Understanding 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 9 summarizes some key physical properties of device interfaces.

Table 9: 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 Understanding Bit Error Rate Testing.
bert-period	Bit error rate test (BERT) time period over which bit errors are sampled. See Understanding Bit Error Rate Testing.
chap	Challenge Handshake Authentication Protocol (CHAP). Specifying chap enables CHAP authentication on the interface. See Understanding CHAP Authentication on a PPPoE Interface.
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 Understanding 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 Understanding 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 Understanding Frame Check Sequences.
mtu	Maximum transmission unit (MTU) size. 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 TCP uses MTU to determine the maximum size of each packet in any transmission. 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. See Configuring Keepalives in the Junos Network Interfaces Configuration Guide.
pap	Password Authentication Protocol (PAP). Specifying pap enables PAP authentication on the interface. See Understanding CHAP Authentication on a PPPoE Interface.
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. See Configuring E3 HDLC Payload Scrambling, Configuring T3 HDLC Payload Scrambling, and payload-scrambler in the Junos Network Interfaces Configuration Guide.

Understanding Interface Physical Properties

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