07-29-2008 05:21 AM - edited 03-06-2019 12:30 AM
Hi,
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.
Regards,
Sham.
07-29-2008 05:24 AM
Hi,
Have a look at this url;
http://www.cisco.com/en/US/tech/tk389/tk689/technologies_tech_note09186a0080094665.shtml
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.
HTH
LR
07-29-2008 05:33 AM
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
07-29-2008 08:14 AM
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
Giuseppe
07-08-2016 11:08 PM
Hi Sham,
VLAN identifier is a special tag that is encapsulated in a Ethernet frame. There are two main types of encapsulation protocols called ISL (Inter Switch Link) which is Cisco proprietary protocol and 802.1q which is an IEEE Standard.
ISL
802.1q
Note :
Cisco 1900 seriea switches uses ISL and Cisco 2900 series and others use 802.1 q by default.
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