ISL vs 802.1q

Unanswered Question
Sep 2nd, 2009

Hi, a couple of queries regarding trunking.

1. PVST+, CST, MSTP, RSTP are supported on 802.1q but not on ISL trunks. Is my understanding right?

2. Can Cisco as well as non-Cisco VoIP work over ISL trunk?


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Collin Clark Wed, 09/02/2009 - 05:28

1. Most non-Cisco 802.1q switches maintain only a single instance of spanning tree, known as Mono Spanning Tree (MST), that defines the spanning-tree topology for all VLANs. When a Cisco switch is connected to a non-Cisco switch through an 802.1q trunk, the MST of the non-Cisco switch and the native VLAN spanning tree of the Cisco switch combine in order to form a single spanning tree topology, known as the Common Spanning Tree (CST).

2. ISL is Cisco's VLAN encapsulation method and supported only on Cisco's equipment through Fast and Gigabit Ethernet links.

Hope that helps.

Peter Paluch Wed, 09/02/2009 - 05:43


Please note that the Multiple-instance Spanning Tree Protocol also uses the acronym MST or MSTP - be careful not to confuse these two.

Best regards,


Jon Marshall Wed, 09/02/2009 - 06:49


1) PVST, RSTP and MST are supported over ISL trunks. Note that PVST+ isn't because PVST+ was a modification to PVST for use on 802.1q trunks.


Giuseppe Larosa Thu, 09/03/2009 - 05:54

Hello Jon,

it may be a mistype

PVST+ supports ISL trunks I tested this on C6509 with CatOS 7.x

it supports also 802.1Q trunks.

It is the older PVST (that I never used) that cannot run over 802.1Q trunks.


have you got experience of MST over ISL trunks?

Hope to help


Jon Marshall Thu, 09/03/2009 - 06:16

Hi Giuseppe

Thanks for letting me know. I suppose logically there is no reason why PVST+ should not run over ISL.

"have you got experience of MST over ISL trunks?"

Not directly but i remember a conversation that Francois was involved in a while back where he talked about running MST over ISL.

And it's not wise to doubt Francios on STP :-)


Jon Marshall Thu, 09/03/2009 - 06:59


I'm having a bit of a moment so you could either or both of you guys just sanity check what i have written in this thread -

I would appreciate you giving it the once over just to make sure i'm not talking nonsense !!

Many thanks


Giuseppe Larosa Wed, 09/02/2009 - 07:30

Hello Dhananjay,

1) PVST+ can run on ISL trunks actually original PVST supported only ISL trunks and PVST+ added support over 802.1Q trunks.

CST has only one instance, MST 802.1s and RSTP should work well only on 802.1Q trunks.

Rapid PVST should support ISL trunks being proprietary.


A cisco ip-phone uses an implicit 802.1Q mini-trunk with

native-vlan = data vlan

voice-vlan = tagged vlan

because you cannot configure it directly but it relies on CDP for learning how to treat pc frames and voip frames ISL trunks are not supported.

Of course, being ISL proprietary you cannot ask a non cisco voip phone to support it.

Hope to help



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