COS values and outgoing interface queues.

[sarahr202]

Hi everybody.

The switch uses its queues, which are available on a per-port basis, to  buffer incoming and outgoing frames. The switch can use the CoS values  to place the frames in the appropriate queues. Voice frames should be  placed in the priority queue for minimal delay.

Let say i a switch trust cos of incoming frames and places in the priority queue for minimal delay. The switch does not alter cos values. Will this switch also place these frames with cos values in the appropriate queues for outgoing interface ?

thanks and have a great weekend.

[smehrnia · Accepted solution]

Hi Sarah,

I hope i'll be a good help for u to get it this time;

1) many ip phones have a built-in three port switch in them, one for PC, one for the phone itself, and one for upstream connection. when using separate voice n data van for ip phones the internal switch will tag voice traffic with the respective voice van ID and applies a COS 5 value.

since the ip phone connection to upstream switch is 802.1q trunk, it considers the Data vlan, its native vlan and send PC traffic untagged. (Correct)

2) using 802.1p, my understanding is that the ip phone sends the voice and data traffic with 802.1p header set and the frames carry a vlan ID equal to Zero (not literally VLAN 0) and switch accepts the frames with VLAN zero as if they are in the access vlan set on the switchport.

3) u r right, this is probably IP Precedence 5, not the CoS 5.

4) This feature is complementary to the "port prioritization for untagged packets" available on supported Cisco Catalyst switches.

whats the use of it? it lets us make sure that the traffic coming from a PC connected to the iphone will never get as high priority as Voice packets generated from the ip phone itself.

me not in the States but Happy 4th of july to U

HTH,

Soroush.

[sarahr202]

Thanks Soroushm.

4) sw--------------ipphone

                           |

                         Pc

Let say we configure sw as:

int f1/1

switchport access vlan 2

switchport voice vlan 3

switchport priority extend cos 3

Ip  phone will use native vlan2 for Pc which means no dot1q tag and  vlan 3  for voip traffic. The last command tells ipphone to set COS value to 3  for frames received on its port connected to PC.

My question is  what is the point as ip phone  will send Pc 'frames using native vlan 2  to sw, without 8021q tag as a result , the cos value 3 can not be sent  with PC data frames to SW.

If the above is true why  should one use "switchport priority extend cos 3"  command ?

4) This feature is complementary to the "port prioritization for  untagged packets" available on supported Cisco Catalyst switches.

whats  the use of it? it lets us make sure that the traffic coming from a PC  connected to the iphone will never get as high priority as Voice packets  generated from the ip phone itself.

My question is since Ip phone will use native vlan to send PC 's frames to sw, therefore it should not matter whether we use the command "switchport priority extend cos 3" or not because the switch will not receives any COS if Ip phone uses native vlan to send attached Pc's frames.

If above is true, then the command "switchport priority extend cos 3" simply tells attached ip phone to set cos value to 3 for received frames from attached PC . Then could we say the command "switchport priority extend cos 3"  just helps internal switch of ip phone to perform qos for its ingress port?( i.e the data port of ip phone connected to PC)

Thanks and have a nice upcoming weekend.

"

[smehrnia · Accepted solution]

I guess u are getting confused with the term of "Native VLAN or 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 (802.1p). in such case's or as a precaution, to make sure we don't ruin our voice priority accidentally or by a rogue user, intentionally, we could use this command.

Hope it Helps,

Soroush.

[sarahr202]

Thanks Soroushm.

let me eloboarate on it

sw-------------ipphone

                         |

                     PC

we configure our switch as:

int f1/1( port connected to ip phone)

switchport access vlan 22

switchport voice vlan 23

switchport priority extend trust

Ip phone will use native vlan 22( which means no 802.1q tag, hence no cos value) for attached pc.

Let say PC is capable of inserting 8021q tag and currently using cos=4

But when iphone sends the frame from pc to sw, it will use native vlan 22 which means cos value will not be carried because there is no 8021q tag.

Is this  correct?

thanks

[smehrnia · Accepted solution]

Sarah,

Priority is 802.1p totally different from 802.1Q header

check this url:

http://www.cisco.com/en/US/products/hw/switches/ps607/products_qanda_item09186a0080116ffe.shtml#q9

Soroush.

[sarahr202]

thanks for the link Soroushm.

Dot1p marrkings are carried by dot1q tag . So If there is no dot1q tag, there is no dot1p marking either.

if ip phone is using native vlan for attached PC, that means there is no 802.1p markings because switch does not insert dot1q tag for native vlan.

Feel free to correct me If I am wrong.

thanks

[smehrnia]

hahaha, its getting complicated and too technicall, im looking arround to find a solid documented chart or sth.

the case is the ip phone itself doesnt tag the packets comming from the PC or the device attached. the device itself could make the frame in such way to carry the 802.1 header.

HTH,

Soroush.

[siddhartham · Accepted solution]

I like this discussion, learned few things that I don't know before.

"Dot1p marrkings are carried by dot1q tag . So If there is no dot1q tag, there is no dot1p marking either.

That is correct.

if  ip phone is using native vlan for attached PC, that means there is no  802.1p markings because switch does not insert dot1q tag for native  vlan"

I am not sure what you exactly mean here but IP phone can't influece what PC can use.

usualy PCs can't tag the traffic and can't mark the traffic unless you use any special settings, so if the switch port is configured as a trunk port the outgoing or incomming PC traffic will go to the native VLAN defined.

that means there is no  802.1p markings because switch does not insert dot1q tag for native  vlan"

Yes

Siddhartha

[smehrnia]

hey Sid,

from cisco.com:

Q. Can the Cisco EtherSwitch EHWICs support 802.1p and port prioritizations at the same time?

A. Yes. The switch uses 802.1p prioritization for the incoming tagged packets and port-based prioritization for the incoming untagged packets. The two modes function in a mutually exclusive and complementary manner.

know anything about it?

this objective is on my nerve now lol gotta look into it in more depth, maybe ask my frnds @ Cisco! will post if anything.

Soroush.

[siddhartham]

Soroush,

Its new to me also, I didn't know that we can prioritize the untagged traffic.

"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 ..."

Siddhartha

[sarahr202]

if   ip phone is using native vlan for attached PC, that means there is no   802.1p markings because switch does not insert dot1q tag for native   vlan"

I am not sure what you exactly mean here but IP phone can't influece what PC can use.

i apologize for not being very articulate.

Let me expound on it.

Sw1--------------------------------ip phone

                                          |

                                       PC

By using command " switchport voice vlan 2"  on sw1, we instruct ip phone to use native vlan for all traffic from attached PC, It also tells ip phone to use vlan 2  for voip traffic.Native vlan means no dot1q tag is inserted by ip phone and therefore no 8021p markings either.

Are you following me Siddhartham? If not let me know.

I assume you underrstood my point. Let me recap the discussion I was having with our friend Soroush.

The command"switchport priority extend cos 3 ' on sw1 tells ip phone to set cos value in frames received from attached pc to 3 . Let assume it is possible that attached Pc can set cos value .

But since Ip phone is instructed to use native vlan for attached pc's traffic. therefore all these cos values will not be carried because ip phone does not insert dot1q tag for native vlan.

It implies " the command " switch priority extend cos 3" or with any other option such as " trust " does not cause sw1 to perform qos for frames originated from attached pc because  these frames are sent over native vlan which means no dot1q tag and therefore no cos markings.

So what is the purpose of this command then?  you might say  by using this command we are telling ip phone to trust or set cos value. I agreed but what is the benefit? It was mentioned in earlier post internal switch inside ip phone does not perform qos.

thanks and have a great day

[siddhartham]

"

Sw1--------------------------------ip phone

                                          |

                                       PC

By  using command " switchport voice vlan 2"  on sw1, we instruct ip phone  to use native vlan for all traffic from attached PC, It also tells ip  phone to use vlan 2  for voip traffic.Native vlan means no dot1q tag is  inserted by ip phone and therefore no 8021p markings either."

Yes, switchport voice vlan 2 command tells the IP phone to send its voice traffic to VLAN2. But I don't think we instruct IP phone to use native VLAN for the PC. It happens by default since the PC sends untagged (no 802.1Q) frames they will be sent to the native VLAN ( which is defined on the SW1 either by switchport access VLAN or by switch trank native VLAN command.)

The command"switchport priority extend cos 3 ' on sw1 tells ip phone to set cos value in frames received from  attached pc to 3 . Let assume it is possible that attached Pc can set  cos value .

But since Ip phone is instructed to use native vlan  for attached pc's traffic. therefore all these cos values will not be  carried because ip phone does not insert dot1q tag for native vlan.

It  implies " the command " switch priority extend cos 3" or with any other  option such as " trust " does not cause sw1 to perform qos for frames  originated from attached pc because  these frames are sent over native  vlan which means no dot1q tag and therefore no cos markings.

I don't have answer to this question, I read few documents but couldn't find anything usefull.

switchport priority extend cos 3- Command tells the IP phone to assign cos 3 (802.1P tag) to the frames that the switch in the ip phone receives on the port connected to the PC

Since 802.1P is a field in 802.1Q,  ip phone should tag the data traffic from the PC with 802.1Q. In that case what will be the VID (VLAN ID) field in the frame set to.

Traffic will be forwarded to native VLAN only if its untagged traffic, does this mean all our PC traffic is going to be drooped?

Sarah,

Sorry for asking more questions instead of answering yours

Soroush,

Please correct me if my questions are not valid.

Siddhartha

[Reza Sharifi · Accepted solution]

Hi Sarah,

1) My book says this command tells ip phone to use native vlan 22 ( also called data vlan) and vlan 24 for voip frames.

ip phone will send pc data without any tag and hence the word native vlan

Is this correct?

Yes

2) If we use " switchport voice vlan dot1p" the switch tells ip phone  to use vlan 0 for voip traffic and use native vlan ( which is vlan 22)  for PC.

My question is since we are using vlan 0 for voip, it should be used for voip throughout switched network for example:

Ip phone1-----sw1--------sw2------ipphone2

    |                                            |

  PC                                       PC

If  we use " switchport voice vlan dot1p" on sw2,  voip frames will be sent  in vlan 0 by iphone2 to sw2.  If ipphone2 has to communicate with  iphone1  and they are on same subnet, then should  ipphone1 be on vlan 0  as well?( that means we must also configure  SW1 with " switchport  voice vlan dot1p")

Yes, you would need to configure switchport  voice vlan dot1p on all your switches, but remember vlan 0 is an internal voice vlan to the phone, when the switch receives vlan 0, it will change it to whatever access vlan you have configured (in your case 22).

3) If we use " switchport voice vlan untagged" on sw2, my book says both  voip and pc data are sent using native vlan. then book also says COS  bits still sent.  This boggles my mind. When we use the option "  untagged"  the switch should not insert any dot1q tag . So without dot1q  tag, how COS bits can still be carried?

Since the dot1p is a 3 bit priority tag and it is inside dot1q, when you configure "switchport voice vlan untagged" than there is no tag and no COS.

happy 4th.

HTH

[sarahr202]

Thanks Reza.

2) If we use " switchport voice vlan dot1p" the switch tells ip  phone  to use vlan 0 for voip traffic and use native vlan ( which is  vlan 22)  for PC.

My question is since we are using vlan 0 for voip, it should be used for voip throughout switched network for example:

Ip phone1-----f1/1sw1--------sw2 f1/1------ipphone2

    |                                                                         |

  PC                                                                      pc

Yes, you would need to configure switchport  voice vlan dot1p on all  your switches, but remember vlan 0 is an internal voice vlan to the  phone, when the switch receives vlan 0, it will change it to whatever  access vlan you have configured (in your case 22).

we have configured both sw2 and sw1 with :

switchport access vlan 22

switchport voice vlan dot1p

Iphone2 sends voip frame lets call it f1 using vlan 0

sw2 receives it perform qos and replaces vlan 0 by vlan 22 and switches it out of trunk port to sw1.

Sw1 receives the frame f1 and performs qos and replaces vlan 22 id by vlan 0 id and forwards it out f1/1 to ip phone.

Is my understanding correct?

Happy 4th to you as well.

[Reza Sharifi · Accepted solution]

Hi Sarah,

Yes, your understanding is correct.

Good Luck with the switching exam!

Make sure to get some hands-on practice on routers and switches before you take it.

Reza