@Rob_Passey Please see my notes below. This is my understanding of how they work. I hope this helps.
1. DNS SRV method (Not all device types are supported)
Below are some sample SRV records under DNS Domain=example.com
Internal DNS SRV Records using TCP (For non-secure registration)
Service: _sip Protocol: _tcp Priority: 5 Weight:10 port: 5060 Server: expresswayC1.company.com
Service: _sip Protocol: _tcp Priority: 10 Weight:10 port: 5060 Server: expresswayC2.company.com
Public DNS SRV Records using TLS (For secure registration. This is needed if you register devices over internet)
Service: _sips Protocol: _tls Priority: 5 Weight:10 port: 5061 Server: expresswayE1.company.com
Service: _sips Protocol: _tls Priority: 10 Weight:10 port: 5061 Server: expresswayE2.company.com
Now, let's say you are trying to register a video endpoint at extension 1000 over TCP. In the SIP URI field of the device, you enter 1000@example.com, and set the protocol to TCP. The SIP device sends a "_sip._tcp.example.com" request to IP Address of it's configured DNS server. The DNS server comes back with the FQDN, Protocol type and the port number of an expressway C server
,in this example, expresswayE1.company.com. Then the device will send a registration request to this server. If the endpoint is on the internet, and you set the protocol to TLS, _sips._tls.example.com record is used and the device will register via the expressway E over TLS. Obviously, TLS can be used for local registrations as well if the internal SRV records are configured. Also, You can use the Priority and Weight values to control the distribution algorithm which is the benefit that comes with this method. In the example above, the devices always register to the primary node first, and use the secondary for failover, but if you set the weight to 7 on the first SRV record, and 3 on the second SRV record, then set priority to 10 on both SRV records, 70 percent of devices will register to the Primary node, and 30 percent to the secondary node.
2. DNS Round Robbin Method.
In this method, you use the FQDN of the Expressway cluster in the Proxy 1 address field of the endpoint. This way, the device will have access to all the Expressway server nodes in the cluster, and if one goes down, the device fails over to the next available node. You cannot control what devices register to which node since the registration requests are distributed in a Round Robbin fashion.
3. Using Static Addresses
You enter the IP Address or FQDN of the primary Expressway in the Proxy 1 Address field, and enter the IP address or FQDN of the secondary Expressway in the Proxy 2 Address field. Devices will always register to Proxy 1 Server, and use Proxy 2 Server for failover. Here you, you have the same level of control as the DNS SRV method but you need to enter the addresses manually.
