The use of the coupling links to exchange STP messages has the following advantages:  Between any two servers that are intended to exchange STP messages, configure each  server so that at least two coupling links exist for communication between the servers. This  configuration prevents the loss of one link from causing the loss of STP communication  between the servers. If a server does not have a CF LPAR, timing-only links can be used to  provide STP connectivity. The z14 server does not support attachment to the IBM Sysplex Timer. A IBM z14™ server  cannot be added into a Mixed CTN. It can participate in an STP-only CTN only. Important:  For more information about configuring an STP CTN with three or more  servers, see the  Important Considerations for STP server role assignments  white paper  that is available at the IBM Techdocs Library website. If the guidelines are not followed, it might result in all the servers in the CTN becoming  unsynchronized. This condition results in a sysplex-wide outage. STP on z14 features the following enhancements: With HMC 2.14.1, Coordinated Timing Network split or merge are supported. CTN split  defines a system-assisted method for dynamic splitting of a CTN, while CTN merge  automates the time synchronization process and the STP role assignment in the merged  CTN.  The limit was Stratum level 3 before z14. The extra stratum allows CPCs to operate as  part of CTN stratum level 4, which can avoid the extra complexity and expense of system  reconfiguration.  Warning:  This extra stratum level should be used only as a temporary state during  reconfiguration. Customer should not run with machines at stratum level 4 for extended  periods because of the lower quality of the time synchronization. This graphical display improved the user interface to STP controls. This type of visual  display of the CTN status, which provided a clearer view of CTNs, can avoid outages, such  as a user intentionally took down the CTS, but did not realize the BTS was down. GLYPH<SM590000> By using the same links to exchange STP messages and CF messages in a Parallel  Sysplex, STP can scale with distance. Servers that are exchanging messages over short  distances, such as IFB or ICA SR links, can meet more stringent synchronization  requirements than servers that exchange messages over long IFB LR links (distances up  to 100 km (62 miles)). This advantage is an enhancement over the IBM Sysplex Timer  implementation, which does not scale with distance. GLYPH<SM590000> Coupling links also provide the connectivity that is necessary in a Parallel Sysplex.  Therefore, a potential benefit can be realized of minimizing the number of cross-site links  that is required in a multi-site Parallel Sysplex. GLYPH<SM590000> CTN split and CTN merge GLYPH<SM590000> Extra stratum level GLYPH<SM590000> Graphical display of a Coordinated Timing Network (CTN) STP enhancements on z14 Chapter 4. Central processor complex I/O system structure  191