Cisco Systems 8540 appendix Automatic Reverting of Clock Source, Ip-address subnet-mask, Hdlc ppp

Chapter 4 Configuring Interfaces

Configuring the POS OC-12c Uplink Interface (Catalyst 8540)

To initially configure the POS OC-12c uplink interface, perform the following steps, beginning in global configuration mode:

	Command	Purpose
Step 1
	Router(config)# ip routing	Enables IP routing.
Step 2
	Router(config)# interface pos	Enters interface configuration mode and specifies the POS
	slot/subslot/interface	interface to configure.
	Router(config-if)#
Step 3
	Router(config-if)# ip address	Assigns an IP address and subnet mask to the interface.
	ip-address subnet-mask
Step 4
	Router(config-if)# encapsulation	Specifies the encapsulation type.
	{hdlc ppp}
Step 5
	Router(config-if)# clock source	Specifies the clock source for the interface. When clocking is
	{line internal}	derived from the received clock, line (the default) is used. When
		no line clocking source is available, internal is used.
Step 6
	Router(config-if)# no shutdown	Enables the interface with the previous configurations.

Example

The following configuration is an example of the tasks in the preceding table:

Router(config)# interface pos 1/0/0

Router(config-if)#ip address 10.1.2.3 255.0.0.0

Router(config-if)# encapsulation ppp

Router(config-if)# clock source line

Router(config-if)# no shutdown

Automatic Reverting of Clock Source

If your system clock source is set to line clock, it uses the recovered received clock to transmit. Under some conditions, the received clock is not reliable because of severe degradation of the signal quality. Because your system software monitors SF (signal failure), it knows when there is severe degradation in the signal quality and resorts to using the internal clock temporarily. Once the conditions that caused the signal quality to deteriorate clear, your system reverts to the line clock.

When two POS interface modules are connected and configured with the default line clock, the signal quality can degrade over time and both POS interfaces revert to the internal clock. As soon as the signal quality improves, both POS interfaces revert to using the line clock. This cycle repeats itself causing the line protocol on both interfaces to toggle. You can prevent this situation by configuring one end of the connection with the default line clock and the other with the internal clock.

In addition, degradation in the signal quality causes an automatic reverting of the clock source under the following conditions:

•SLOS (section loss of signal)

•SLOF (section loss of frame)

•AIS-L (line alarm indication signal)

•SF (signal failure) due to B2 error rate crossing the SF threshold value

•SF (signal failure) due to B3 error rate crossing the SF threshold value when the pos delay triggers path command is configured

	Layer 3 Switching Software Feature and Configuration Guide
4-16	78-6235-04, Cisco IOS Release 12.0(10)W5(18)