ISL VS DOT1Q – Difference between ISL and DOT1Q

ISL vs DOT1QISL and DOT1Q –

ISL and DOT1Q are 2 encapsulation standards supporting Virtual LANs (VLANs) on an Ethernet network. These are the 2 frame tagging protocols that Cisco supports.

The tagging protocol can be manually specified on a trunk port, or dynamically negotiated using Cisco’s proprietary Dynamic Trunking Protocol (DTP).

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A common query in LAN switching which we generally hear is the difference between ISL encapsulation and DOT1Q encapsulation.

Below table will showcase how both the terms differ from each other & details of ISL and Dot1Q Encapsulation

PARAMETERISLDOT1Q
Abbreviation for
ISL (Inter Switch Link)
-
Standard
Cisco proprietary protocol
IEEE Standard
Vlans Supported
Supports up to 1000 Vlans
Supports 4096 Vlans
Encapsulation
Original frame is encapsulated and a new header is inserted during encapsulation process. A 26 byte header and a 4 byte FCS (frame check sequence) are inserted which makes it total of 30 Bytes of overhead.
802.1q encapsulation inserts a 4 byte tag into original frame and FCS (Frame Check Sequence) is re-calculated.
PVST
Supported
Not Supported
Supported on Cisco NX-OS
Not SupportedSupported
Frame Format
ISL Header
(Image in downloadable table)
802.1Q Tag
(Image in downloadable table)
CPU Intensive
ISL uses more processor cycles than 802.1q because of the modification of the header and recalculation of FCS.
Less intensive than ISL
Native frame
ISL does not tag frames from native Vlans
802.1q does not tag frames from native Vlans
Configuration
Switch(config)#interface Fa1/1
Switch(config-if)#switchport trunk encapsulation isl
Switch(config-if)#switchport mode trunk
Switch(config)#interface Fa 1/1
Switch(config-if)#switchport trunk encapsulation 802.1q
Switch(config-if)#switchport mode trunk
Fields used
* DA (Destination Address)
* Type
* User
* SA (Source Address)
* Len
* SNAP
* HSA (High Bits of Source Address)
* VLAN (Destination VLAN ID)
* BPDU
* Index
* Res
* ISL CRC:
* TPID
* User Priority
* CFI
* VLAN ID
Supported models
ISL is supported in
Cisco 1900 series switches only
All cisco and non-cisco models

Download the difference table here.

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