Foundry Networks Switch and Router, Installation And Configuration Manual

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Using Packet Over SONET (POS) Modules
December 2000 6 - 7
Syntax: clock internal | line
The internal and line parameters specify whether the clock source is on the POS module (internal) or on the
network (line).
Changing the Loopback Path
Foundry POS interfaces can use the following loopback configurations for self tests:
• Internal – Packets that the router transmits on the interface are looped back to the interface’s POS framer.
The internal loopback configuration is useful for checking the POS circuitry.
• Line – The interface’s transmit and receive fibers are logically linked so that packets received on the receive
fiber are sent back out on the transmit fiber. Use this mode on the POS interfaces on both ends of a link to
test the interfaces along with the link.
By default, loopback is disabled. Do not enable loopback unless you are testing the interface.
USING THE CLI
To configure POS interface 2/1 for internal loopback, enter the following commands:
BigIron(config)# interface pos 2/1
BigIron(config-posif-2/1)# loop internal
BigIron(config-posif-2/1)# write memory
Syntax: loop internal | line
The internal and line parameters specify the path for the loopback. The internal parameter loops packets
transmitted on the interface back to the framer. The line parameter loops packets that are received on the receive
fiber of the port back out on the transmit fiber.
Changing the MTU
The MTU (Maximum Transmission Unit) specifies the maximum number of bytes a frame transmitted on the
interface can contain. You can configure the MTU to a value from 60 – 4470 bytes. The default is 4470 bytes.
USING THE CLI
To change the MTU for POS interface 2/1 to 1200 bytes, enter the following commands:
BigIron(config)# interface pos 2/1
BigIron(config-posif-2/1)# mtu 1200
BigIron(config-posif-2/1)# write memory
Syntax: mtu <length>
The <length> can be from 60 – 4470 bytes.
Changing the CRC Length
The CRC (Cyclic Redundancy Check) length specifies whether the CRC portion of each frame transmitted on the
interface is 16 bits or 32 bits long. The default is 32 bits. You can change the CRC length using the following
method.
USING THE CLI
To change the CRC length for POS interface 2/1 to 16 bits, enter the following commands:
BigIron(config)# interface pos 2/1
BigIron(config-posif-2/1)# crc 16
BigIron(config-posif-2/1)# write memory
Syntax: crc 16 | 32
The 16 and 32 parameters specify how many bits in each frame transmitted on the interface contain the CRC
data.