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Wednesday, June 1, 2011

Trunking commands on Catalyst 2950 ver 12.1

We have a Cisco 2950 with a trunk link which reads as follows.

interface FastEthernet0/4
switchport mode trunk
switchport nonegotiate
speed 100
duplex full

1) Why does the encapsulation method not show?
2) When we try to place the same configuration on a 3560 we have to specify the encapsulation method before it will let me configure the port as a trunk.. and when we show the running config is shows encapsulation method in the running config unlike the 2950.. why the difference? and is there any functionality difference?
3) Why does the native VLAN not show under the running config on the 2950?

1) The encapsulation method does not show because the 2950 (along with the 2940, 2955 and 2960) support only Dot1Q.

2) Also 3560 and the rest of the 3XX0 series switches will support both Dot1Q and ISL. In both switches, enter the command "sh interface capabilities" and look under "Trunk encap. type:".

3)The native VLAN does not show because vlan 1 is the default vlan. If you change the native vlan 1 to a different number, it will then show up in the config
have a look at below config from a 2950 switch I have changed vlan 1 to native 20

Current configuration : 129 bytes
interface FastEthernet0/25
switchport trunk native vlan 20
switchport mode trunk
no ip address
spanning-tree portfast


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