1

How can I set the switchport's trunk protocol?

I saw there can use the command to set the trunk mode:

SW(config-if)# switch trunk encapsulation ?
    dot1q
    isl
    negotiate

but I tried bellow command did not found the command.

Switch(config-if)#switchport mode trunk 
Switch(config-if)#switchport trunk en?
% Unrecognized command
Switch(config-if)#switchport ?
  access         Set access mode characteristics of the interface
  mode           Set trunking mode of the interface
  nonegotiate    Device will not engage in negotiation protocol on this
                 interface
  port-security  Security related command
  priority       Set appliance 802.1p priority
  protected      Configure an interface to be a protected port
  trunk          Set trunking characteristics of the interface
  voice          Voice appliance attributes
Switch(config-if)#switchport trunk ?
  allowed  Set allowed VLAN characteristics when interface is in trunking mode
  native   Set trunking native characteristics when interface is in trunking
           mode

I use the Switch 2950T-24 in Cisco Packet Tracer.

  • You can't set the trunking protocol on a layer 2 switch. It'll always use Dot1q (IEEE 802.1Q encapsulation) when setting the mode to trunk. – user36472 Nov 5 '18 at 8:37
  • @Cown: try that on a classic Cat3750G. It won't even let you set the interface to mode trunk before the encapsulation mode isn't fixed. – Marc 'netztier' Luethi Nov 5 '18 at 8:44
  • @Marc'netztier'Luethi not a layer 2 switch. It has layer 3 capabilities. – user36472 Nov 5 '18 at 8:53
  • @Cown Care to elaborate on the relationship of ISL-vs-802.1 trunking support and L2/L3 capabilities? The cat2950s certainly didn't have L3 capability, but supported both ISL and 802.1q. – Marc 'netztier' Luethi Nov 5 '18 at 8:57
  • @Marc'netztier'Luethi the Cisco 2950 never had ISL capabilities. You can check it yourself: cisco.com/c/en/us/support/docs/lan-switching/8021q/… – user36472 Nov 5 '18 at 9:05
4

Only Cisco switch platforms that still support isl trunking need to have the trunking protocol configured.

Newer platforms have no support for isl trunking anymore, so the interface command switchport trunk encapsulation dot1q is default/implicit/unsupported, and there's no need to configure it. Just set switchport mode trunk.

Of the ones that still support isl, Cat3750, 3550, 3560 spring to mind, and the Cat4000 and Cat4500 Series (incl their 4900M and 4948 cousins), and probably a few of the older ones, too.

EDIT: the following section is incorrect as it seems that the 29xx range never had support for isl trunking at all.


I couldn't say when ISL trunking support was dropped from the 2950/2960G/2960-S/2960-X product line.


Eventually, it's just a matter of trying. If the switchport won't accept switchport mode trunk, then setting the trunking encapsulation is probably needed.

| improve this answer | |
  • Again, ISL trunking was never supported on any 2950/2960G/2960-S/2960-X product. – user36472 Nov 5 '18 at 9:12

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