Difference between ISL AND dot1q

Unanswered Question
Jul 29th, 2008


I know ISL is a cisco proprietary one and dot1Q can use for multiple vendors. What are the other differences between these two? This is for my study purpose kindly give answer.



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lee.reade Tue, 07/29/2008 - 05:24


Have a look at this url;


You pretty much know the main differences, others are dot.1q has less overhead and can support 4096 vlans, whereas ISL can only support 1000.

Another difference is that ISL does not use the native vlan concept, whereas 802.1q trunks do and will not trunk correctly if the native vlan is different on each side.

Even Cisco recommend these days to use 802.1q to it being the most widely deployed.



Danilo Dy Tue, 07/29/2008 - 05:33

From what I know there are 3 major differences.

(1) Network Overhead. dot1q has less network overhead than isl. dot1q encapsulates the header of the frame while isl encapsulates the entire frame.

(2) Native VLAN. In dot1q, frames sent across the trunk doesn't have a header. The remote switch will assume that any frame without a header is destined for the native vlan. In isl, every single frame will be encapsulated, regardless of the vlan it's destined for.

(3) isl is Cisco Proproetary while dot1q is Industry Standard

Giuseppe Larosa Tue, 07/29/2008 - 08:14

Hello Sham,

an ISL frame is bigger and isn't an ethernet frame but carries one ethernet frame inside

30 bytes ISL overhead against 802.1Q 4 bytes

only 1024 vlans against 4096 in the original specification of ISL (modified later)

an 802.1Q tagged frame is still an ethernet frame with ethertype 802.1Q.

802.1Q allows for stacking : two 802.1Q headers and then the L3 pdu /user data

this is known as 802.1Q tunneling or Q in Q and is used to provide services in a L2 MAN.

802.1Q may require to enable baby giants of to modify the ethernet MTU to pass traffic with 1500 Byte IP payload

Some entry level switches support only 802.1Q trunks like C2950 some old entry level switches support only ISL

Hope to help



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