ISL and 802.1Q. Do they work hand in hand ?

[Aryanaresh]

What happens if i have 802.1Q configured on my end and the other end runs with ISL?

I want to know what happens in both scenarios, if the device that i am running with 802.1Q is Cisco and if i am running with other vendor devices like HP.

I believe the Trunk will not get established, but the doubt raised in my mind is because i read in a forum where they have mentioned that ISL will over rule 802.1Q and Trunk will get established using ISL for communication. Is that true ???

Kindly explain me the trunking process between devices running with ISL and 802.1Q, when connected together.

Thank you for your time.

[danrya]

ISL is Cisco proprietary, 802.1Q is industry standard.  They are NOT compatible.  Cisco 802.1q is compatible with the industry standard and will operate with other vendors, including servers, switches, firewalls, etc.

Newer Cisco switches, including the Nexus family no longer support ISL.

[Aryanaresh]

Yes, I understand the Fact that ISL is Cisco proprietary and 802.1Q is industry standard, ISL will be deprecated

I just wanted know what happens when two switch running on different protocols, when connected to form trunk.

Does DTPs are exchanged and then tears down the session or DTP exchange will not happen on the first place ? In simple how the DTP is treated in this scenario ?

[Leo Laohoo · Accepted solution]

I just wanted know what happens when two switch running on different protocols, when connected to form trunk.

If the two switch are directly connected to each other, layer 2 will not come up.  It's two people with one  talking in English and the other party can only talk in Klingon.  

The other way around this is to put a "translator" in the middle, like another switch.  You configure this switch to talk to ISL to another ISL switch (like Catalyst 3K) and talk 802.1q to another 802.1q (like Catalyst 2K switches).

[Aryanaresh]

Thank you Leo, I know its not possible. Just got confused by a silly guy's talk in my work place.

Still waiting for someone to explain me on

Does DTPs are exchanged and then tears down the session or DTP exchange will not happen on the first place ? In simple how the DTP is treated in this scenario ?

Hope somebody will clear my doubt

[devils_advocate]

DTP is Cisco proprietary so if you are connecting a Cisco switch to an HP switch, only the Cisco will send the DTP packets.

In this case you would statically configure a Trunk on the Cisco side with the 'switchport mode trunk' command and also issue the 'switchport nonegotiate' command which will stop the Cisco sending the DTP messages.

If you let the Cisco send these messages, the HP will just drop them as it will not know what they are.

Most modern Cisco switches do not even come with ISL as an option so the DTP messages are just there to negotiate whether the port Will or Will Not be a Trunk. If you were to connect an older Cisco switch which was configured with ISL to another switch which is using Dot1Q and leave DTP running, they would not negotiate a Trunk.

[Aryanaresh]

Thank you all for your reply.... I got the clear picture now...

Once again thank you all for your time and efforts....

[danrya · Accepted solution]

To elaborate a little further.  The default DTP mode for (most) Cisco switches that support DTP is:

Dynamic Desirable

If you don't set an encapsulation on the interface, the default is ISL on switches that support it.

If you connect to a device that doesn't participate in DTP to a device that is set to default configuration the negotiation fail's and the Cisco switch will enable trunk mode with ISL (if it supports it).

If the Cisco switch is set to "switchport mode access" or "switchport nonegotiate" then DTP will not run.

If a Cisco switch is connected to a non-Cisco device and the Cisco switch is set to encapsulation dot1q with everything else default, the trunk will come up as a Dot1q trunk when DTP fails to find a neighbor.

If you connect two Cisco switches, and one is default and the other is default except for "encapsulation dot1q".  The DTP will fail to negotiate a trunk and the interface will not come up.

DTP is not supported on Nexus and the default switchport mode is access.