Native VLAN question

Quick question.. how exactly do cisco switches know that two connected trunk ports have mismatched native VLANs? The native vlan mismatch error you get in logs also seems to appear between two cisco switches only, am i correct?

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FloOz

Native vlans I believe do not have a tag.

networker050184

CDP communicates this information.

boobobobobob

Ah thank you, this also explains why cisco's only show this error.

Jackace

I believe STP also communicates it and STP will block the port if the native vlans do not match.

Edit- below is the message I got after disabling cdp and then changing the native vlan on one side of the trunk to 5.

*Mar 1 00:05:55.939: %SPANTREE-2-RECV_PVID_ERR: Received BPDU with inconsistent peer vlan id 1 on FastEthernet1/0 VLAN5.
*Mar 1 00:05:55.943: %SPANTREE-2-BLOCK_PVID_PEER: Blocking FastEthernet1/0 on VLAN1. Inconsistent peer vlan.
*Mar 1 00:05:55.995: %SPANTREE-2-BLOCK_PVID_LOCAL: Blocking FastEthernet1/0 on VLAN5. Inconsistent local vlan.

boobobobobob

That's interesting, can anyone explain why STP would block VLANs with different native VLANs like in the above example? I was under the impression it would still work.

If SW1's Fa0/1 Native VLAN is 1 & SW2's Fa0/1 Native VLAN is 5. Wouldn't the untagged vlan from SW1 turn unto VLAN 5 once it reached SW2?

powmia

CDP is only an informational protocol, it doesn't negotiate anything. (you might argue that a phone negotiates via CDP, but it only uses the information to alter its own settings). STP may block a vlan (though it's native, the BPDU still contains the information). The only item that actually prevents a trunk from forming due to a native vlan mismatch is the other Cisco proprietary protocol... DTP. Issue a "switchport nonegotiate" and you will be able to change native vlans midstream... also need to use this to interoperate w/ other vendors.

networker050184

The BPDUs carry the VLAN ID in the extended ID field. If those mismatch on an untagged frame it is put in the inconsistent state.