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Cisco ASR 1000 Series Aggregation Services Routers SIP and SPA Software Configuration Guide
OL-14127-08
Chapter 9 Overview of the Ethernet SPAs
Restrictions
SSM and ESMC
Network clocking uses these mechanisms to exchange the quality level of the clock between the network
elements :
• Synchronization Status Message
• 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. Non-Ethernet interfaces, such as optical interfaces and
SONET/T1/E1 SPA framers, use SSM. The key benefits of the SSM are:
• Prevents timing loops.
• Provides quick recovery when a part of the network fails.
• Ensures that a node derives 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 Ethernet Synchronization Messaging Channel (ESMC) based on IEEE 802.3
Organization-Specific Slow Protocol 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 those 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 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. The algorithm has a nonrevertive behavior among 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 QL-enabled and QL-disabled modes. When multiple selection
processes are present in a network element, all 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.
Note • If no external commands are active, the algorithm selects the reference (for clock selection) with the
highest quality level that does not experience a signal fail condition. If multiple inputs have the same
highest quality level, the input with the highest priority is selected. For multiple inputs having the
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