Native vlan doesn't need to be on switch?

Okay, I'm curious because I've
never done this in production. I was doing some lab work and changed the
native vlan on a trunk. Then I started getting cdp errors on the other
side, so on the other side I changed the default to match up with the
other end. CDP stopped complaining and spanning-tree started forwarding
again. The vlan that I had specified existed on SW1, but it doesn't
exist on SW3. I was under the impression that the vlan, native or not,
had to be on the switch in order to forward anything:

 

R3 before native changed:

 

*Mar 
1 00:23:36.647: %CDP-4-NATIVE_VLAN_MISMATCH: Native VLAN mismatch
discovered on FastEthernet0/13 (1), with Rack1SW1 FastEthernet0/16
(146).

*Mar  1 00:23:36.675: %CDP-4-NATIVE_VLAN_MISMATCH: Native
VLAN mismatch discovered on FastEthernet0/14 (1), with Rack1SW1
FastEthernet0/17 (146).

*Mar  1 00:23:36.695:
%CDP-4-NATIVE_VLAN_MISMATCH: Native VLAN mismatch discovered on
FastEthernet0/15 (1), with Rack1SW1 FastEthernet0/18 (146).

 

R3 after native changed:

 

*Mar  1 00:24:43.879: %SPANTREE-2-UNBLOCK_CONSIST_PORT: Unblocking FastEthernet0/13 on VLAN0001. Port consistency restored.

*Mar  1 00:24:43.879: %SPANTREE-2-UNBLOCK_CONSIST_PORT: Unblocking FastEthernet0/14 on VLAN0001. Port consistency restored.

*Mar  1 00:24:43.883: %SPANTREE-2-UNBLOCK_CONSIST_PORT: Unblocking FastEthernet0/15 on VLAN0001. Port consistency restored.

 

1    default                          active    Fa0/1, Fa0/2, Fa0/3, Fa0/4

                                                Fa0/6, Fa0/7, Fa0/8, Fa0/9

                                                Fa0/10, Fa0/11, Fa0/12, Fa0/16

                                                Fa0/17, Fa0/18, Fa0/19, Fa0/20

                                                Fa0/21, Fa0/22, Fa0/23, Gi0/1

                                                Gi0/2

5    VLAN0005                         active    Fa0/5

9    VLAN0009                         active

43   VLAN0043                         active    Fa0/24

79   VLAN0079                         active

 

Here's the port config:

 

 

Rack1SW3#sh run int fa0/13

Building configuration...

 

Current configuration : 129 bytes

!

interface FastEthernet0/13

switchport trunk encapsulation dot1q

switchport trunk native vlan 146

switchport mode trunk

 

How is this allowing 146 when it doesn't even exist on the switch?

 

Thanks!

John

Comments

  • The issue here is that by default Cisco will send out STP BPDU's that have the VLAN id encompassed in it, this therefore tells the other switch what VLAN is set on the remote end and it will moan at you if this is the case.

    If you were to use something like MST then this wouldn't happen as you don't then run one STP instance per VLAN.

    David

    --
    David Rothera
    [email protected]
    W: networkbroadcast.co.uk


    On Friday, 31 August 2012 at 11:37, john.blakley wrote:

    > Okay, I'm curious because I've never done this in production. I was doing some lab work and changed the native vlan on a trunk. Then I started getting cdp errors on the other side, so on the other side I changed the default to match up with the other end. CDP stopped complaining and spanning-tree started forwarding again. The vlan that I had specified existed on SW1, but it doesn't exist on SW3. I was under the impression that the vlan, native or not, had to be on the switch in order to forward anything:
    >
    >
    > R3 before native changed:
    >
    >
    > *Mar 1 00:23:36.647: %CDP-4-NATIVE_VLAN_MISMATCH: Native VLAN mismatch discovered on FastEthernet0/13 (1), with Rack1SW1 FastEthernet0/16 (146).
    > *Mar 1 00:23:36.675: %CDP-4-NATIVE_VLAN_MISMATCH: Native VLAN mismatch discovered on FastEthernet0/14 (1), with Rack1SW1 FastEthernet0/17 (146).
    > *Mar 1 00:23:36.695: %CDP-4-NATIVE_VLAN_MISMATCH: Native VLAN mismatch discovered on FastEthernet0/15 (1), with Rack1SW1 FastEthernet0/18 (146).
    >
    >
    > R3 after native changed:
    >
    >
    > *Mar 1 00:24:43.879: %SPANTREE-2-UNBLOCK_CONSIST_PORT: Unblocking FastEthernet0/13 on VLAN0001. Port consistency restored.
    > *Mar 1 00:24:43.879: %SPANTREE-2-UNBLOCK_CONSIST_PORT: Unblocking FastEthernet0/14 on VLAN0001. Port consistency restored.
    > *Mar 1 00:24:43.883: %SPANTREE-2-UNBLOCK_CONSIST_PORT: Unblocking FastEthernet0/15 on VLAN0001. Port consistency restored.
    >
    >
    > 1 default active Fa0/1, Fa0/2, Fa0/3, Fa0/4
    > Fa0/6, Fa0/7, Fa0/8, Fa0/9
    > Fa0/10, Fa0/11, Fa0/12, Fa0/16
    > Fa0/17, Fa0/18, Fa0/19, Fa0/20
    > Fa0/21, Fa0/22, Fa0/23, Gi0/1
    > Gi0/2
    > 5 VLAN0005 active Fa0/5
    > 9 VLAN0009 active
    > 43 VLAN0043 active Fa0/24
    > 79 VLAN0079 active
    >
    >
    > Here's the port config:
    >
    >
    >
    >
    > Rack1SW3#sh run int fa0/13
    > Building configuration...
    >
    >
    > Current configuration : 129 bytes
    > !
    > interface FastEthernet0/13
    > switchport trunk encapsulation dot1q
    > switchport trunk native vlan 146
    > switchport mode trunk
    >
    >
    > How is this allowing 146 when it doesn't even exist on the switch?
    >
    >
    > Thanks!
    > John
    >
    >
    >
    >
    > INE - The Industry Leader in CCIE Preparation
    > http://www.INE.com
    >
    > Subscription information may be found at:
    > http://www.ieoc.com/forums/ForumSubscriptions.aspx
  • I think I may have just run into a bug. I tried to configure this again this morning in GNS and I get an error that the vlan doesn't exist.

     

    R1(config-if)#
    *Mar  1 00:11:04.747: %DTP-5-TRUNKPORTON: Port Fa1/5 has become dot1q trunk
    *Mar  1 00:11:05.251: %LINEPROTO-5-UPDOWN: Line protocol on Interface Vlan1, changed state to up
    *Mar  1 00:11:05.359: %SPANTREE-2-RECV_PVID_ERR: Received BPDU with inconsistent peer vlan id 42 on FastEthernet1/5 VLAN1.
    *Mar  1 00:11:05.359: %SPANTREE-2-BLOCK_PVID_LOCAL: Blocking FastEthernet1/5 on VLAN1. Inconsistent local vlan.
    *Mar  1 00:11:07.159: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet1/5, changed state to up

     

    1    default                          active    Fa1/0, Fa1/1, Fa1/2, Fa1/3
                                                    Fa1/4, Fa1/6, Fa1/7, Fa1/8
                                                    Fa1/9, Fa1/10, Fa1/11, Fa1/12
                                                    Fa1/13, Fa1/14, Fa1/15
    15   VLAN0015                         active   
    20   VLAN0020                         active   
    25   VLAN0025                         active   
    30   VLAN0030                         active   

     

    R1(config-if)#switchport trunk native vlan 42
    % VLAN 42 does not exist. Please add it to vlan database
    R1(config-if)#

     

    This makes more sense...I just wish I knew what was going on when I was doing the lab...

     

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