Dot1q and ISL

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Dec 14th, 2009
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May i inquire if dot1q and isl can work on the same network (i.e. switch to switch connection are ISL and my AP to switch connection is Dot1q).

Antbody here who have tried this setup. Does it work?


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Ganesh Hariharan Mon, 12/14/2009 - 00:44
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Trunk encapsulation work only between switch to switch on trunk port so you can run different trunk encapsulation protocol between devices in your network.



vvasisth Mon, 12/14/2009 - 01:18
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This will work perfectly fine, only thing that matters is both ends of the trunk link should have same protocal either ISL or Dot1q.

Your setup will work just fine



rc.castillo Mon, 12/14/2009 - 01:46
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Hi sir,

     Do i have to create a separate vlan for the dot1q trunk connected through AP? Or i can directlty have the ap have the same vlan but different encapsulation (i.e dot1q). Because when i tried to put the ap on the same vlan but different encapsulation, it didnt work.

vvasisth Mon, 12/14/2009 - 01:49
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when we talk about ISL and Dot1Q we are refering to trunking which means the port can carry data for more than one vlan

but i think you want 1 vlan to move on the link wher AP is connected, if thats the case then you would need the port to be an access port rather than a trunk link.

please clarify ..



vvasisth Mon, 12/14/2009 - 01:50
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and if you are tranfering same vlans on ISL and dot1q it would work fine as well you wont need to create seperate vlan.



saurabh_knl Mon, 12/14/2009 - 02:53
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Just have this simplified:-

1. You need trunks on both the ends if there are more than one VLAN supposed to be traversing through it.

2. ISL does not supports native vlan, by default. Thus, if you are using voice make sure it aint going in the native vlan, in a setup as yours. Have the voice packets tagged into a vlan, and have it traversed in the network.

3. The trunk encapsulation type (ISL or DOT1Q) should be same on both the ends of the trunk. However, on different trunks it can be different, and can thus coexist in the same environment.




Giuseppe Larosa Mon, 12/14/2009 - 03:27
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Hello Rc.Castillo,

the trunking protocol must match on both  ends of a single link.

Sw1 ---- ISL ---- Sw2 ---- dot1q---- AP

this can work but both ends of second link has to be configured for 802.1Q.

SW2 can perform all the necessary conversions between the two protocol formats without any additional config.

int type x/y

desc versus AP

switchport trunk enc dot1q

switchport mode trunk

switchport trunk allowed vlan 10,20,30

But Sw2 port towards AP should have:

switchport trunk enc dot1q

if the switchport mode is desirable and the access point is Cisco it may work also with

switchport mode desirable because they might be able to negotiate with DTP the trunk status and the trunking protocol.

But it is more safe to configure explicitly

Hope to help



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