switchport mode trunk
switchport nonegotiate

На некоторых модулях поддержка ISL убрана сто лет назад, проверить можно командой:

sh interfaces gigabitEthernet x/y capabilities:

  Trunk encap. type:     802.1Q
  Trunk mode:            on,off,desirable,nonegotiate

  Trunk encap. type:     802.1Q,ISL
  Trunk mode:            on,off,desirable,nonegotiate

также рекомендовал бы почитать про архитектуру модулей - на многих порты ДАЖЕ НА ОДНОМ МОДУЛЕ не равнозначны как по функционалу, так и по переподписке:

2    18  1000BaseX (GBIC)                       WS-X4418-GB        JAE0931H5J8

sh int gigabitEthernet 2/1 capabilities
  Model:                 WS-X4418-GB-Gbic
  Type:                  No Gbic
  Speed:                 1000
  Duplex:                full
  Auto-MDIX:             no
  Trunk encap. type:     802.1Q,ISL
  Trunk mode:            on,off,desirable,nonegotiate
  Channel:               yes
  Broadcast suppression: percentage(0-100), hw
  Flowcontrol:           rx-(off,on,desired),tx-(off,on,desired)
  VLAN Membership:       static, dynamic
  Fast Start:            yes
  CoS rewrite:           yes
  ToS rewrite:           yes
  Inline power:          no
  SPAN:                  source/destination
  UDLD:                  yes
  Link Debounce:         no
  Link Debounce Time:    no
  Port Security:         yes
  Dot1x:                 yes
  Maximum MTU:           9198 bytes (Jumbo Frames)
  Multiple Media Types:  no
  Diagnostic Monitoring: N/A
  Queuing:               rx-(N/A), tx-(1p3q1t, Sharing/Shaping)

sh int gigabitEthernet 2/3 capabilities
   Model:                 WS-X4418-GB-Gbic
  Type:                  No Gbic
  Speed:                 1000
  Duplex:                full
  Auto-MDIX:             no
  Trunk encap. type:     802.1Q
  Trunk mode:            on,off,desirable,nonegotiate
  Channel:               yes
  Broadcast suppression: percentage(0-100), hw
  Flowcontrol:           rx-(off,on,desired),tx-(off,on,desired)
  VLAN Membership:       static, dynamic
  Fast Start:            yes
  CoS rewrite:           yes
  ToS rewrite:           yes
  Inline power:          no
  SPAN:                  source/destination
  UDLD:                  yes
  Link Debounce:         no
  Link Debounce Time:    no
  Port Security:         yes
  Dot1x:                 yes
  Maximum MTU:           1500 bytes
  Multiple Media Types:  no
  Diagnostic Monitoring: N/A
  Queuing:               rx-(N/A), tx-(1p3q1t, Sharing/Shaping)

С наступающим!