ISL vs Dot1Q

The differences between ISL and Dot1Q are as follows –

PARAMETER	ISL	DOT1Q
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 Supported	Supported
Frame Format
CPU Intensive	ISL uses more processor cycles than 802.1q because of the modification of the header and recalculation of FCS.	Les 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

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