ISL Basic Concept

[fabio.marino]

Hi to all i have doubts about basic concept of ISL.

First of all, what is going to happen in the case of ISL trunk enabled only in one side of the trunked link?

More, all catalyst switch support ISL trunk?

Finally, does ISL trunk support all types of media (Ethernet, Fiber Optic)?

Thanks in advance.

[Latchum Naidu]

Hi,

First of all, what is going to happen in the case of ISL trunk enabled only in one side of the trunked link?
If one side of the link is configured as an ISL trunk, the other side of the link should also be configured as ISL. Similarly, if one side of the link is configured as 802.1Q, the other side of the link should also be configured as 802.1Q.

More, all catalyst switch support ISL trunk?
If they are with CatOS then yes, they will support for ISL.

NOTE: The Catalyst 4000 family (including Catalyst 2948G and Catalyst 2980G) running CatOS only supports 802.1Q trunking, not ISL trunking.

Please rate the helpfull posts.

Regards,
Naidu.

[fabio.marino]

Hello, thanks for your time.

I agree with you that both side of a trunk link need to adopt the same encapsulation protocol.

But in the case of ISL misconfiguration, i mean one side isl enable and the other one not, what is going to happen?

[Amit Singh · Accepted solution]

Hi,

If you have DTP (Dynamic trunking protocol) enable on the ports, the ports will auto-matically negotiate the trunking properties and it will form a  trunk link. If DTP is not enabled or supported by the switch, the link wont come up and you will see a lots of trunking related errors.

Also, to your original question, most of the Cisco IOS switches supports ISL trunking as well.

http://www.cisco.com/en/US/prod/collateral/switches/ps5718/ps5023/prod_qas09186a00801b0971.html

http://www.cisco.com/en/US/tech/tk389/tk689/technologies_tech_note09186a008017f86a.shtml#subtopic2b

HTH,Please rate if it does.

-amit singh

[Latchum Naidu]

Hi,

If one side is trunking and the other side is not, then the switch ports won't understand each other's traffic. One side will be encapsulating frame sin ISL whereas the other side is not doing any encapsulation. You shold avoid the following combinations:

Nonegotiate (enable ISL but don't negotiate) and off (don't enable ISL)
Nonegotiate (enable ISL but don't negotiate) and auto (enable ISL only if the other side says to)

Hope this cleare you.

Please rate the all helpfull posts.

Regards,
Naidu.