Enabling application switchover across subnets with IPv6

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.

Address Resolution Protocol (ARP) 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 Next Generation (RIPng) for IPv6.

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.

  1. Select an IPv6 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.
    • It is recommended that this address is defined with a shorter IPv6 address prefix than any other IPv6 address that shares the same IPv6 prefix to ensure that the correct address is chosen for the source address in outbound packets.
    • Add the takeover interface (for example, 2001:0DB8:1234::1. Create it with a line description of *VIRTUALIP, maximum transmission unit of 1500 (any number in the range 576-16388), and autostart of *NO.
  2. 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.
  3. Use the Change RIP Attributes (CHGRIPA) command to set the RIPng attributes. Run the command: CHGRIPA AUTOSTART(*YES) IP6COND(*NEVER) IP6ACPDFT(*NO) IP6SNDONLY(*VIRTUAL).
  4. Ensure there is an IPv6 link-local address active on the system. An IPv6 link-local address starts with 'fe80:'.
  5. Use the Add RIP Interface (ADDRIPIFC) command add a RIP interface used by the OMPROUTED server to advertise the virtual address used for the takeover IP address. For example, if fe80::1 is the active IPv6 link-local address, run the command: ADDRIPIFC IFC('fe80::1') RCVDYNNET(*YES) SNDSTTRTE(*YES) SNDHOSTRTE(*YES) SNDONLY(*VIRTUAL).
  6. Restart the OMPROUTED server using the following commands:
  7. Ensure that all the commercial routers in the network that interconnect the recovery domain LANs are accepting and advertising host routes for RIPng.
    • This is not necessarily the default setting for routers. The language varies with router manufacturer, but the RIPng 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.
  8. 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 that the takeover address is ended again.
    (Clustering will start the address on the specified primary node when the CRGs are started.)
  9. Start the application CRGs.
    • The takeover address is started by clustering on the specified, preferred node, and RIPng advertises the routes throughout the recovery domain. RIPng might take up to 5 minutes to update routes across the domain. The RIPng function is independent from the start CRG function.
  • 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.