• A Trunk carries nultiple VLANs aby adding a special encapsulation; ISL/802.1Q
  • Both ISL and 802.1Q accomplish the same goal of encoding the VLAN number in frame header to seperate traffic

Main Differences:

  •  ISL
       * Cisco Propietary
       * 30 byte encapsulation on all frames ( 26 byte header + 4 byte trailer)
       * has support for Ethernet, Token Ring, and FDDI
       * does not modify original frame
       * no concept of Native VLAN
  • 802.1Q
       * IEEE Standard
       * 4 byte Header
       * modifies original frame
       * more accepted because of interoperability


On most Newer Switches ISL support has been dropped


SW01(config-if)#switchport trunk encapsulation ?
                   dot1q      Interface uses only 802.1q trunking encapsulation when trunking
                    isl        Interface uses only ISL trunking encapsulation when trunking
                    negotiate  Device will negotiate trunking encapsulation with peer on interface


SW01#sh interface trunk / show interface gi2/0/1 trunk

Port        Mode             Encapsulation  Status        Native vlan
Gi0/1     on                   802.1q                trunking   1

Port        Vlans allowed on trunk
Gi0/1       1-4094

Port        Vlans allowed and active in management domain
Gi0/1       1-14,98-114,121,250,800

Port        Vlans in spanning tree forwarding state and not pruned
Gi0/1       1-14,98-114,121,250,800


DTP: Dynamic Trunking Protocol

  • Cisco proprietary
  • Supports ISL and Dot1q
  • 5 Modes:
         1) Trunk
         2) Dynamic Desirable
         3) Access
         4) Dynamic Auto
         5) Nonegotiate

Trunk Port: forces interface to be trunk, sends DTP to otherside even if it cant be trunk
Dynamic Desirable: Port attempts to be a trunk, using DTP if the other side is trunk, dynamic desirable, or dynamic auto
Access: Forces interface not to trunk
Dynamic Auto: becomes Trunk if teh otherside starts trunking with DTP
Nonegotiate: becomes a trunk and does not send DTP, used when connecting to non-cisco device


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