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Native VLAN mismatch

Pavel Doronin
Level 3
Level 3

‎08-02-2011 03:29 AM - edited ‎03-07-2019 01:30 AM

If there is a NATIVE VLAN mismatch on either side of an 802.1Q trunk, layer-2 loops may occur because VLAN 1 STP bridge protocol data units (BPDUs) are sent to the IEEE STP MAC adress (0180.c200.0000) untagged.

Can you explain this with examples?

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Latchum Naidu
VIP Alumni
VIP Alumni

‎08-02-2011 05:30 AM

Hi Pavel,

For switches running 802.1Q as both end trunking, the native VLAN of each port on the trunk must match. By default all COS ports are in VLAN 1; and the native VLAN on the IOS devices is also configured for VLAN 1, so the native VLAN does match. If you choose to change the native VLAN, use the set vlan command for COS switches or the switchport trunk native vlan command for IOS switches to specify the native VLAN. Remember that the native VLAN must match on both sides of the trunk link for 802.1Q; otherwise the link will not work. If there is a native VLAN mismatch, Spanning Tree Protocol (STP) places the port in a port VLAN ID (PVID) inconsistent state and will not forward on the link.

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Regards,
Naidu.

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