Link Controller - Create Link(ISP) object

The link definition will specify the next hop for outgoing connection from the LC´s perspective.

So it will be the "internal" interface of your ISP router which can be reached locally from your LC.

These interfaces (typically you are using two or even more ISP links) will be grouped as well in a "default_gateway_pool" which should be assigned as default route to your LC in the network settings.

(At least in the past this was a mandatory step to get the LC automatically configured as a server in the GTM configuration [LC is a hybrid of LTM & GTM].)

When forwarding requests (initiated by internal) clients to servers in the public internet somehow you need to make sure the responses will be routed back properly and symmetrically (return via same ISP link).

This can be achieved by applying SNAT for outgoing traffic. To capture outgoing traffic a wildcard network virtual server 0.0.0.0/0 has to be configured to listen on the LC´s internal interface. It will run in PerformanceL4 mode, with SNAT automap (requires floating self IPs on your interfaces towards the ISP router) use the default_gateway_pool and has the destination address affinty profile as recommended by Chris.

By default, the BIG-IP will SNAT only tcp and udp. To SNAT i.e. ICMP or ESP as well, you need to enable it first, please:

tmsh modify ltm global-settings general snat-packet-forward enabled
tmsh save sys config