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Layer 3 Switching Software Feature and Configuration Guide
78-6235-04, Cisco IOS Release 12.0(10)W5(18)
Chapter4 Configuring Interfaces
Configuring the ATM Uplink Interface (Catalyst8540)
Example
The following configuration is an example of the tasks in the preceding table:
Router(config)# interface atm 2/0/0
Router(config-if)# ip address 10.1.2.4 255.0.0.0
Router(config-if)# atm clock internal
Router(config-if)# no shutdown

Configuring the Clock Source

The ATMuplink interfaces support internal and line clock source. The default mode for the clock is no
internal, which is the same as the line clock. If your system clock source is set to line clock, it uses the
recovered received clock to transmit.
When two ATMuplink interfaces are connected and set to line clock, both interfaces at each end of the
link cannot accurately synchronize the clock. This causes transfer of corrupt data, which might cause
the line protocol on both interfaces to go down. Toprevent this situation, make sure you configure one
end of the connection with internal clock and the other end with no internal clock.
When your system is configured to use the line clock, the following conditions cause the clock to
automatically revert to internal:
•SLOS (section loss of signal)
•SLOF (section loss of frame)
•AIS-L (line alarm indication signal)
•S1 (synchronizing status) byte in the SONET line overhead is equal to 0xF
When these conditions clear, the clock automatically restores to line clock.
Customizing the Configuration
This section describes how to configure your ATM uplink interface to match your network
configuration.

Setting the MTU Size

To set the maximum transmission unit (MTU), perform the following steps, beginning in global
configuration mode:
Command Purpose
Step1 Router(config)# interface atm
slot/subslot/interface
Router(config-if)#
Enters interface configuration mode and specifies the ATM
interface to configure.
Step2 Router(config-if)# mtu bytes Configuresthe MTU size with a value from 64 to 9188 bytes. The
default MTU size is 4478 bytes.
Step3 Router(config-if)# no shutdown Enables the interface with the previous configuration.