Configuring Synchronized Clocking

To make sure the network is synchronized, configure the attached network device that obtains its clocking from the Cisco MC3810 (from the T1/E1 controller clock source set to internal) to derive its clock from the T1/E1 signal sent by the Cisco MC3810. If the T1/E1 signal received from the attached network device is not synchronous with the Cisco MC3810 network clock, then frame and clock slips will occur at the T1/E1 controller, causing loss of data.

To configure the Cisco MC3810 to obtain its clock source from a network device attached to controller T1/E1 0, use the following commands beginning in global configuration mode:

	Command	Purpose
Step 1	Router(config)# controller {T1 | E1} 0	Enters controller configuration mode for controller T1/E10.
Step2	Router(config-controller)# clock source line	Configures controller T1/E1 0 to obtain the CiscoMC3810 clock source from an attached network device.
Step3	Router(config)# controller {T1 | E1} 1	Enters controller configuration mode for controller T1/E11.
Step4	Router(config-controller)# clock source internal	Configures controller T1/E1 1 to obtain its clocking from the internal network clock PLL.
Step5	Router(config-controller)# network-clock base-rate {56k | 64k}	Sets the network clock base rate for the serial ports. The default is 56k.
Step6	Router(config)# interface serial 0	Enters interface configuration mode for serial 0.
Step7	Router(config-if)# clock rate network-clock rate	Configures the network clock speed for DCE mode. The rate must be a multiple of the value set with the network-clock base-rate command in Step 5. Repeat Steps 6 and 7 for serial port 1.
Step8	Router(config)# exit	Exits configuration mode.
Step9	Router# show network-clocks	Displays the network clock configuration.

To configure the Cisco MC3810 to obtain its clock source from a network device attached to controller T1/E1 1, use the following commands beginning in global configuration mode:

	Command	Purpose
Step1	Router(config)# controller {T1 | E1} 1	Enters controller configuration mode for controller T1/E11.
Step2	Router(config-controller)# clock source line	Configures controller T1/E1 1 to obtain the CiscoMC3810 clock source from an attached network device.
Step3	Router(config)# controller {T1 | E1} 0	Enters controller configuration mode for T1/E1 0 to configure the clock source for the MFT.
Step4	Router(config-controller)# clock source internal	Configures controller T1/E1 1 to obtain its clocking from the internal network clock PLL.
Step5	Router(config-controller)# network-clock base-rate {56k | 64k}	Sets the network clock base-rate for the serial ports. The default is 56k.
Step6	Router(config)# interface serial 0	Enters interface configuration mode for serial 0.
Step7	Router(config-if)# clock rate network-clock rate	Configures the network clock speed for DCE mode. The rate must be a multiple of the value set with the network-clock base-rate command in Step 5. Repeat Steps 6 and 7 for serial port 1.
Step8	Router(config)# exit	Exits configuration mode.
Step9	Router# show network-clocks	Displays the network clock configuration.

To configure the Cisco MC3810 to use loop-timed clock mode on controller T1/E1 0, use the following commands beginning in global configuration mode:

	Command	Purpose
Step1	Router(config)# controller {T1 | E1} 0	Enters controller configuration mode for T1/E1 0.
Step2	Router(config-controller)# clock source loop-timed	Configures controller T1/E1 0 to take the clock from the receive line and send it back to the source.
Step3	Router(config)# controller {T1 | E1} 1	Enters controller configuration mode for T1/E1 1.
Step4	Router(config-controller)# clock source internal	Configures controller T1/E1 1 to obtain its clocking from the internal network clock PLL. NoteTo configure controller T1 1 for loop-timed mode, follow the same configuration procedure, but change the controller that will be configured for loop-timed mode.
Step5	Router(config-controller)# network-clock base-rate {56k|64k}	Sets the network clock base rate for the serial ports. The default is 56k.
Step6	Router(config)# interface serial 0	Enters interface configuration mode for serial 0.
Step7	Router(config-if)# clock rate network-clock rate	Configures the serial 0 network clock speed for DCE mode. The rate must be a multiple of the value set with the network-clock base-rate command in Step5. Repeat Steps 6 and 7 for serial port 1.
Step8	Router(config)# exit	Exits configuration mode.
Step9	Router# show network-clocks	Displays the network clock configuration.

To configure the Cisco MC3810 to use a network device attached to serial port 0 as the clock source, use the following commands beginning in global configuration mode:

	Command	Purpose
Step1	Router(config)# network-clock base-rate {56k|64k}	Sets the network clock base rate for the serial ports. The default is 56k.
Step2	Router(config)# network-clock-select 1 serial 0	Configures the network clock PLL to use the multiplied 2Hz. clock from serial 0.
Step3	Router(config)# interface serial 0	Enters interface configuration mode for serial 0.
Step4	Router(config-if)# clock rate line rate	Configures the network clock line rate on serial 0 acting in DTE mode. The rate value is the rate of the incoming clock, and this value must be a multiple of 8 KHz.
Step5	Router(config)# interface serial 1	Enters interface configuration mode for serial 1.
Step6	Router(config-if)# clock rate network-clock rate	Configures the network clock line rate for serial 1 acting in DCE mode. The rate must be a multiple of the value set with the network-clock base-rate command and must match the value set in Step 1.
Step7	Router(config)# controller {T1 | E1} 0	Enters controller configuration mode for T1/E1 0.
Step8	Router(config-controller)# clock source internal	Configures controller T1/E1 0 to obtain its clocking from the internal network clock PLL.
Step9	Router(config)# controller {T1 | E1} 1	Enters controller configuration mode for T1/E1 1.
Step10	Router(config-controller)# clock source internal	Configures controller T1/E1 1 to obtain its clocking from the internal network clock PLL.
Step11	Router(config)# exit	Exits configuration mode.
Step12	Router# show network-clocks	Displays the network clock configuration.

To configure the Cisco MC3810 to use its internal 2 MHz clock as the clock source, use the following commands beginning in global configuration mode:

	Command	Purpose
Step1	Router(config)# network-clock base-rate {56k|64k}	Sets the network clock base rate for the serial ports. The default is 56k.
Step2	Router(config)# interface serial 0	Enters interface configuration mode for serial 0.
Step3	Router(config-if)# clock rate network-clock rate	Configures the network clock line rate on serial 0 acting in DCE mode. The rate must be a multiple of the value set with the network-clock base-rate command and must match the value set in Step 1.
Step4	Router(config)# interface serial 1	Enters interface configuration mode for serial 1.
Step5	Router(config-if)# clock rate network-clock rate	Configures the network clock line rate on serial 1 acting in DCE mode. The rate must be a multiple of the value set with the network-clock base-rate command and must match the value set in Step 1.
Step6	Router(config)# controller {T1 | E1} 0	Enters controller configuration mode for T1/E1 0.
Step7	Router(config-controller)# clock source internal	Configures controller T1/E1 0 to obtain its clocking from the internal network clock PLL.
Step8	Router(config)# controller {T1 | E1} 1	Enters controller configuration mode for T1/E1 1.
Step9	Router(config-controller)# clock source internal	Configures controller T1/E1 1 to obtain its clocking from the internal network clock PLL.
Step10	Router(config)# exit	Exits configuration mode.
Step11	Router# show network-clocks	Displays the network clock configuration.

NoteWhen using the internal Cisco MC3810 clock source as the master clock, make sure to configure any other network devices directly attached to the Cisco MC3810 T1/E1 controllers and serial ports to obtain their clocking from the Cisco MC3810.

Using the network-clock-select command, you can configure a dynamic hierarchy of clock sources that are used if the primary clock source fails. Each clock source is assigned a priority. A higher priority number of a clock source places that source higher in the clocking hierarchy. The highest clock source priority is used as the default.

When a clock source fails, the Cisco MC3810 switches to the clock source in the hierarchy with the next highest priority. For example, if the clock source with priority 1 (the highest priority) fails, the Cisco MC3810 switches to the clock source with priority 2. Then, if the clock source with priority2 fails, the Cisco MC3810 then switches to the clock source with priority 3 (assuming that the clock source with priority 1 has not become active in the meantime.)

If the module providing the clock experiences a failure (for example, if the T1/E1 controller experiences a Loss-of-Signal or Loss-of-Frame), then the clock source will be switched.

NoteIf you shut down a controller that is the current clock source, the shutdown will not cause the clock source to be switched.

To configure a hierarchy of clock sources for backup purposes, use the following commands beginning in global configuration mode:

	Command	Purpose
Step1	Router(config)# network-clock-select 1-4 [serial0 | system | controller]	Specifies the highest priority clock source that will provide timing to the system backplane pulse code modulation (PCM) bus. Specify 1 and select the clock source to be assigned the highest priority. This clock source will be used as the default clock source unless it fails. Repeat Step 1, specifying each clock source with a lower priority. If the clock source with the highest priority fails, the clock source with the next priority is used. You can configure four clock sources: one primary clock source and up to three backup clock sources. For example, if serial 0 is priority 1 and controller T1 0 is priority2, when serial 0 goes down the system will switch to controller T1 0 as the clock source.
Step2	Router(config)# network-clock-switch [switch-delay | never] [restore-delay | never]	Configures the amount of time the network clock will wait before switching to a different clock, and the amount of time the current network clock will wait before recovering. The switch-delay option sets the duration the system waits before switching to the clock source with the next highest priority (as configured with the network-clock-select command). The restore-delay option sets the duration before the current network clock source recovers.
Step3	Router(config)# controller {T1 | E1} {0|1}	If one of the controllers will be used as a clock source in the hierarchy, enter controller configuration mode for the T1/E1 controller.
Step4	Router(config-controller)# clock source line	Configures the controller to obtain the Cisco MC3810 clock source from an attached network device. If the other controller will be used as a potential clock source in the hierarchy, repeat Steps 3 and 4. NoteTo prevent clock source conflicts, make sure to configure both controllers to clock source line after configuring the network-clock-select commands. For more information about how clock source conflicts are resolved using this feature, see the section following this procedure.
Step5	Router(config)# interface serial 0	If serial interface 0 will be used as a potential clock source in the hierarchy, enter interface configuration mode for serial 0.
Step6	Router(config-if)# clock rate line rate	Configures the network clock line rate on serial 0 acting in DTE mode. The rate must be a multiple of the value set with the network-clock base-rate command.
Step7	Router(config)# exit	Exits configuration mode.
Step8	Router# show network-clocks	Displays the network clock configuration.

When you configure a hierarchy of clock sources, each potential clock source must be preconfigured to a mode that enables the Cisco MC3810 to derive the clock from that source. For example, if a controller will be a potential clock source, the controller clock source must be configured to line. If the controller clock source is configured to internal, the controller cannot be configured as a potential backup clock source using the network-clock-select command.

In the normal configuration, configuring both controllers to clock source line causes clocking conflicts. However, when configuring a hierarchy of clock sources, because only one controller is used as the primary clock source at one time, the conflict is prevented.

The following rules apply to configuring the clock source hierarchy:

• If a controller is a potential clock source in the hierarchy, the controller clock source must be configured to line.

• If a controller is a potential clock source in the hierarchy but is not currently being used as the clock source, the clock source setting for that controller is automatically switched to loop-timed. This is a temporary state set by the software to prevent a clocking conflict. If the controller becomes the clock source because another clock source fails, the clock source setting for the controller switches to line.

In this situation, even though the setting for the controller clock is switched to loop-timed, the actual configuration remains line. This is the difference between the preconfigured state and the temporary "set state" of the controller.

• If either controller is the active clock source, the network clock PLL switch is thrown in the direction of the active clock. The system clock is recovered from the controller with the active clock source.

• If serial interface 0 is the active clock source, the clock source settings for both controllers are automatically set to loop-timed, and the network clock PLL switch is thrown in the direction of the serial port. The system clock is driven by a clock recovered from the DTE serial 0 interface, which has been multiplied from (n x 8000) Hz to 2 MHz.

• If the internal system clock is the active clock source, the clock source settings for both controllers are automatically set to loop-timed, and the network clock PLL switch is thrown in the direction of the controllers. Because both controllers are in the loop-timed state, neither clock provides a recovered clock to drive the PLL, resulting in a free-running, or internally timed, system clock.

The following is a configuration example showing a hierarchy of clock sources:

network-clock-select 1 t1 0
network-clock-select 2 t1 1
network-clock-select 3 serial0
network-clock-select 4 system
network-clock-switch 10 10
 
controller t1 0
clock source line
 
controller t1 1
clock source line
 
interface serial0
clock rate line 64000
 

In this configuration, controller T1 0 is the primary clock source, and the clock source is configured to line. Controller T1 1 is a backup clock source and although the clock source is configured to line, the system temporarily sets the clock source to the loop-timed state.

If the controller T1 0 clock source fails, the system switches to use controller T1 1 as the clock source. The clock source loop-timed "set state" on controller T1 1 is switched to the preconfigured line state.