Switching design assistance?

[UK_PSE]

I have 8 switches, 5 in one location and the other 3 all in 3 separate buildings with fibre links from these buildings to the main cabinet. I have a pair of FG-100F's in the same cabinet as the 5 switches and want to know the best option of connecting these.

Im thinking 5 in an IDF ring dual honed to the FG-100F's on 10Gb (top switch/bottom switch). I then need to connect the 3 x 108F's back to the main stack but not sure on the best topology here?

At the moment this is conceptual as im still working out which FortiSwitches to use to replace my Cisco switches but ive already got the Gates. I need 48 ports in the 5 switches in the main cabinet and only 8 port switches in the other buildings (108F's).

Possibility to use 248's in an MCLAG with 3 x 148's in an IDF for the main cabinet? How would i then connect the 3 x 108's back to the main cabinet with resilience? Trying to keep cost down but maintain redundancy. obviously i run out of ports trying to connect all 108's to the MCLAG. I want to use FortiLink and the same VLAN's are present in all locations with fibre connected between locations.

[gfleming]

Do you want the three switches in the three separate buildings to backhaul all L3 routing to the FortiGates? Because by default that is what is going to happen.

If you want to use MC-LAG (what is the driver for this?) then you can review some of the topologies here: https://docs.fortinet.com/document/fortiswitch/7.2.1/fortilink-guide/780635/switch-redundancy-with-m...

You might not need MC-LAG in which case the 5-switch IDF ring would work just fine:

A mix of these topologies:

https://docs.fortinet.com/document/fortiswitch/7.2.1/fortilink-guide/801190/ha-mode-fortigate-units-...

https://docs.fortinet.com/document/fortiswitch/7.2.1/fortilink-guide/801188/ha-mode-fortigate-units-...

[UK_PSE]

Hi Graham, thanks for this. My main concern was connecting the 3 standalones direct to the IDF ring and if there were any limitations? Layer 3 on the Gates for sure for everything. 
Any issue with the standalones having redundant links to differing switches in the IDF ring?
Would the standalones still form in to the fabric OK?

Thanks

[gfleming]

As far as I know it shouldn't be a problem—FortiLink will detect the ring topology of the 5 IDF switches. And any standalone switches hanging off of the "Stack" should be treated as leaf nodes, essentially. I'm not a FortiSwitch expert though so might be worth confirming with someone else.