VLANs and trunking

Trunking modes

Each port is is one of 4 trunking modes. Ports in trunk or dynamic desirable modes will send out DTP (Dynamic Trunking Protocol) frames to negociate a trunk.

Port mode Description
access forced to operate as an access port
trunk forced to operate as a trunk.
dynamic auto will accept trunk negociation, but default as an access port
dynamic desirable will initiate the negociation of a trunk

VLAN summary

SW2#show vlan brief

VLAN Name                             Status    Ports
---- -------------------------------- --------- -------------------------------
1    default                          active    Fa1/0/33, Fa1/0/34, Fa1/0/35
                                                Fa1/0/36, Fa1/0/37, Fa1/0/38
                                                Fa1/0/39, Fa1/0/40, Fa1/0/41
                                                Fa1/0/42, Fa1/0/43, Fa1/0/44
                                                Fa1/0/45, Fa1/0/46, Fa1/0/48
                                                Gi1/0/1, Gi1/0/2, Gi1/0/3
100  ADMIN                            active    Fa1/0/1, Fa1/0/2, Fa1/0/3
                                                Fa1/0/4, Fa1/0/5, Fa1/0/6
                                                Fa1/0/7, Fa1/0/8, Fa1/0/9
                                                Fa1/0/10, Fa1/0/11, Fa1/0/12
                                                Fa1/0/13, Fa1/0/14, Fa1/0/15
200  PRIV                             active    Fa1/0/17, Fa1/0/18, Fa1/0/19
                                                Fa1/0/20, Fa1/0/21, Fa1/0/22
                                                Fa1/0/23, Fa1/0/24, Fa1/0/25
                                                Fa1/0/26, Fa1/0/27, Fa1/0/28
                                                Fa1/0/29, Fa1/0/30, Fa1/0/31
300  SERV                             active
1002 fddi-default                     act/unsup
1003 token-ring-default               act/unsup
1004 fddinet-default                  act/unsup
1005 trnet-default                    act/unsup


Fa1/0/47 is missing in the example above. That’s because it is currently configured as a trunk.

Set VLAN name

SW2(config)#vlan 100
SW2(config-vlan)#name ADMIN

Delete a VLAN

SW2(config)#no vlan 100

Note that the VLANs will not be deleted if you erase the configuration of the switch. They are store in a separate file, vlan.dat, within the flash memory.

SW2#show flash

Directory of flash:/

    2  -rwx        2408   Mar 1 1993 02:58:12 +00:00  config.text
    3  -rwx           5   Mar 1 1993 02:58:12 +00:00  private-config.text
    4  -rwx         736   Mar 1 1993 01:07:47 +00:00  vlan.dat
    5  drwx         192   Mar 1 1993 00:05:52 +00:00  c3750-ipbase-mz.122-25.SEB2

15998976 bytes total (8887296 bytes free)

If you want to fully erase the switch, including VLANs, you need to issue two commands.

SW2#write erase
SW2#delete flash:vlan.dat

Set the VLAN for a (range of) port

SW2(config)#int fa 1/0/4
SW2(config-if)#switchport access vlan 100

Or you can select a port range instead

SW2(config)#int range fa 1/0/1 - 16
SW2(config-if-range)#switchport access vlan 100

Show trunking summary

SW2>show int trunk

Port        Mode         Encapsulation  Status        Native vlan
Fa1/0/47    desirable    802.1q         trunking      400

Port      Vlans allowed on trunk
Fa1/0/47    1-4094

Port        Vlans allowed and active in management domain
Fa1/0/47    1,100,200,300

Port        Vlans in spanning tree forwarding state and not pruned
Fa1/0/47    100,200,300

Show trunking status for a port

SW2>show int fa 1/0/47 switchport
Name: Fa1/0/47
Switchport: Enabled
Administrative Mode: dynamic desirable
Operational Mode: trunk
Administrative Trunking Encapsulation: dot1q
Operational Trunking Encapsulation: dot1q
Negotiation of Trunking: On
Access Mode VLAN: 1 (default)
Trunking Native Mode VLAN: 400 (Inactive)
Administrative Native VLAN tagging: enabled
Voice VLAN: none
Administrative private-vlan host-association: none
Administrative private-vlan mapping: none
Administrative private-vlan trunk native VLAN: none
Administrative private-vlan trunk Native VLAN tagging: enabled
Administrative private-vlan trunk encapsulation: dot1q
Administrative private-vlan trunk normal VLANs: none
Administrative private-vlan trunk private VLANs: none
Operational private-vlan: none
Trunking VLANs Enabled: ALL
Pruning VLANs Enabled: 2-1001
Capture Mode Disabled
Capture VLANs Allowed: ALL

Protected: false
Unknown unicast blocked: disabled
Unknown multicast blocked: disabled
Appliance trust: none

Change the encapsulation for a trunk

SW2(config-if)#switchport trunk encapsulation dot1q

Change the native vlan for a trunk

SW2(config-if)#switchport trunk native vlan 100

Change the trunking mode for a port

SW2(config-if)#switchport mode dynamic desirable