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ISL vs dot1Q

Hi,

I know this basic difference between the ISL and dot1q. But i'm trying to understand the difference between them as per the operation.

Can someone please explain ?

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ISL is cisco proprietary and presently it is supported only by few high end switches only.

Dot1Q is an IEEE’s open standard, which be used to create trunk connection between switches of different vendors.

In ISL the original ethernet frame is not modified, it is encapsulated between an ISL header and an FCS.

In dot1Q the orignal ethernet fram is modified, a 4 bytes field called as TAG is inserted between the Source MAC field and the Length/Type field of the ethernet frame.

In ISL all the VLAN frames are encapsulated with ISL header and FCS.

In dot1Q the frames of all the VLANs are modified i.e., a vlan tag is inserted into the frame, except for the native VLAN.

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Thanks in advance.

Regards,

Chandu

Regards, Chandu
Everyone's tags (3)
6 REPLIES
Hall of Fame Super Gold

ISL vs dot1Q

ISL was developed by Cisco and later ratified by IEEE as 802.1q standard.

Nowadays, NO ONE uses ISL. 

Not all switches support ISL.  All switches support 802.1q.

ISL vs dot1Q

You will find a few older switches support ISL, modern ones tend not to as Dot1Q is the way forward.

This link explains a lot of the questions you asked:

http://www.cisco.com/en/US/tech/tk389/tk689/technologies_tech_note09186a0080094665.shtml

Do they still ask about ISL on the current CCNA R&S exam still?

New Member

ISL vs dot1Q

----

Do they still ask about ISL on the current CCNA R&S exam still?

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I'm not sure about that, the thing that i'm learning it is i want to learn and understand it...

I'm not preparing in such a way as to get certificate ...

Becuase... having a driving license doesn't mean that he is a good driver... and doen't having a driving license doen't mean that he is a poor driver..:)

Regards,

Chandu

Regards, Chandu
Hall of Fame Super Gold

ISL vs dot1Q

I'm not sure about that, the thing that i'm learning it is i want to learn and understand it...

I'm not preparing in such a way as to get certificate ...

Don't waste your time on ISL.  Like I've said before, no one is using ISL anymore.

New Member

ISL vs dot1Q

And my acutal question is "Operational difference between them as per the switches i mean how the switches read & understand ISL frames & dot1q frames.. what they do first when a frame is received it may be ISL/1q's "

Regards, Chandu

Re: ISL vs dot1Q

Chandu,

both protocols can be seen as layer-2 "containers" with additional information like VLAN-IDs and QoS-markings (that information cannot be exchanged with standard ethernet frames).

ISL was a Cisco proprietary approch before the 802.1Q standard was published for the first time in 1998, other vendors had other protocols, e.g. 3Com's VLT (Virtual LAN Trunk) tagging.

Very old Cisco switches (e.g. Catalyst 1900) supported only ISL, then, for a long time, switches supported both protocols and by now a backwarts-compatibility is no longer needed, so most newer platforms only support 802.1Q.

Link: System Requirements to Implement Trunking

What kind of encapsulation (interesting keyword, by the way, as .1Q is no encapsulation at all) a switchport "understands" is an either-or decision, normally defined by configuration:

(config-if)# switchport trunk encapsulation { dot1q | isl | ... }

However, for the sake of completeness, there is also a dynamic way for a port to learn which encapsulation is used by the neighbor switch: DTP (Dynamic Trunking Protocol, keyword: dynamic {auto|desirable} ).

But with DTP it's the same like with ISL: One should know them but they are of no great importance nowadays. The recommendations are to disable DTP and to use 802.1Q tagging.

HTH

Rolf

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