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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 POS OC-12c Uplink Interface (Catalyst8540)

Additional Configurations

Toconfigure additional properties to match those of the interface at the far end, perform the following
steps, beginning in global configuration mode:
Note The above steps apply both to the POS OC-12c uplink interface on the switch router and
to the interface to which it connects at the far end.
Customizing the Configuration
This section describe how to customize the configurationof the POS OC-12c uplink interface to match
your network environment.

Setting the MTU Size

To set the maximum transmission unit (MTU), perform the following steps, beginning in global
configuration mode:
Note The POS OC-12c uplink interface supports IP unicast and IP multicast fragmentation. For
IP unicast fragmentation, the packet must ingress on a POS interface and egress on any
interface. For IP multicast fragmentation, IP multicast data packets greater than
1500 bytes are fragmented to 1500 bytes on the ingress POS interface before being
switched to other members in the multicast group. All the members in the multicast group
must have a MTU equal to or greater than 1500 bytes.
Command Purpose
Step1 Router(config-if)# no keepalive Turns off keepalive messages. Keepalive messages, though not
required, are recommended.
Step2 Router(config-if)# no cdp enable Turns off CDP, which is not required.
Step3 Router(config-if)# crc {16 | 32} Sets the CRC value. If the device to which the POS module is
connected does not support the default CRC valueof 32, set both
devices to use a value of 16.
Command Purpose
Step1 Router(config)# interface pos
slot/subslot/interface
Router(config-if)#
Enters interface configuration mode and specifies the POS
interface to configure.
Step2 Router(config-if)# mtu bytes Configuresthe MTU size up to a maximum of 9188 bytes. Default
MTU size is 4470 bytes.