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Interface Physical Properties

The physical properties of a network interface are the characteristics associated with the physical link that affect the transmission of either link-layer signals or the data across the links. Physical properties include clocking properties, transmission properties, such as the maximum transmission unit (MTU), and encapsulation methods, such as point-to-point and Frame Relay encapsulation.

The default property values for an interface are usually sufficient to successfully enable a bidirectional link. However, if you configure a set of physical properties on an interface, those same properties must be set on all adjacent interfaces to which a direct connection is made.

Table 20 summarizes some key physical properties of device interfaces.

Table 20: Interface Physical Properties

Physical Property	Description
bert-error-rate	Bit error rate (BER). The error rate specifies the number of bit errors in a particular bit error rate test (BERT) period required to generate a BERT error condition. See Bit Error Rate Testing.
bert-period	Bit error rate test (BERT) time period over which bit errors are sampled. See Bit Error Rate Testing.
chap	Challenge Handshake Authentication Protocol (CHAP). Specifying chap enables CHAP authentication on the interface. See PPP Authentication.
clocking	Clock source for the link. Clocking can be provided by the local system (internal) or a remote endpoint on the link (external). By default, all interfaces use the internal clocking mode. If an interface is configured to accept an external clock source, one adjacent interface must be configured to act as a clock source. Under this configuration, the interface operates in a loop timing mode, in which the clocking signal is unique for that individual network segment or loop. See Interface Clocking.
description	A user-defined text description of the interface, often used to describe the interface's purpose.
disable	Administratively disables the interface.
encapsulation	Type of encapsulation on the interface. Common encapsulation types include PPP, Frame Relay, Cisco HDLC, and PPP over Ethernet (PPPoE). See Physical Encapsulation on an Interface.
fcs	Frame check sequence (FCS). FCS is an error-detection scheme that appends parity bits to a digital signal and uses decoding algorithms that detect errors in the received digital signal. See Frame Check Sequences.
mtu	Maximum transmission unit (MTU) size. The MTU is the largest size packet or frame, specified in bytes or octets, that can be sent in a packet-based or frame-based network. The Transmission Control Protocol (TCP) uses the MTU to determine the maximum size of each packet in any transmission. For MTU values on J-series interfaces, see MTU Default and Maximum Values.
no-keepalives	Disabling of keepalive messages across a physical link. A keepalive message is sent between network devices to indicate that they are still active. Keepalives help determine whether the interface is operating correctly. Except for ATM-over-ADSL interfaces, all interfaces use keepalives by default.
pap	Password Authentication Protocol (PAP). Specifying pap enables PAP authentication on the interface. To configure PAP, use the CLI or J-Web configuration editor. PAP is not available in the J-Web Quick Configuration pages.
payload-scrambler	Scrambling of traffic transmitted out the interface. Payload scrambling randomizes the data payload of transmitted packets. Scrambling eliminates nonvariable bit patterns (strings of all 1s or all 0s) that generate link-layer errors across some physical links.

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