Native VLAN and Trunk

[bapatsubodh]

Hi,

Is it mandatory that Native VLAN on the both sides of the trunk shoud be same? By default VLAN1.

If it is configured in-correctly will the trunk form?

If native VLAN must be same - what is the reason for this, probably DTP will not communicate or anything else?

CASE 1.

If on one side  native VLAN is 100 and on second side it is configued as VLAN 200.

In this case frames from one for VLAN 100 will not be tagged while from second VLAN 200 frames will not be tagged. so VLAN 100 and also VLAN 200 frames will not be working and will be dropped.

Remainng VLAN id frames will cross the trunk with tagged as usual.

CASE 2 .

No trunk will be fromed no frames will cross the trunk.

What is the status of BPDU frames in this case?

please share the experince.

Thanks in advance

Subodh

[jgraafmans]

The VLAN id is send within the BPDU’s. If the switch receives a BPDU with the wrong VLAN id it will block both VLAN’s on the trunk. All other VLAN’s will work fine.

This are the logging messages you get when you configure a wrong native VLAN (I have set native VLAN to 301 on this switch):

%SPANTREE-2-RECV_PVID_ERR: Received BPDU with inconsistent peer vlan id 1 on GigabitEthernet0/25 VLAN301.
%SPANTREE-2-BLOCK_PVID_PEER: Blocking GigabitEthernet0/25 on VLAN0001. Inconsistent peer vlan.
%SPANTREE-2-BLOCK_PVID_LOCAL: Blocking GigabitEthernet0/25 on VLAN0301. Inconsistent local vlan.

[Ganesh Hariharan]

Hi,
Is it mandatory that Native VLAN on the both sides of the trunk shoud be same? By default VLAN1.
If it is configured in-correctly will the trunk form?
If native VLAN must be same - what is the reason for this, probably DTP will not communicate or anything else?
CASE 1.
If on one side  native VLAN is 100 and on second side it is configued as VLAN 200.
In
this case frames from one for VLAN 100 will not be tagged while from
second VLAN 200 frames will not be tagged. so VLAN 100 and also VLAN
200 frames will not be working and will be dropped.
Remainng VLAN id frames will cross the trunk with tagged as usual. 
CASE 2 .
No trunk will be fromed no frames will cross the trunk.
What is the status of BPDU frames in this case?
please share the experince.
Thanks in advance
Subodh

Hi Subodh,

In 802.1Q trunking, all VLAN packets are tagged on the trunk link, except the native VLAN. The native VLAN packets are sent untagged on the trunk link. Therefore, the native VLAN should be the same on both switches configured for trunking. This way, you can deduce to which VLAN a frame belongs when you receive a frame with no tag. By default, VLAN 1 is the native VLAN on all switches.

Hope to Help !!

Ganesh.H

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