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
Gi1/0/4
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
Fa1/0/16
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
Fa1/0/32
300 SERV active
1002 fddi-default act/unsup
1003 token-ring-default act/unsup
1004 fddinet-default act/unsup
1005 trnet-default act/unsup
Note
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