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I am getting CDP NATIVE VLAN MISMATCH logs on my Cisco 3750x switch stack. Gig 2/0/5 and Gig 4/0/6 are in no way physically connected to each other (neither is 1/0/44 and 2/0/13 as you'll see below). I interconnect each switch via Stackwise cabling. I haven't seen any performance issues regarding this. Possibly a bug? I'm running 15.2(4)E9 on all of the following switches

Dec  6 17:06:11: %CDP-4-NATIVE_VLAN_MISMATCH: Native VLAN mismatch discovered on GigabitEthernet2/0/5 (10), with 3750X_STACK GigabitEthernet4/0/6 (50).
Dec  6 17:06:11: %CDP-4-NATIVE_VLAN_MISMATCH: Native VLAN mismatch discovered on GigabitEthernet2/0/5 (10), with 3750X_STACK GigabitEthernet4/0/6 (50). (3750X_STACK-2)
Dec  6 17:07:07: %CDP-4-NATIVE_VLAN_MISMATCH: Native VLAN mismatch discovered on GigabitEthernet4/0/6 (50), with 3750X_STACK GigabitEthernet2/0/5 (10).
Dec  6 17:07:07: %CDP-4-NATIVE_VLAN_MISMATCH: Native VLAN mismatch discovered on GigabitEthernet4/0/6 (50), with 3750X_STACK GigabitEthernet2/0/5 (10). (3750X_STACK-4)
Dec  6 17:07:10: %CDP-4-NATIVE_VLAN_MISMATCH: Native VLAN mismatch discovered on GigabitEthernet2/0/5 (10), with 3750X_STACK GigabitEthernet4/0/6 (50).
Dec  6 17:07:10: %CDP-4-NATIVE_VLAN_MISMATCH: Native VLAN mismatch discovered on GigabitEthernet2/0/5 (10), with 3750X_STACK GigabitEthernet4/0/6 (50). (3750X_STACK-2)
Dec  6 17:07:59: %CDP-4-NATIVE_VLAN_MISMATCH: Native VLAN mismatch discovered on GigabitEthernet2/0/5 (10), with 3750X_STACK GigabitEthernet4/0/6 (50).
Dec  6 17:07:59: %CDP-4-NATIVE_VLAN_MISMATCH: Native VLAN mismatch discovered on GigabitEthernet2/0/5 (10), with 3750X_STACK GigabitEthernet4/0/6 (50). (3750X_STACK-2)
Dec  6 17:08:07: %CDP-4-NATIVE_VLAN_MISMATCH: Native VLAN mismatch discovered on GigabitEthernet4/0/6 (50), with 3750X_STACK GigabitEthernet2/0/5 (10).
Dec  6 17:08:07: %CDP-4-NATIVE_VLAN_MISMATCH: Native VLAN mismatch discovered on GigabitEthernet4/0/6 (50), with 3750X_STACK GigabitEthernet2/0/5 (10). (3750X_STACK-4)

3750X_STACK#show switch detail
Switch/Stack Mac Address : xxxx.xxxx.xxxx
                                           H/W   Current
Switch#  Role   Mac Address     Priority Version  State
----------------------------------------------------------
*1       Master xxxx.xxxx.xxxx     15     3       Ready
 2       Member xxxx.xxxx.xxxx     1      3       Ready
 3       Member xxxx.xxxx.xxxx     1      3       Ready
 4       Member xxxx.xxxx.xxxx     1      3       Ready



         Stack Port Status             Neighbors
Switch#  Port 1     Port 2           Port 1   Port 2
--------------------------------------------------------
  1        Ok         Ok                2        4
  2        Ok         Ok                3        1
  3        Ok         Ok                4        2
  4        Ok         Ok                1        3

3750X_STACK#show cdp neighbors
Capability Codes: R - Router, T - Trans Bridge, B - Source Route Bridge
                  S - Switch, H - Host, I - IGMP, r - Repeater, P - Phone,
                  D - Remote, C - CVTA, M - Two-port Mac Relay

Device ID        Local Intrfce     Holdtme    Capability  Platform  Port ID
3750X_STACK
                 Gig 2/0/5         171             R S I  WS-C3750X Gig 4/0/6
3750X_STACK
                 Gig 4/0/6         131             R S I  WS-C3750X Gig 2/0/5
3750X_STACK
                 Gig 2/0/13        150             R S I  WS-C3750X Gig 1/0/44
3750X_STACK
                 Gig 1/0/44        147             R S I  WS-C3750X Gig 2/0/13

Switch Ports Model                     SW Version            SW Image
------ ----- -----                     ----------            ----------
*    1 54    WS-C3750X-48P             15.2(4)E9             C3750E-UNIVERSALK9-M
     2 54    WS-C3750X-48P             15.2(4)E9             C3750E-UNIVERSALK9-M
     3 54    WS-C3750X-48P             15.2(4)E9             C3750E-UNIVERSALK9-M
     4 54    WS-C3750X-48P             15.2(4)E9             C3750E-UNIVERSALK9-M

3750X_STACK#show run interface gigabitEthernet 2/0/5
Building configuration...

Current configuration : 151 bytes
!
interface GigabitEthernet2/0/5
 switchport access vlan 10
 switchport trunk encapsulation dot1q
 switchport mode access
end

3750X_STACK#show run interface gigabitEthernet 4/0/6
Building configuration...

Current configuration : 106 bytes
!
interface GigabitEthernet4/0/6
 switchport access vlan 50
 switchport mode access
end
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2
  • 3
    Is it possible that someone connected two switch connections on a wall plate? Did someone connect two interfaces of a PC that are bridged? Somehow, you have those switch interfaces connected as CDP reports.
    – Ron Maupin ♦
    Commented Dec 6, 2019 at 22:36
  • 3
    What I do not see in your configuration are portfast and bpduguard. You should enable those. You can do it globally (recommended) or per interface. You are not getting a broadcast storm because those are different VLANs, but enabling those will shut down switch interfaces that see BPDUs from another switch, and that can really save you in the event of a spanning tree loop. Also, you now have created a larger broadcast domain (at least with VLAN 10 and 50) by combining the VLANs.
    – Ron Maupin ♦
    Commented Dec 6, 2019 at 22:42

2 Answers 2

Reset to default
6

This happens when something downstream of the switch doesn't understand CDP and mishandles the traffic. So, something is connecting 2/0/5 to 4/0/6; chase the wiring and you'll find it. (someone has a dumb switch, or dumb VM leaking traffic)

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2
  • 1
    I've seen some really odd results from CDP in mixed vendor environments.
    – Ricky
    Commented Dec 6, 2019 at 22:59
  • 2
    Confirming this answer. CDP frames can be forwarded/flooded by switches which do not speak the CDP protocol. Worth understanding what is physically out these ports. If you use per vlan spanning tree it may be worth verifying that the vlans are not “merged” into one spanning tree instance (look for the root bridge on each vlan).
    – Darrell Root
    Commented Dec 7, 2019 at 1:46
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This can happen for example when two switches are connected with access ports, and those access ports are in different VLANs. In some cases this is on purpose, in some cases it's a mistake and that's why the warning. Although the ethernet frames themselves don't have a VLAN tag, the native VLAN of the port is included in the CDP frame, so when a switch receives a CDP neighbor announcement from another switch with VLAN X in the CDP announcement, and the port where that announcement was received has a different native VLAN from X, then this warning is logged.

If this VLAN mismatch is on purpose it's possible to silence these warnings for that port with:

no cdp log mismatch native
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