Core issue
The Incorrect trunk type specified for port error message is received when the set trunk negotiate command is issued to configure the type of trunk encapsulation.
When the set trunk negotiate command is issued and the module of the switch supports only one type of trunk encapsulation (either ISL or 802.1Q), the error message will be dispalyed. Because the switch cannot form the trunk by negotiating if the trunk encapsulation is configured differently in other end of the switch.
All Cisco Catalyst ports do not support 802.1Q encapsulation. Currently, Cisco Catalyst 4500/4000 switches support only 802.1Q. Ports of the Cisco Catalyst 6500/6000 families are able to use 802.1Q or ISL encapsulation. Depending on the module, Catalyst 5500/5000 trunk-capable ports are able to use 802.1Q encapsulation, ISL encapsulation, or both.
Resolution
To verify the trunk encapsulation type, issue the show port capabilities command. Issue the set trunk negotiate command only if the port supports both the encapsulation type.
The trunk capacity is explicitly stated as in the output:
Switch> (enable) show port capabilities 1/1
Model WS-X5530
Port 1/1
Type 1000BaseSX
Speed 1000
Duplex full
Trunk encap type 802.1Q,ISL
Trunk mode on,off,desirable,auto,nonegotiate
Channel no
Broadcast suppression percentage(0-100)
Flow control receive-(off,on,desired),send-(off,on,desired)
Security no
Membership static
Fast start yes
Rewrite no
