F5 LTM HA Link Best Practices

Your HA link can either be connected directly to the peer device or using a switch. If your devices are close enough then there is no problem using directly connected cables. The benefit of the switch is that you can have each device in a different rack location - which is good from a datacentre design perspective. Functionally they are the same.

With regards to your GTM - when you say VIP I presume this is this on the LTM. GTM and LTM peer using a self IP address, not a VIP address, this is done over iQuery. The only consideration you need when using internal IP addresses for a GTM listener, is that when configuring your DCs, you specify the real Public IP as well as the NAT address. GTM will resolve to a client a routable (generally public) IP address for name resolution. If GTM answers with a public IP address and LTM VIPs are using internal addressing, you just need to make sure requests are translated on an external firewall as they ingress the environment.

MAC Masquerading is not mandatory but can can make failover events happen more quickly