phone to become a member of via DHCP options then as long as you set the IP phone to full DHCP the DHCP server will pass the sig server IP addresses / VLAN with the DHCP request and allow the phone to register on the network on the correct VLAN.

The Cisco switch config we use for a Nortel phone is as follows and we use DHCP options 128 & 144 with scripts supplied by Nortel;

switchport trunk encapsulation dot1q
switchport trunk native vlan 2
switchport mode trunk
switchport voice vlan 701
switchport priority extend cos 0
spanning-tree portfast

I think Avaya use DHCP option 176 but not familiar with them as much as Cisco / Nortel.

Hmm. I thought the switchport voice vlan <vlan-id> was a "special" type of 802.1q trunk, so I didnt think that you could use regular trunking as well.

This is true for Cisco IP phones which accomplish this with the use of Dynamic Trunking Protocol & CDP. A non Cisco phone does not perform these actions as CDP is proprietry to Cisco, hence the port will not be trunked by the IP phone and you require to use switchport mode trunk command.

kpjungle wrote:

Also, for data from the PC (connected to the ip-phone), i thought the switchport access vlan <vlan-id> was used, but i guess that since its a regular trunk, everything not-tagged will go into vlan 2?

correct

switchport priority extend cos 0

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