Preventing looping when using connect statements

Figure 1 shows definitions for a 3-node network with one non-JES2 node (N3). Each of these connections has a resistance value defined to prevent transmitted data from looping through the system if multiple outages occur.

To understand the effect of simultaneous multiple outages on incorrectly defined connections in a network involving a non-JES2 node, assume that:

The N1 to N3 and N2 to N3 connections are unavailable.
N3 is a non-JES2 node.
Only the connections listed exist.

Figure 1. Three-Node Network Configuration CONNECT Statements to Avoid Looping

The path managers at both N1 and N2 discover that the path to N3 is down. Both path managers attempt to route data to the alternate path. When data arrives at N2, the path manager there knows the N2 to N3 path has failed. Therefore N1 sends the data to N1 to make use of the N1 to N3 path. When data arrives at N1, the path manager there knows the N1 to N3 path has failed. Therefore, N1 sends the data to N2 to make use of the N2 to N3 path. The path managers continue to send the data back and forth (looping) until one of the unavailable links restarts or the NJEDEF MAXHOP= limit at one of the sites has been reached.

Note: In the case where both paths are prone to failure and require alternate paths, IBM® suggests a low setting, such as NJEDEF MAXHOP=2. In the event of a failed connection, data travels back-and-forth only one time.

An installation can avoid looping by defining a NJEDEF RESTMAX= parameter value that eliminates all alternate paths, as shown in Figure 1. That is, the path from N1 through N2 to N3 has a total resistance of 200. Data from N1 will not take this path to N3 because the resistance exceeds the 150 maximum.

If one of the paths is particularly prone to failure, an installation can add static connections between both N1 and N3 and N2 and N3 as illustrated in Figure 2.

Figure 2. CONNECT Statements that Avoid Looping in 3-Node Network

CONNECT NODEA=N1,NODEB=N3,REST=100
CONNECT NODEA=N2 NODEB=N3,REST=100

The CONNECT statements advise the path managers at both N1 and N2 when the N3 connection is down, so both path managers hold the data.

Note: If both paths require alternate paths but CONNECT statements like those in Figure 2 do not exist, IBM suggests a low setting for the NJEDEF MAXHOP= parameter (such as MAXHOP=2). In the event of a failed connection, data travels back-and-forth only one time.