Enabling application switchover across subnets with IPv4

Clustering, in general, requires that all cluster nodes in the recovery domain of an application cluster resource group reside on the same LAN (use the same subnet addressing). Cluster resource services supports a user configured takeover IP address when configuring application CRGs.

Start of change Address Resolution Protocol (ARP) End of change is the network protocol that is used to switch the configured application takeover IP address from one node to another node in the recovery domain. To enable the application switchover across subnets, you need to use the virtual IP address support and the Routing Information Protocol (RIP) for IPv4.

The following manual configuration steps are required to enable the switchover environment. This set of instructions must be done on all the nodes in the recovery domain, and repeated for the other nodes in the cluster that will become nodes in the recovery domain for the given application CRG.

Select an IPv4 takeover IP address to be used by the application CRG.
- To avoid confusion, this address should not overlap with any other existing addresses used by the cluster nodes or routers. For example, if choosing 19.19.19.19, ensure that 19.0.0.0 (19.19.0.0) are not routes known by the system routing tables.
- Add the takeover interface (for example, 19.19.19.19. Create it with a line description of *VIRTUALIP, subnet mask of 255.255.255.255 (host route), maximum transmission unit of 1500 (any number in the range 576-16388), and autostart of *NO. This takeover address (for example, 19.19.19.19) does must exist as a *VIRTUALIP address before identifying it as an associated local interface in next step. It does not, however, must be active.
Associate the intended takeover IP address with one or both of the IP addresses that you specify to be used by cluster communications when you create the cluster or add a node to the cluster.
- For example, this means that you make the 19.19.19.19 takeover address an associated local interface on the IP address for the cluster node. This must be done for each cluster address on each cluster node.
  Note: The cluster addresses must be ended to accomplish this change under the Configure TCP/IP (CFGTCP) command.
Create the cluster and create any CRGs. For the application CRG, specify QcstUserCfgsTakeoverIpAddr for the Configure takeover IP address field. Do not start any application CRGs.
Using Configure TCP/IP applications (option 20) from the Configure TCP/IP menu, then Configure RouteD (option 2), then Change RouteD attributes (option 1), ensure that the Supply field is set to *YES. If not, set it to *YES. Then start or restart RouteD (RIP or RIP-2) on each cluster node.
- NETSTAT option 3 shows the RouteD using a local port if currently running. RouteD must be running and advertising routes (ensure that the Supply field is set to *YES) on every cluster node in the CRG recovery domain.
Ensure that all the commercial routers in the network that interconnect the recovery domain LANs are accepting and advertising host routes for RIP.
- This is not necessarily the default setting for routers. The language varies with router manufacturer, but the RIP interfaces settings should be set to send host routes and receive dynamic hosts.
- This also applies to both the router interfaces that point to the systems as well as the router-to-router interfaces.
Note: Do not use an IBM i machine as the router in this configuration. Use a commercial router (IBM® or otherwise) that is designed for routing purposes. IBM i routing cannot be configured to handle this function.
Manually activate the takeover address on one of the cluster nodes:
1. Wait up to 5 minutes for RIP to propagate the routes.
2. Ping the takeover address from all nodes in the CRG recovery domain and from selected clients on the LANs who will be using this address.
3. Ensure the takeover address is ended again.
(Clustering will start the address on the specified primary node when the CRGs are started.)
Start the application CRGs.
- The takeover address is started by clustering on the specified, preferred node, and RIP advertises the routes throughout the recovery domain. RIP might take up to 5 minutes to update routes across the domain. The RIP function is independent from the start CRG function.

Important:

If the above procedure is not followed for all cluster nodes in the application CRG recovery domain, the cluster hangs during the switchover process.
Even though you do not perform a failover to replica nodes, it is a good idea to perform the procedure on the replica nodes in the event that they might be changed at a later date in time to become a backup.
If you want to use multiple virtual IP addresses, then each one will require a separate application CRG and a separate IP address with which to be associated. This address may be another logical IP address on the same physical adapter or it may be another physical adapter altogether. Also, care must be taken to prevent ambiguities in the routing tables. This is best achieved by doing the following:
- Add a *DFTROUTE to the routing table for each virtual IP address.
- To use multiple IP address use CFGTCP (option 2).
- Set all parameters, including the next hop, the same to reach the router of choice; however, the Preferred binding interface should be set to the local system IP address that is associated with the virtual IP address that is represented by this route.