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Recommended methods for Testing STP Recovery

Document Author:
Additional Author(s):

Gregory Hutchison
Clay HarshbergerIII

Document ID:


Doc. Organization:

Advanced Technical Sales

Document Revised:


Product(s) covered:

z9 BC; z9 EC; zSeries 890; zSeries 990

Abstract: IBM recommends the following procedure for testing STP Recovery scenarios that simulate a loss of clock source for a server in a diad configuration. A diad configuration consists of only one Stratum-1 (PTS/CTS) and one Stratum-2 (BTS).

One of the most common techniques used to simulate this scenario is to force the loss of the last timing link.

To prevent an accidental misconfiguration of the last timing link that is being used to deliver STP timing between any two servers, safeguards were put in place. These safeguards have made it difficult for customers wishing to test STP recovery functions such as Backup Time Server (BTS) taking over as Current Time Server (CTS) or z/OS messages posting WTOR when BTS becomes unsynchronized.

While it may seem like a simple/easy test method to unplug the last timing link to circumvent these safeguards, IBM does not recommend that link cables be unplugged while they are on-line, as unexpected problems may occur.

If you desire to remove the last timing link, it should be done in one of two ways.
  1. For z/OS LPARs you must use the Hardware Management Console (HMC).
    When you attempt to vary off the last link from the HMC, a 'details' panel will provide a checkbox that will allow this 'last link' to be varied off.
    A detailed procedure is described in the STP Implementation Guide, SG247281.
  2. For CFCC LPARs a new command has been implemented. "con xx off forcestp"

STP information can be found and downloaded from




Backup and Recovery


IBM System z Family




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