CCIE Pursuit Blog

August 8, 2007

Native VLAN Can Be Configured For Nonexistent VLAN

Filed under: Cisco,Cisco Certification,IOS,Switching,Tech Tips — cciepursuit @ 11:07 am

Interesting….you  can set the native vlan without having created the vlan first and IOS will not automatically create it for you:

sw1(config)#do sh run int fa0/13
Building configuration…

Current configuration : 128 bytes
!
interface FastEthernet0/13
 switchport trunk encapsulation dot1q
 switchport trunk native vlan 10
 switchport mode trunk
end

sw1(config)#do sh vlan br [Note: VLAN 10 does not exist on this switch]

VLAN Name                             Status    Ports
—- ——————————– ——— ——————————-
1    default                          active    Fa0/1, Fa0/2, Fa0/3, Fa0/4
                                                Fa0/5, Fa0/6, Fa0/7, Fa0/8
                                                Fa0/9, Fa0/10, Fa0/11, Fa0/12
                                                Fa0/14, Fa0/15, Fa0/22, Fa0/23
                                                Fa0/24, Gi0/1, Gi0/2
1002 fddi-default                     act/unsup
1003 token-ring-default               act/unsup
1004 fddinet-default                  act/unsup
1005 trnet-default                    act/unsup

A good way to verify whether or not the native VLAN exists on the switch is to view the switchport details for the interface:

When the vlan is not configured:
sw1#sh int fa0/13 switch
Name: Fa0/13
Switchport: Enabled
Administrative Mode: trunk
Operational Mode: trunk
Administrative Trunking Encapsulation: dot1q
Operational Trunking Encapsulation: dot1q
Negotiation of Trunking: On
Access Mode VLAN: 1 (default)
Trunking Native Mode VLAN: 10 (Inactive)
—–output truncated—–
sw1#sh span vlan 10

Spanning tree instance(s) for vlan 10 does not exist.
After configuring the vlan:
sw1#sh int fa0/13 switch
Name: Fa0/13
Switchport: Enabled
Administrative Mode: trunk
Operational Mode: trunk
Administrative Trunking Encapsulation: dot1q
Operational Trunking Encapsulation: dot1q
Negotiation of Trunking: On
Access Mode VLAN: 1 (default)
Trunking Native Mode VLAN: 10 (VLAN0010)
—-output truncated—-
sw1#sh span vlan 10

VLAN0010
  Spanning tree enabled protocol ieee
  Root ID    Priority    32778
             Address     0018.ba55.5b00
             This bridge is the root
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32778  (priority 32768 sys-id-ext 10)
             Address     0018.ba55.5b00
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time 300
—-output truncated—-

*Note: in the case above adding the vlan to one side of the link does NOT bring the other side out of inactive (VTP notwithstanding):

 sw2#sh int fa0/13 switch | i Native
 Trunking Native Mode VLAN: 10 (Inactive)

Unless you are running vtp, you will need to configure the vlan on both sides of the link.

This is another good point to clarify with the lab proctor.  The task may not specify that the native VLAN be created and it may not have already been created in another task.  You will want to ask the proctor whether or not the VLAN needs to be created to get credit for the task.  You may also want to read the rest of the exam closely to see if there are other tasks that depend on the existence of that VLAN.

Advertisements

Leave a Comment »

No comments yet.

RSS feed for comments on this post. TrackBack URI

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

Blog at WordPress.com.

%d bloggers like this: