Create a host group object and name it "Chicago"
Create a host group object and name it "Firewalls"
Create a host group object and name it "switches"

Create a host and name it "firewall". Place this object in "Firewalls" and also in "Chicago"

Create a ping item and place it on the firewall host. Create a trigger with the following value:

{firewall:icmpping[].max(180)}<1

Create a host and name it "switch". Place this object in "Switches" and also in "Chicago".

Create a ping item and place it on the switch host. Create a trigger with the following value:

({switch:icmpping[].max(180)}<1 & {firewall:icmpping[].min(180)}>0)

When the switch goes offline, and is unpingable, you will see the dashboard report issues in the following host groups:

Switches, Chicago AND Firewalls.

Keep in mind, the host that has the issue (switch) is ONLY in Switches and Chicago. It is NOT a member of Firewalls.