Innovative Technologies

Statically set the trunking encapsulation of SW1’s inter-switch links to ISL.

SW1(config)#int ra fa 0/13 - 21
SW1(config-if-range)#switchport trunk encapsulation isl 

Verify that SW2, SW3, & SW4 are negotiating ISL as the trunking encapsulation to SW1, and that SW1 is not negotiating ISL to SW2, SW3,and SW4.

SW1#sh int trunk 
Port        Mode         Encapsulation  Status        Native vlan
Fa0/13      desirable    isl            trunking      1
Fa0/14      desirable    isl            trunking      1
Fa0/15      desirable    isl            trunking      1
Fa0/16      desirable    isl            trunking      1
Fa0/17      desirable    isl            trunking      1
Fa0/18      desirable    isl            trunking      1
Fa0/19      desirable    isl            trunking      1
Fa0/20      desirable    isl            trunking      1
Fa0/21      desirable    isl            trunking      1

SW2#sh int trunk
Port        Mode         Encapsulation  Status        Native vlan
Fa0/6       on           802.1q         trunking      1
Fa0/13      auto         n-isl          trunking      1
Fa0/14      auto         n-isl          trunking      1
Fa0/15      auto         n-isl          trunking      1

SW3#sh int trunk
Port        Mode         Encapsulation  Status        Native vlan
Fa0/13      auto         n-isl          trunking      1
Fa0/14      auto         n-isl          trunking      1
Fa0/15      auto         n-isl          trunking      1

SW4#sh int trunk
Port        Mode         Encapsulation  Status        Native vlan
Fa0/13      auto         n-isl          trunking      1
Fa0/14      auto         n-isl          trunking      1
Fa0/15      auto         n-isl          trunking      1