ISL tags

Unanswered Question
mahmoodmkl Sat, 02/10/2007 - 04:58

Hi Sayeed,

ISL is the encapsulation method which is used for carrying traffic for multiple vlans.ISL encapsulates the frame with 26 bit and a 4 bit FCS.



sachinraja Sat, 02/10/2007 - 05:02

Hello sayed,

Good to see you use Netpro a lot.. this is the best place to be in !!!

Trunks are used to carry traffic that belongs to multiple VLANs between devices over the same link. A device can determine which VLAN the traffic belongs to by its VLAN identifier. The VLAN identifier is a tag that is encapsulated with the data. ISL and 802.1Q are two types of encapsulation that are used to carry data from multiple VLANs over trunk links.

Anyway, ISL is a trunking protocol, which will tag all the VLANs flowing through two switches... ISL is a Cisco proprietary protocol for the interconnection of multiple switches and maintenance of VLAN information as traffic goes between switches. ISL provides VLAN trunking capabilities while it maintains full wire-speed performance on Ethernet links in full-duplex or half-duplex mode. These useful info is been taken from the following URL:

Hope this helps.. all the best.. rate replies if found useful.


ahmednaas Sat, 02/10/2007 - 08:11

Please be aware that ISL is a CISCO proprietary protocol and that it has been superceded by the IEEE 802.1Q standard. Cisco is recommending the use of .1q and there are cisco products now that support .1q but not ISL. So, unless you have legacy cisco equipment that support only ISL, stay away from it.

If interoperability with other manufacturer switches is a concern, you must use .1q.

ahmednaas Sun, 02/11/2007 - 06:01

There is no document the states explicitly that cisco recommends .1q. I came to this conclusion from discussions with cisco employees and others. The main argument is of course interoperability with non-cisco equipment.

If I recall correctly, the 2950/2960 series of switches does not support isl.

sachinraja Sun, 02/11/2007 - 17:15

Hello sayed,

As a general practice, network consultants/design people, normally recommend dot1q as the trunking protocol, since it is an open standard, as other said... but it all depends on your company's requirement and security standards.. Just remember that ISL is not supported on the following switches, as ahmed was mentioning:

1) 2940

2) 2950

3) 2960

4) Cat 500

Otherwise, there are some few differences.. ISL supports upto 1000 VLANs and dot1x upto 4096 VLANs.... ISL normally uses Per VLAN spanning tree (PVST), which offers great flexibility & load balancing..dot1q normally works on Mono spanning tree (MST), but is enhanced now to work with PVST+ for flexibiligy and load balancing.... So, if you see all the pro's , it is always good to use dot1q than ISL.... You can point out these points to your seniors.. also have a look at the following URL, for frame formats of ISL and dot1q:

Hope this helps.. all the best..


jain.nitin Sun, 02/11/2007 - 08:40

Hi Further to ur discussion, actually ISL is cisco proprietary encapsulation protocol whereas .1q is open standard which is supported by all the vendor's product.



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