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WAN Switching Network Synchronization

Fundamentals

Document ID: 10755

Contents

Introduction

Prerequisites

Requirements

Components Used

Conventions

Cisco WAN Synchronization Methodology, Guidelines, and Definitions

Related Information

Introduction

When you design the synchronization plan for a network of Cisco WAN switches, the fundamental design

goals include:

Synchronize the greatest number of network elements to the smallest number of independent clock

sources. Ideally, all network elements are synchronized to a single clock source.

Use clock sources of the highest quality (in terms of stability and long−term accuracy). This implies

that available clock sources are used with this priority:

Clock sources (typically stratum 2) provided by a national or international carrier.♦

The internal (stratum 3) clock source provided by a BPX node.♦

The internal (stratum 4) clock source provided by an IGX node, a PBX, or other customer

premises equipment (CPE).

♦

In order to ensure resiliency of the synchronization, plan in the face of possible failures of clock

sources, network elements, or network trunks.

Prerequisites

Requirements

There are no specific requirements for this document.

Components Used

This document is not restricted to specific software and hardware versions.

Conventions

Refer to Cisco Technical Tips Conventions for more information on document conventions.

Cisco WAN Synchronization Methodology, Guidelines, and

Definitions

Summary of content (6 pages)

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WAN Switching Network Synchronization Fundamentals Document ID: 10755 Contents Introduction Prerequisites Requirements Components Used Conventions Cisco WAN Synchronization Methodology, Guidelines, and Definitions Related Information Introduction When you design the synchronization plan for a network of Cisco WAN switches, the fundamental design goals include: 1. Synchronize the greatest number of network elements to the smallest number of independent clock sources.
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It is the responsibility of the network administrator to identify and define the available network clock sources during the creation of the network synchronization architecture 1. Network versus Node Clock Sources It is critical to note that the network administrator does not explicitly identify which clock source is to be used by each node in an IGX or BPX network. Instead, the network, as a part of switch software, automatically selects the best available clock source and path for each node.
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Once the available clock sources are defined for the network, the system software of the network automatically determines the specific clock source to be used by each node in the network. In order to ensure resiliency, the node clock source selection algorithm is re−executed as a result of these items: ♦ The addition or deletion of a network clock source by the network administrator. ♦ The failure of any defined network clock source. ♦ The failure or repair of any node in the network.
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♦ If, for any reason, the network administrator wishes to prevent the trunk from being included in the clock path between any nodes in the network. This is sometimes done for trunks that are prone to frequent outages. Note: Subrate trunks by definition cannot pass clocks and are therefore blocked from being configured as Pass Sync. Virtual trunks are physically unable to pass clocking information but are not restricted from being configured as "Pass Sync: Yes.
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♦ A trunk that is clocked by the carrier and the carrier's clock is not used as the clock source for the node. This configuration prevents uncontrolled frame slips (and the corresponding data loss) in the carrier's facilities. 6. Is it OK to Have Multiple Clock Sources in Use in the Network? In some cases, it is unavoidable for some nodes and trunks in a network to synchronize to one clock source and to synchronize other nodes and trunks in the network to another clock source.