Voice VLAN - CDP role

[snarayanaraju]

Hi All, Though it is basics, I am confused a little to understand the VOICE VLAN. Can you help me to get insight on the below

There is a IP Phone and a PC is connected to the Phone

    interface GigabitEthernet1/0/12
    description **** Cisco IP Phone ****
    switchport access vlan 10
    switchport mode access
    switchport voice vlan 20
 

1. I understand using CDP the switch inform IP Phone that the IP Phone has to send packet with VLAN 20. What will be the impact if CDP is disabled? Will the Phone work if I manually configure the VLAN as 20 by disabling CDP?

2. I understand the PC will be in VLAN 10. Does the Native VLAN concept plays a role here?

Thanks in advance

SAIRAM

[Peter Paluch]

Hi Sairam,

1. I understand using CDP the switch inform IP Phone that the IP Phone has to send packet with VLAN 20. What will be the impact if CDP is disabled? Will the Phone work if I manually configure the VLAN as 20 by disabling CDP?

Yes, the IP phone will work even with deactivated CDP if you configure it with the voice VLAN manually. CDP has two primary purposes here: it informs the IP phone about the voice VLAN to be used, and if the phone is required to rewrite the CoS field in the data frames sent by the PC to a specific value, CDP also advertises this CoS value to the phone.

2. I understand the PC will be in VLAN 10. Does the Native VLAN concept plays a role here?

It does although you do not configure it explicitly: Because the PC communicates without tagging, and is supposed to be placed in VLAN 10, the VLAN 10 is in fact the native VLAN on this port. The port basically operates as a trunk port with two allowed VLANs: VLAN 10 (the native VLAN; PC) and VLAN 20 (voice VLAN).

On old switches like 2900XL, the switchport voice vlan command caused CDP to advertise the voice VLAN but did not actually allow an access port to also send and receive tagged frames in the voice VLAN. Therefore, on these old switches, ports toward IP phones were actually configured as so-called mini-trunks. Your configuration would be accomplished on these old switches as follows:

interface GigabitEthernet1/0/12
 description **** Cisco IP Phone ****
 switchport mode trunk
 switchport trunk allowed vlan 10,20
 switchport trunk native vlan 10
 switchport voice vlan 20

Best regards,
Peter

[snarayanaraju]

Hi Peter - Thank you so much for the reply. The answer to first query is very clear

About the 2nd query you explain that the native is VLAN 10. Here I do have a small confusion. The output of "show interface gig 1/0/12 switchport" shows Native VLAN=1 (default) and Access VLAN=10. Does this give meaning only to Older IOS switches?  Does this mean that in the switches like 3750 the frame comes untagged in Gig 1/0/12 is tagged with Access VLAN 10

SAIRAM

[christopher jardine]

Is the port configured as a trunk or access?

if it shows native vlan it sounds like it's configured as a trunk port.

[Peter Paluch · Accepted solution]

Hello Sairam,

The term "native VLAN" is used in different contexts and it is sometimes not entirely clear.

By definition, the native VLAN is the VLAN on a switchport that does not use frame tagging when transmitting and receiving frames. This definition is simple and clear but it also creates interesting overlaps: On an access port, the access VLAN configured by the switchport access vlan command does not use tags. Thus, it meets the definition of the native VLAN as well. In other words, the access VLAN on an access port is also its native VLAN.

This fact causes confusion when talking about what VLAN operates as a native VLAN and when checking what VLAN is configured as a native VLAN. On Cisco Catalyst switches, a native VLAN is configured using the switchport trunk native vlan command - but this command is in effect only if the ports operates in the trunk mode. Otherwise, its value is stored but unused. Nevertheless, on access ports, the access VLAN operates as the native VLAN although it is not reported and mostly not even described as such. This is at the core of misunderstandings regarding the native VLAN.

Now, the native VLAN indicated in the show interface switchport and show interface trunk command outputs only refers to the particular value of the switchport trunk native vlan command. The show commands thus tell you which VLAN would be the native VLAN if the port was operating as a trunk mode port. However, if the port is an access port with or without the voice VLAN configured, it is the access VLAN itself that operates as a native VLAN.

Would this clarify the issue?

Best regards,
Peter

[snarayanaraju]

Hi Peter - Thank you again for the response. Really the explanation "the access VLAN on an access port is also its native VLAN" is clarifying the Native VLAN concept in Voice VLAN scenarios.

By the way, I am ordering you authored book "CCIE Routing and Switching v5.0 Official Cert Guide, Volume 1 (5th Edition)". Thank you for you contributions to the Network Society :)

SAIRAM

[christopher jardine]

I like this post because I didn't know of many people that had to do that to get ip Phones working on a 2900 switch.

2924 in my case, good stuff.