# Brocade Fibre Channel Networking Community

## What is the maximum distance difference between ISL

• #### 1.  What is the maximum distance difference between ISL

Posted 03-21-2014 07:30 AM

Hello everybody,

I am looking for a matrix with the max. Distance difference for ISL Trunking according to the link speed.

Can anybody supply this information?

RGDS

Klaus

• #### 2.  Re: What is the maximum distance difference between ISL

Posted 03-21-2014 12:53 PM

Hi Klaus,

AFAIK, the link speed has no influence on trunking.(but should be the same for all ports)

Max deskew is somewhat around 230 deskew values (trunkshow)

if the deskew is too high, the trunk does not form.

A lot of people toldl me that 15deskew shoud be max difference for best performance,within a trunking group

Hope this helps

Uli

PS: one deskesw equals 1ns divided by 10,  5 ns equals one meter of fiber; so 15 deskew ~30m

• #### 3.  Re: What is the maximum distance difference between ISL

Posted 03-23-2014 03:38 AM

Could you please tell where you got that deskew value of 230 from?

Thanks,

Felipon

#deskew
#trunking

• #### 4.  Re: What is the maximum distance difference between ISL

Posted 03-24-2014 08:10 AM

If memory serves me right, the shortest ISL has a default deskew value of 15.

The difference should not be more than 30 meters for performance reasons and the max supported difference is 300 meters.

• #### 5.  Re: What is the maximum distance difference between ISL

Posted 03-25-2014 12:49 AM

Is this a written rule, or best practice? Any Document on this?

We need to calculate the length of a fiber coil to compensate two different dark fibers.

We have a measured difference of 15 Km, but we didn't know the exact length of the coil. 30 m accuracy on a 15 Km coil - not easy to achieve... We try our luck! :smileyhappy:

Anyway, thank you!

• #### 6.  Re: What is the maximum distance difference between ISL

Posted 04-05-2014 04:44 AM

All info I have something I picked up in training and I can't share the docs though for obvious reasons.

Its in the BCFA training on the routing and trunking module.

There it states (rev 611)

-trunking max difference is actually 400 meters (so you have some room to play with)

-the shortest ISL has the value of 15.

-the difference in deskew values across ISL's divided by 2 gives the rough cable length difference

-crossing 30 meters difference (ie having a deskew value of 30, a difference of 15 with the shortest ISL and 1 counts for ~2meters), COULD introduce performance problems.

Perhaps you can find this info in the BCFA nutshell as well, or have another forum member confirm the information.

• #### 7.  Re: What is the maximum distance difference between ISL

Posted 04-09-2014 10:37 PM

From the broacde official training material:

* The maximum supported difference in cable length between the shortest and the longest ISL in a trunk group is 400 meters
- Consider this when creating long distance trunks over WDMs

* A two meter difference is approximately equal to one deskew unit
- Differences greater than 30 meters could introduce performance degradation
- Since the shortest ISL is set to a deskew of 15, an ISL with a difference of 30 meters has a deskew of approx. 30
- A 400 meter difference would yield a deskew value of 215

If you have a large difference beween ISL cable length i personally would consider not to trunk them.
In that case maybe you can take a look at protection switching which provides transparant failover between ISLs but that's only what i heard of it in theory, unfortunately i have no hands-on experience with this..