IPv6 static routing

Static routing requires that routes are configured manually for each router or destination; this is a significant reason system administrators avoid this technique if given a choice. Static routing has the disadvantage that network reachability is not dependent on the state of the network itself. If a destination is down, or unreachable through a statically configured route, the static routes remain in the routing table and traffic continues to be sent toward that destination without success.

To minimize network administrator tasks, configuration of static routes is to be avoided, especially in a large network. However, certain circumstances make static routing more appropriate. For example, static routes can be used:

To define a route that will not be learned dynamically from OMPROUTE or from router advertisements received from routers as part of the router discovery protocol
In conjunction with dynamic routes to provide backup routes when a dynamic route to the destination cannot be found. In this case, the static route must be configured as replaceable.

If static routing is used, only the PROFILE.TCPIP data set has to be updated with BEGINROUTES statements. The only ways to modify static routes are:

Replace the routing table using the VARY TCPIP,,OBEYFILE command
Use incoming ICMPv6 redirect packets
If a static route is defined on a BEGINROUTES statement as being replaceable, it can be replaced by a dynamic route

Note that the first BEGINROUTES statement in PROFILE.TCPIP, or in the data set referenced by a VARY TCPIP,,OBEYFILE command, replaces all static routes in the TCP/IP stack routing table. Subsequent statements within the same data set append to the routing table.

Rule: If you use static routes and want to honor ICMPv6 redirect messages (that is, you do not code IPCONFIG6 IGNOREREDIRECTS), then you must code the first hop address using the link-local address of the router. This is required because all redirect messages are sent using the router's link-local address, and if the source address of the redirect message does not match the address of the first hop in the routing table, the redirect message will be ignored.

For more information on the VARY TCPIP,,OBEYFILE command, see z/OS Communications Server: IP System Administrator's Commands. For more information on the IPCONFIG6 statement, and the IGNOREREDIRECTS parameter on the IPCONFIG6 statement, see z/OS Communications Server: IP Configuration Reference.

Tip: You can use the IBM® Health Checker for z/OS® to check whether the total number of indirect routes in a TCP/IP stack routing table exceeds a maximum threshold. When this threshold is exceeded, OMPROUTE and the TCP/IP stack can experience high CPU consumption from routing changes. For more information about IBM Health Checker for z/OS, see z/OS Communications Server: IP Diagnosis Guide and IBM Health Checker for z/OS: User's Guide.