How works the native VLAN?

cjrchoi11 · ‎08-04-2005

Pls guide me to clarify the native VLAN.

sw1 & sw2 trunk connected and configure native VLAN100 & VLAN200 each as mismatch.

Q1) Is the management information such as VTP via native VLAN? Can be exchangable L2 information even native VLAN mismatch?

Q2) what happen the STP? sw1 sending to sw2 w/untagged but sw2 send w/tagged and it looks looping. please explain.

Thanks in advance,

amit-singh · ‎08-04-2005

Hi,

Native vlan carried the traffic as untagged for particular vlan defined as native over the trunks. Cisco's implementaion has made is madatory that you Native vlans has to be same on both the sides of the trunk.

The management information and the control plane traffic like PAGP, LACP,STP, CDP,VTP travel through Vlan 1 on layer 2 or low-end switches. Switches like 3550,3750,4K and 6K supports Vlan 1 minimization which means that the traffic for CDP and VTP will travel on native vlan.

If you have a native vlan mismatch and if the vlan is diffrent from Vlan 1, then it doesnot matter as the info will travel on vlan1. But if your Native vlan is same then the info will not travel from switch1 to switch2.

HTH,

-amit singh

andrew.butterworth · ‎08-05-2005

Amit

I believe you are incorrect with your second statement regarding control-plane traffic and VLAN #1. To quote Cisco:

"In summary, it is worth noting that on trunks, Cisco Discovery Protocol, VTP, and PAgP updates are always forwarded on trunks with a VLAN 1 tag. This is the case even if VLAN 1 has been cleared from the trunks and is not the native VLAN. Clearing VLAN 1 for user data has no impact on control plane traffic that is still sent using VLAN 1.

802.1q IEEE BPDUs are forwarded untagged on the common spanning tree VLAN 1 for interoperability with other vendors, unless VLAN 1 has been cleared from the trunk. Cisco Per-VLAN Spanning Tree (PVST+) BPDUs are sent and tagged for all other VLANs."

The management interface (on Layer-2 only switches) can be on any VLAN you choose. There are some bugs detailed regarding older IOS images for the 2900/3500XL as well as the 2950 so you may need to check these out (or simply upgrade the code) before changing the VLAN the management interface is on.

HTH

Andy

amit-singh · ‎08-05-2005

Andy,

Yes, you are right about the control plane traffic that it will forward it with a Vlan 1 tag if vlan 1 is removed from the trunk, I read the same a way back. I just mentioned the native vlan here quickly to ease the things and couldnot remember it quickly.

Anyways thanks for letting it know !!! :o)

regards,

-amit singh

cjrchoi11 · ‎08-05-2005

Thanks guys,

My understanding based on you guys comment is L2 traffic forwards through native VLAN with VLAN1 tagging and it works even filtering VLAN1 in the trunk interface.

Q1) still not clear in case of native VLAN mismatch between switches(ex, vlan1 filtered and vlan100/van200 each). Can you explain some detail for match and mismatch case to forward L2 control/management traffic?

Q2) what happen STP when native VLAN mismatch w/vlan1 clearing? looping condition?

Regards,

John.

cjrchoi11 · ‎08-08-2005

please advise. Txs.