Cisco ASR1002-10G-VPN/K9 Software Guide - Page 299

Chapter 10 Synchronous Ethernet Support Synchronization Status Message and Ethernet Synchronization Messaging Channel Synchronization Status Message Network elements use Synchronization Status Messages (SSM) to inform the neighboring elements about the Quality Level (QL) of the clock. The non-ethernet interfaces such as optical interfaces and T1 or E1 SPA frames, use SSM. The key benefits of the SSMs are: • Prevents timing loops. • Provides fast recovery when a part of the network fails. • Ensures that a node get timing from the most reliable clock source. Ethernet Synchronization Messaging Channel In order to maintain a logical communication channel in synchronous network connections, Ethernet relies on a channel called the Ethernet Synchronization Messaging Channel (ESMC), which is based on the IEEE 802.3 Organization-Specific Slow Protocol (OSSP) standards. ESMC relays the SSM code that represents the quality level of the Ethernet Equipment Clock (EEC) in a physical layer. The ESMC packets are received only for the ports configured as clock sources, and transmitted on all the SyncE interfaces in the system. These packets are then processed by the clock selection algorithm on route processors (RP) and are used to select the best clock. The Tx frame is generated based on the QL value of the selected clock source, and sent to all the enabled SyncE ports. Clock Selection Algorithm The clock selection algorithm selects the best available synchronization source from the nominated sources. This algorithm exhibits nonrevertive behavior among the clock sources with the same QL value, and always selects the signal with the best QL value. For clock option 1, the default is revertive, and for clock option 2, the default is nonrevertive. The clock selection process works in the following modes: • QL-Enabled Mode, page 10-3 • QL-Disabled Mode, page 10-4 When multiple selection processes are present in a network element, all the processes work in the same mode. QL-Enabled Mode In QL-enabled mode, the following parameters contribute to the selection process: • Quality level • Signal fail via QL-FAILED • Priority • External commands. If no external commands are active, the algorithm selects the reference (for clock selection) with the highest QL that does not experience a signal fail condition. If multiple inputs have the same highest QL, the input with the highest priority is selected. For multiple inputs having the same highest priority and QL, the existing reference is maintained (if it belongs to highest priority and QL group). Otherwise, an arbitrary reference from highest priority and QL group is selected. OL-16506-10 Cisco ASR 1000 Series Aggregation Services Routers Software Configuration Guide 10-3