Cisco BPX/IGX/IPX WAN Software


Cisco BPX/IGX/IPX WAN Software

The intended audience for this document is the user who needs an introduction to clock synchronization in BPX, IPX/IGX and MGX networks or someone who wants an overview of network-wide clock synchronization. A basic understanding of BPX, IPX, IGX and MGX functionality is assumed.

Network Clock Sources for Cisco WAN Switches

  • Internal Clock Sources

  • External Clock Sources

  • Trunk Clock Sources

  • Circuit Line Clock Sources

Switch software allows you to configure external, trunk, and circuit line types of clock sources at any of the hierarchy levels using the cnfclksrc command. The internal clock source is used as the default clock source in the absence of a user-configured clock source. It is also used when the user-configured clock source is disrupted or unreachable.

Cisco recommends basing the clock source configuration in a given hierarchy level on its stratum level. To verify the hierarchy level of a clock source, issue the dspclksrcs command. Stratum levels are used to describe the accuracy and stability of a clock. This document references stratum levels that range from 1 (most accurate) to 4 (least accurate). A stratum level 4 clock is not as stable as a stratum level 1 clock.

Typically, external clock sources are of the highest stratum level and, at a given hierarchy level, an external clock source would be configured in preference to some other type of clock source. Cisco recommends Y-cabling external clock sources when redundant processors are used.

The internal clock source of the BPX 8600 and MGX 8850 (PXM45) meets stratum Level 3 requirements for accuracy and stability.

These clock source configurations are not supported:

  • A circuit line on the MGX 8220 platform.

  • These port interfaces on BPX, IGX and MGX platforms:

    • V.35

    • X.21

    • RS-232

    • RS-449

    • Frame Relay

Clock Selection in BPX/IPX/IGX Networks

The highest numbered reachable node in a BPX/IPX/IGX network makes the selection of reference clock source for each node in the network. In a 10 node network, node number 10 determines the path to the closest, highest-available stratum clock source for every node in the network. The highest numbered reachable node performs this computation and instructs other nodes to perform any needed switching of clock source references as a result of these network changes.