ISL and 802.1q trunking
Adam B
Member Posts: 108 ■■□□□□□□□□
in CCNA & CCENT
Hey guys, was just reading the Todd Lammle Book on the CCENT, and I read about 802.1q and ISL, but they seem very much alike besides the fact that ISL is only associated with cisco and 802.1q is associated with multiple vendors. What are the primary differences between these two as far as the cisco CCENT objectives go? Thanks 
2015 Goals: CCNP SWITCH [] SEC+ [ ] CCNP ROUTE [ ] CCNP TSHOOT [ ]
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jahaziel
Member Posts: 175 ■■■□□□□□□□
As you mentioned ISL is only cisco. It adds a 24 byte header and 4 byte trailer I believe and doesn't modify the frame. 802.1q does modify the frame and added a 4 byte frame I believe in the middle of the Ethernet frame.
Double check on that information since i'm going off the top of my memory right now from my ccent studies. -
Adam B
Member Posts: 108 ■■□□□□□□□□
Yup, seems about right, thanks man I appreciate it. Also, I'm assuming that ISL then must have more overhead since its basically encapsulating the whole frame, while the 802.1q is basically only encapsulating the header2015 Goals: CCNP SWITCH [] SEC+ [ ] CCNP ROUTE [ ] CCNP TSHOOT [ ]
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DoubleNNs
Member Posts: 2,015 ■■■■■□□□□□
The main points you want to know to differentiate them is:
ISL is Cisco proprietary whereas 802.1q is IEEE standardized.
802.1q inserts a 4-byte field directly into the Ethernet frame header in order to ID the VLAN the frame is for. This is important because if the frame is not tagged w/ the 4-byte field, the frame is designated for the *Native VLAN*, which is useful when dealing w/ devices that don't support trunking. ISL does not support Native VLANs.
ISL isn't widely used and a lot of Cisco devices don't bother to support it.
-Pretty much what jahaziel mentioned, but the Native VLAN concept is important to know.Goals for 2018:
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Adam B
Member Posts: 108 ■■□□□□□□□□
Thank you I appreciate it DoubleNNs Thank you both, Means a lot. Onto ACLS now in my reading 2015 Goals: CCNP SWITCH [] SEC+ [ ] CCNP ROUTE [ ] CCNP TSHOOT [ ]
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late_collision
Member Posts: 146
As you mentioned ISL is only cisco. It adds a 24 byte header and 4 byte trailer I believe and doesn't modify the frame. 802.1q does modify the frame and added a 4 byte frame I believe in the middle of the Ethernet frame.
Double check on that information since i'm going off the top of my memory right now from my ccent studies.
26 byte header. ISL adds 30 bytes to a frame.
I think what everybody has offered up will get you through CCENT and CCNA. Know that ISL exists, it's Cisco proprietary, and that it is not widely used or supported.