Sql-server – Availability groups NIC best practices

We'd like to introduce AlwaysOn Availability Groups from ClusterP to ClusterS. I've read multiple guides to this and there is one question I can't really answer. We do NOT have the ability to have cross-datacenter shared storage (no SAN mirroring across datacenters). With this setup, is it possible to setup AlwaysOn Availability Groups?

No, that is not possible. Outside of a migration, an availability group cannot straddle multiple failover clusters (WSFC).

If you want to leverage an AG, you'll have to ensure that all participating replicas reside within the same WSFC cluster. As for SQL Server failover cluster instances participating in the AG, that is not a problem. A FCI can be an AG replica.

You can achieve a geographically dispersed cluster with a few different strategies. If you need to have a witness vote (which, if you have three node clusters then that wouldn't help and you'd be best with a node majority quorum model) you could utilize a file share witness.

If what you're looking for is your availability group to span two data centers, then yes that is possible. The only requirement for this situation would be a single WSFC cluster (which is possible to span multiple data centers).

How would one approach this?

With a whole bunch of pre-move work :)

My initial thoughts are: /snipped for brevity

You have the right ideas, let me add a few more.

Pre-Work

• The cluster is now Multisubnet, we'll need to add an IP for the cluster name in the second subnet. This will be added to the cluster name resource as an 'OR' dependency as you've stated above. This can be added at any time (I'd add it pre-move). The cluster CAP will now have 2 IP addresses, one for each subnet.
• Firewall rules, etc.
• The listener (as you've already stated) will become multi-subnet. Another IP address will need to be added in the 'OR' dependency.
• Double check the setting of the client access point (CAP [aka VNN, aka network name]) for: HostRecordTTL and RegisterAllProvidersIP to make sure they are setup the way you want them. Note that you may want to use multiple listeners to facilitate clients that use older connection libraries not supporting the new keywords.
• Decide if you want to remove the replica from the AG (if it's going to take a while to physically move the server) or to just pause the AG. If the AG is removed, the databases will go back into a "restoring..." state and you'll be able to catch them up with restores at a later point after the server arrives and is setup.
• Quorum: There isn't much to say here, it's a 2 node cluster with 1 node getting airline miles. Included because we'll hit this later and it's a common question.

Post-Work

• The server should (with a new IP and proper firewall rules) be able to contact the cluster and join.
• Run cluster validation wizard. Save this report as the initial move report. We may need to look at it later. This will also verify most cluster related items and runs very quickly.
• Verify network latency. Set CrossSubnetDelay and CrossSubnetThreshold appropriately for the latency and health of the connection. No changes may be needed but it's good to double and triple check.
• Restore transaction logs/diffs/etc.
• Add the replica back into the AG
• Revisit Quorum. Even though there are just two nodes in the cluster, we'll want to double check we're not using a disk witness. Depending on the version of Windows we should/could use either a fileshare witness or Azure witness.

There may be additional items specific to your environment but that should be the gist of it. You've pretty much hit the nail on the head in your original question/post, this just adds in a little filler :)