auto-isl-port-group best practice

[Dan_Eng52]

Hi all, 

Wonder if someone can help me understand the use case for the auto-isl-port-group command on FortiSwitch. Taking the below diagram as an example, if the the distribution layer layer is already configured for MCLAG ICL when the access layer switches are attached the distribution layer switches will automatically establish automatically a trunk as per below:

edit "_FlInK1_MLAG0_"
set mode lacp-active
set auto-isl 1
set mclag enable
set members "port29" "port30"

Since these trunks are automatically established when connecting switches to an exiting MCLAG ICL peer group what is the use case for the auto-isl-port-group command and what difference is there in letting the trunks establish automatically rather than manually setting the auto-isl-port-group? 

Thanks in advance. 

Dan_Eng52

[Anthony_E]

Hello Dan,

Thank you for using the Community Forum. I will seek to get you an answer or help. We will reply to this thread with an update as soon as possible.

Thanks,

[Dan_Eng52]

Hi Anthony_E, 

I've been trying to find an answer but haven't managed to find anything as of yet so that would be greatly appreciated. 

Thanks, 
Dan. 

[Anthony_E]

Hello Dan,

We are contacting experts to provide the best answer :)!

Regards,

[Anthony_E]

Hi Dan,

Did you already have a look at this document?:

https://docs.fortinet.com/document/fortiswitch/6.4.2/devices-managed-by-fortios/617516/network-topol...

Tell me if it is helping. If not, we will continue to look for an answer.

Regards,

[Dan_Eng52]

Hi Anthony, 

Yes, it was actually that document that sparked my curiosity but did not answer my question. If you're able to find an answer for me, that would be greatly appreciated. 

Regards, 

Dan. 

[Anthony_E]

Oh ok!

Let me find an FSW expert!

[sachitdas_FTNT]

Hi Dan,

We configure auto-isl-port-group when we configure multi tier mclag setup. For eg. in tier1 and tier2 mclag, we will configure auto-isl-port-group on tier1 mclag FSWs. In the auto-isl-port-group, we will add the ports as member that are connecting to tier2 mclag FSWs.

Technical Tip: Three-tier MCLAG configuration on m... - Fortinet Community

[Dan_Eng52]

Hi Sachit Das, 

Thanks for your response. 

What does this achieve compared to leaving the MCLAG tiers automatically establish their trunks automatically? If I have two tiers of MCLAG peer switches already setup and I connect them together, they will automatically create the trunks as below: 

edit "_FlInK1_MLAG0_"
set mode lacp-active
set auto-isl 1
set mclag enable
set members "port29" "port30"

If I already have a multi tier MCLAG setup with the above automatically created trunks, is there any merit or difference to issuing the auto-isl-port-group and would this cause any issues on a live environment? 

Am I correct in stating that if we issue the auto-isl-port-group the name must match across the MCLAG peer switches but the ports themselves in that group will be local to that device? 

Many thanks in advance, 

Dan. 

[sachitdas_FTNT]

Hi Dan, it will create STP loops and inconsistency in the network if you do not configure auto-isl-port-group (as per design). Yes, names should be the same on both MCLAG-ICL peers. This setting instructs the switches to group ports from MCLAG peers together into one MCLAG when the inter-switch link (ISL) is formed.