802.1q and 802.1p standard,dtp and cdp isl

Answered Question
Dec 28th, 2008

hi every body!

I have few questions:

The context is a switch is connected to cisco ip phone. My book says" If an 802.1q trunk is needed, a special-case trunk automatically is negotiated by dtp and cdp"

1) Can cdp be used to negotiate the trunking?

2) I understand that the cos bits in 802.1q were undefined. 802.1p defines those bits.

My book refers to 802.1q encapsulation with 802.1p bits as 802.1p trunk.

My argument is 802.1p is not trunk encapsulation , it is just the standard that define cos bits in 802.1q. Am i correct?

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The command to configure a port connected to ip phone:

switch(config-if)# switchport voice vlan{vlan id/dotip/untagged/none}

The book explains" The default condition for every switch port is none,where a trunk is not used.All modes except for none use the special-case 802.1q trunk"

Here is the assumption is dot1q was negotiated . how about If it were ISL not dot1q?

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How can i configure switch port,connected to ip phone, use isl trunk ?

Can i do the following:

switch(config-if)# switchport trunk encapsulation isl

switch(config-if)# switchport voice vlan 4

But above command assumes dot1q is in use.

thanks a lot!

I have this problem too.
0 votes
Correct Answer by lejoe.thomas about 7 years 11 months ago

Hi Sarah,

I know most cisco switches do support it (2950/2960,3550,3650,3750,6500s to name a few). By going to configuration guide of particular model and looking at the option of configuring voice vlans, this can be confirmed.

Most IP phones (Cisco 7960,7940) are marking COS using the 802.1q header's priority bits and never ISL. ISL is proprietary (still existing to maintain compatibility with older switches I guess) and 802.1q is standards based, which could be one of the reasons.

Happy new year as well!

HTH

Lejoe

Correct Answer by lejoe.thomas about 7 years 11 months ago

Hi Sarah,

IP phones support 802.1q/802.1p standard for reasons as mentioned, with switch informing phones to do marking based on these standards. If you go into a switch port configuration available for the IP phones (which has been covered in one your previous posts: ip phone native), you'll observe 802.1q/802.1p standards are used.

HTH

Lejoe

Correct Answer by Giuseppe Larosa about 7 years 11 months ago

Hello Sarah,

ISL and IP phones are not good companions I think.

ISL overhead is 30 bytes instead of 4 bytes of 802.1Q (in which are the 3bits CoS 802.1p)

for all the other questions Jon's answer rules

Hope to help

Giuseppe

Correct Answer by Jon Marshall about 7 years 11 months ago

Sarah

1) No it can't but CDP is used to recognise that a Cisco phone has been connected to the port. Then DTP is used to automatically for the trunk.

2) Correct - 802.1p defines the CoS markings not the encapsulation on the trunk link.

Jon

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Correct Answer
Jon Marshall Mon, 12/29/2008 - 02:22

Sarah

1) No it can't but CDP is used to recognise that a Cisco phone has been connected to the port. Then DTP is used to automatically for the trunk.

2) Correct - 802.1p defines the CoS markings not the encapsulation on the trunk link.

Jon

Correct Answer
Giuseppe Larosa Mon, 12/29/2008 - 03:06

Hello Sarah,

ISL and IP phones are not good companions I think.

ISL overhead is 30 bytes instead of 4 bytes of 802.1Q (in which are the 3bits CoS 802.1p)

for all the other questions Jon's answer rules

Hope to help

Giuseppe

sarahr202 Mon, 12/29/2008 - 05:18

Thanks a lot Giuseppe!

So we don't or can not use isl with cisco ip phone. Is it correct t assume?

thanks a lot!

Correct Answer
lejoe.thomas Mon, 12/29/2008 - 14:47

Hi Sarah,

IP phones support 802.1q/802.1p standard for reasons as mentioned, with switch informing phones to do marking based on these standards. If you go into a switch port configuration available for the IP phones (which has been covered in one your previous posts: ip phone native), you'll observe 802.1q/802.1p standards are used.

HTH

Lejoe

sarahr202 Mon, 12/29/2008 - 18:11

Thanks Lejoe !

But i don't find any option for isl when i use the command" switchport voive vlan ?"

1)Should I take as safe bet that ip phone do not support isl?

Do all the switches(of course Cisco brand)support 802.1p?

thanks a lot!

Correct Answer
lejoe.thomas Mon, 12/29/2008 - 18:39

Hi Sarah,

I know most cisco switches do support it (2950/2960,3550,3650,3750,6500s to name a few). By going to configuration guide of particular model and looking at the option of configuring voice vlans, this can be confirmed.

Most IP phones (Cisco 7960,7940) are marking COS using the 802.1q header's priority bits and never ISL. ISL is proprietary (still existing to maintain compatibility with older switches I guess) and 802.1q is standards based, which could be one of the reasons.

Happy new year as well!

HTH

Lejoe

sarahr202 Mon, 12/29/2008 - 20:06

Thanks a lot Lejoe for your reply and wishes.

I wish you a very happy new year as well!

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