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How to know if CISCO CATALYST switch support isl or 802.1q?

bitsnake asked
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Last Modified: 2010-08-05
Where is tha table?
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patch575IT Systems Admin

I would visit the cisco site and look under product specifications or even the documentation.  
What model of Catalyst is it?  Certain series of Catalyst do not have all the functions.  You may need certain modules.  Certain switches like the 2950 and the 4000 will not support the ISL trunking but it will support the 802.1q trunking.  But then again there are switches that will only do ISL or some that do both.  Let me know and I can find out, or you can just search under Cisco's site.

Looking it up is a pain.  Use the following command:

show port capabilities

The output for "trunk encap type" will tell you what it will support.


I am designing a network and i use cisco products quick reference guide,but it is not good
because about 3548XL,it says 3548xl supports isl and no information about 802.1q,so i don't know if it support or not.
Is there a matrix about this capability of catalyst switchs?

If you follow the link from patch575 you will get to Cisco's official site for Catalyst Switches. Here you will
find the product guide to Catalyst 3500XL switches.

Quote from the guide:

"Virtual LAN trunks can be created from any port using either standards-based 802.1Q tagging or the Cisco ISL VLAN architecture"

So, it does indeed support .1q tagging.

IT Systems Admin
All Catalyst 3500 series XL switches feature Cisco IOS software and Cisco Visual Switch Manager (CVSM) software, an easy-to-use Web-based management interface. All high-performance Catalyst 3500 series XL switches are available in either Standard or Enterprise Editions. Enterprise Edition switches offer advanced software features, including complete 802.1Q and ISL VLAN support, TACACS+ security, and fault tolerance through Uplink Fast.


Btw, it looks like it's starting to be a good idea to think about migrating to Dot1q only for trunking.. we're using 2900XL's and 3500XL's here and were using ISL for all the trunking..

Recently we wanted to buy more 2900XL, but they are in End Of Sale status.. grlmb.. the replacement product is the 2950 line, which does NOT support ISL... found that out the hard way during a network change one night..

Havent found anything official yet from Cisco about if they're dropping ISL entirely, but I've heard rumors on different sides..

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