why sh int trunk shows int as native vlan 1

[mahesh18]

hi all,

i have question regarding this  output

3550SMI#sh int trunk

Port        Mode             Encapsulation  Status        Native vlan
Fa0/8       desirable        n-802.1q       trunking      1
Fa0/13      auto             n-802.1q       trunking      1
Fa0/24      on               802.1q         trunking      1

Port        Vlans allowed on trunk
Fa0/8       1-4094
Fa0/13      1-4094
Fa0/24      1-4094

Port        Vlans allowed and active in management domain
Fa0/8       1,10,20,30
Fa0/13      1,10,20,30
Fa0/24      1,10,20,30

Port        Vlans in spanning tree forwarding state and not pruned
Fa0/8       1,10,20,30
Fa0/13      1,10,20,30
Fa0/24      1

Here all 3 trunk  ports show  Native vlan  1 even though i did not config them for vlan 1.

Here is vlan info

3550SMI#sh vlan

VLAN Name                             Status    Ports
---- -------------------------------- --------- -------------------------------
1    default                          active    Fa0/1
10   VLAN0010                         active    Fa0/2, Fa0/3, Fa0/4, Fa0/5
                                                Fa0/6, Fa0/7, Fa0/9, Fa0/10
20   VLAN0020                         active    Fa0/12, Fa0/14, Fa0/15, Fa0/16
                                                Fa0/17, Fa0/18, Fa0/19
30   VLAN0030                         active    Fa0/20, Fa0/21, Fa0/22, Fa0/23
                                                Gi0/1, Gi0/2
1002 fddi-default                     act/unsup
1003 trcrf-default                    act/unsup
1004 fddinet-default                  act/unsup
1005 trbrf-default                    act/unsup

VLAN Type  SAID       MTU   Parent RingNo BridgeNo Stp  BrdgMode Trans1 Trans2
---- ----- ---------- ----- ------ ------ -------- ---- -------- ------ ------
1    enet  100001     1500  -      -      -        -    -        0      0
10   enet  100010     1500  -      -      -        -    -        0      0
20   enet  100020     1500  -      -      -        -    -        0      0
30   enet  100030     1500  -      -      -        -    -        0      0
1002 fddi  101002     1500  -      -      -        -    -        0      0
1003 trcrf 101003     4472  1005   3276   -        -    srb      0      0

VLAN Type  SAID       MTU   Parent RingNo BridgeNo Stp  BrdgMode Trans1 Trans2
---- ----- ---------- ----- ------ ------ -------- ---- -------- ------ ------
1004 fdnet 101004     1500  -      -      -        ieee -        0      0
1005 trbrf 101005     4472  -      -      15       ibm  -        0      0

VLAN AREHops STEHops Backup CRF
---- ------- ------- ----------
1003 0       0       off

Remote SPAN VLANs
------------------------------------------------------------------------------

Primary Secondary Type              Ports
------- --------- ----------------- --------

we see that vlan 1   is down.

thnaks

mahesh

[George Stefanick · Accepted solution]

Mahesh,

Great question. The reason why you are seeing VLAN 1 as native is only because this is the Cisco's default, out of the box. You can change the native vlan with the

config t

c3550(config-if)#switchport trunk native vlan ?

  <1-4094>  VLAN ID of the native VLAN when this port is in trunking mode

Remember only 1 native vlan may exist on a switch. And if you change the native statement you need to makre sure they match on either end of the link.

Does this help?

[mahesh18]

Hi George thanks for reply

can you please explain me

<1-4094>  VLAN ID of the native VLAN when this port is in trunking mode 

Thanks

mahesh

[Jon Marshall]

Mahesh

Just to add to George's post.

Vlan 1 can be down on the switch, you can make sure no ports are allocted to vlan 1 and you can make sure all trunk links do not allow vlan 1 but vlan 1 is still used by Cisco switches primarily for 2 things -

1) the native vlan as you have found out and George has explained

2) Cisco management protocols such as VTP/CDP/PagP

Jon

[mahesh18]

Hi Jon thanks

for reply

So cdp messages and vtp info means when we create vlans or server VTP  mode  it replicate to clients with help of Native vlan .

but if no port is assinged to native vlan then also all these messages go by trunk ports?

thanks

mahesh

[Jon Marshall · Accepted solution]

mahesh18 wrote:
Hi Jon thanks
for reply
So cdp messages and vtp info means when we create vlans or server VTP  mode  it replicate to clients with help of Native vlan .
but if no port is assinged to native vlan then also all these messages go by trunk ports?
thanks
mahesh

Mahesh

VTP and CDP + others are sent between switches using vlan 1. Because the default native vlan is vlan 1 then unless you change the native vlan yes VTP/CDP/PagP will go between switches on the native vlan. But if you change the native vlan to something other than vlan 1 VTP/CDP/PagP will still use vlan 1.

DTP is slightly different, it does the use the native vlan so if you change the native vlan to, for example, vlan 999 then DTP will use vlan 999 to exchange packets between switches.

Jon

[mahesh18]

Hi jon and george,

Many thanks for good explanation.

Best regards

Mahesh