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New Member

ISL vs 802.1q

besides ISL being proprietary, are there any significant funtional differences?

Anyone know of a decent write-up describing the differences?

7 REPLIES
New Member

Re: ISL vs 802.1q

As you already know, the main difference is that ISL is Cisco-proprietary and 802.1q is an IEEE standard and is therefore interoperable between vendors. There are also other subtle differences as follows:

With ISL, the Ethernet frame is encapsultated with a 26-byte header (containing VLAN info) and a 4-byte CRC trailer is added to the frame (this CRC is in addition to any other frame checking the Ethernet frame is implementing).

802.1q however, inserts a 4-byte field into the existing Ethernet frame. This allows 802.1q to work on both access links and trunk links as it is treated as a normal Ethernet frame.

The only other difference I can think of, is that 802.1q also uses 802.1p. 802.1p is part of the 802.1q field and indicates the priority of the frame. You can then provide a element of QoS using the priority fields although I have never used this myself.

Hope this helps,

Darren.

New Member

Re: ISL vs 802.1q

Thanks Darren.

New Member

Re: ISL vs 802.1q

ISL also supports Per-VLAN Spanning-Tree. 802.1q Does not.

Dereck

New Member

Re: ISL vs 802.1q

Just to add to previous posts-

802.1Q tags all vlans except the native vlan. This is why it works on both "access links and trunk links". Therefore any hosts on the access link that belong to the native vlan will work.

PVST (per-vlan spanning tree) is supported on 802.1Q too. The 802.1Q spec recommends one spt for all vlans. However, it left the option open for vendors to implement one per vlan if they so chose. And Cisco chose to implement PVST.

I would use 802.1Q instead of ISL. Less overhead and it appears Cisco is pushing to go that direction.

HTH

New Member

Re: ISL vs 802.1q

Hi Richard, I haven't seen any evidence of cisco going down the dot1q road, can you elaborate? In the UK they still bang on about ISL, they never mention dot1q except when talking about interoperability.

New Member

Re: ISL vs 802.1q

AFAIK ISL is not supported by all Cisco devices (Cat2948G or Cat4000 e.g.) So if you want to do a tagging with that boxes you have to choose 802.1Q! I'm not sure if the are also other devices which are only ISL capable beside some old Cat3000. Rgds. Stephan

New Member

Re: ISL vs 802.1q

the 4000's only run dot1q

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