Course contents

Chapter 4: VLANs and TrunksLesson 4 of 18

ISL vs 802.1Q

Two ways to mark VLANs on a trunk: Cisco ISL encapsulation and the IEEE 802.1Q tag.

Beginner · 6 min read

Technical definition

Trunk encapsulation is the method a trunk uses to mark which VLAN each frame belongs to. IEEE 802.1Q inserts a 4-byte tag into the Ethernet frame. Cisco ISL wrapped the whole frame in a 26-byte header and added a 4-byte trailer.

Two ways to mark a VLAN

On a trunk, frames from many VLANs share one cable, so every frame has to say which VLAN it belongs to. There have been two methods on Cisco gear. Both do the job; they differ in how they add the VLAN information.

802.1Q inserts a tag; ISL wraps the whole frame
802.1Q inserts a tag; ISL wraps the whole frameAn 802.1Q frame is the original Ethernet frame with a 4-byte tag inserted after the source MAC address, and the frame check sequence recalculated. A Cisco ISL frame is the whole original Ethernet frame, FCS included, placed inside a 26-byte ISL header and a 4-byte ISL CRC.IEEE 802.1Q (standard)Dest MAC 6Src MAC 6802.1Q tag 4Type 2PayloadFCS 4 (new)TPID 0x8100 + priority + VLAN IDCisco ISL (legacy)ISL header 26Original Ethernet frame, untouched (including its FCS)ISL CRC 4802.1Q: +4 bytes, standard, native VLAN untagged.ISL: +30 bytes, Cisco only, every VLAN tagged.On a modern switch you configure only: switchport trunk encapsulation dot1q (if asked).
ISL was Cisco proprietary, tagged every VLAN including the native one, and added 30 bytes. IEEE 802.1Q is the standard, adds 4 bytes and leaves the native VLAN untagged. Current Catalyst switches support only 802.1Q.

802.1Q: insert a tag

IEEE 802.1Q adds a 4-byte tag between the source MAC address and the EtherType. It contains a 16-bit tag protocol identifier (0x8100), a 3-bit priority field, a drop-eligible bit and a 12-bit VLAN ID. Because the frame has changed, the switch recalculates the frame check sequence. The maximum frame size goes from 1518 to 1522 bytes. Frames of the native VLAN are sent without a tag.

ISL: wrap the frame

Inter-Switch Link was Cisco's earlier method. It left the original frame alone and put it inside a new one: a 26-byte ISL header in front and a 4-byte CRC after, 30 bytes in all. Every VLAN, including VLAN 1, was encapsulated, so ISL had no native VLAN. It supported normal-range VLANs only, and it worked only between Cisco devices.

Comparison

ISL and 802.1Q compared
ISL and 802.1Q compared; ISL; 802.1Q. Standard: Cisco proprietary IEEE standard Method: Encapsulates the whole frame Inserts a tag in the frame Added bytes: 30 (26 header, 4 trailer) 4 Native VLAN: None: every frame is tagged One VLAN is sent untagged VLAN range: Normal range only 1–4094 Works with other vendors: No Yes Current platforms: Removed Supported everywhereISL802.1QStandardCisco proprietaryIEEE standardMethodEncapsulates the whole frameInserts a tag in the frameAdded bytes30 (26 header, 4 trailer)4Native VLANNone: every frame is taggedOne VLAN is sent untaggedVLAN rangeNormal range only1–4094Works with othervendorsNoYesCurrent platformsRemovedSupported everywhere

On small screens, swipe sideways or focus the diagram and use the arrow keys.

Read each row across. The exam usually asks about standard vs proprietary, tag vs encapsulation, and the native VLAN.

Configuring and verifying the encapsulation

Commands are based on Cisco IOS documentation and have not been run on a lab device here.

interface GigabitEthernet0/1 switchport trunk encapsulation dot1q switchport mode trunk

On platforms that support both methods, choose the encapsulation first, then set trunk mode. On current switches that only support 802.1Q this encapsulation command is not accepted and is not needed.

Example output · based on Cisco documentation; exact format varies by platform and software version
SW1#show interfaces trunk
Port        Mode         Encapsulation  Status        Native vlan
Gi0/1       on           802.1q         trunking      1

Port        Vlans allowed on trunk
Gi0/1       1-4094
Example output. The Encapsulation column shows 802.1q (or isl on older gear). A mode of on means a static trunk; with DTP it would show desirable or auto.

Where mismatches show up

  • One side ISL, the other 802.1Q: no working trunk, with no useful error beyond missing connectivity.
  • One side trunk, the other access: tagged frames arrive at a port that expects untagged ones.
  • Native VLAN differs: 802.1Q frames land in the wrong VLAN; see the native VLAN lesson.

ISL vs 802.1Q in short

  • 802.1Q inserts a 4-byte tag and leaves the native VLAN untagged; it is the standard.
  • ISL encapsulated the whole frame (30 bytes) and tagged every VLAN; it is Cisco-only and obsolete.
  • Verify with show interfaces trunk.

Test yourself

Predict · scenario 1

How many bytes does 802.1Q add to a frame?

Predict · scenario 2

Which method has no native VLAN?

Predict · scenario 3

Which trunking method works with non-Cisco switches?

Predict · scenario 4

What does the 12-bit VLAN ID field allow?

Predict · scenario 5

Which command shows the encapsulation of operational trunks?

FAQ

Is ISL still used?
Almost never. It is Cisco proprietary and has been dropped from current Catalyst platforms. You may still see the command on older switches, or ISL in exam questions that compare the two.
Why does 802.1Q need a native VLAN but ISL did not?
ISL tagged every frame, including VLAN 1, so there was nothing untagged. 802.1Q leaves one VLAN (the native VLAN) untagged for compatibility with devices that do not understand tags.
What happens if the two ends use different encapsulations?
The trunk does not come up. Each side sees the other's frames as malformed. Static configuration on both ends, or DTP, must agree.