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Configuring PPP

Understanding PPP

Point-to-Point Protocol (PPP) is a communications protocol. To configure PPP for subscriber access, see PPP Subscriber Access Networks Overview.

Supported PPP Interface Standards

Junos OS substantially supports the following RFCs, which define standards for PPP interfaces.

  • RFC 1332, The PPP Internet Protocol Control Protocol (IPCP)

  • RFC 1334, PPP Authentication Protocols

  • RFC 1661, The Point-to-Point Protocol (PPP)

How to Configure PPP

Configure the Maximum Number of LCP Configure-Requests to be Sent

Link Control Protocol (LCP) Configure-Request is used to establish a link. You can configure the maximum number of LCP Configure-Requests to send. The router stops sending LCP Configure-Requests after the specified maximum number is sent. To configure the LCP Configure-Request maximum, use the lcp-max-conf-req statement at the [edit interfaces interface-name unit number ppp-options] hierarchy level. The number range is from 0 to 65,535; where 0 specifies no limit and the LCP Configure-Request is sent indefinitely. The default is 254.

Configure the Maximum Number of NCP Configure-Requests to be Sent

Network Control Protocol (NCP) Configure-Request is used to establish a link. You can configure the maximum number of NCP Configure-Requests to send. The router stops sending NCP Configure-Requests after the specified maximum number is sent. To configure the NCP Configure-Request maximum, use the ncp-max-conf-req statement at the [edit interfaces interface-name unit number ppp-options] hierarchy level. The number range is from 0 to 65,535; where 0 specifies no limit and NCP Configure-Request is sent indefinitely. The default is 254.

Configure the LCP and NCP PPP Restart Timers

You can configure a restart timer for the Link Control Protocol (LCP) and Network Control Protocol (NCP) components of a PPP session. You can configure the LCP restart timer on interfaces with PPP, PPP TCC, PPP over Ethernet, PPP over ATM, and PPP over Frame Relay encapsulations. You can configure the NCP restart timer on interfaces with PPP and PPP TCC encapsulations and on multilink PPP bundle interfaces.

To configure the restart timer for the NCP component of a PPP session, include the ncp-restart-timer statement, and specify the number of milliseconds.

To configure the restart timer for the LCP component of a PPP session, include the lcp-restart-timer statement, and specify the number of milliseconds:

You can include these statements at the following hierarchy levels:

  • [edit interfaces interface-name unit logical-unit-number ppp-options]

  • [edit logical-systems logical-system-name interfaces interface-name unit logical-unit-number ppp-options]

To monitor the configuration, issue the show interfaces interface-name command. Configured options are displayed in the PPP parameters field for the physical interface.

Configure the PPP Clear Loop Detected Timer

When a PPP session detects a loop, the loop detected flag is set. If the flag is not cleared by the protocol after the loopback is cleared, the clear loop detected timer clears the flag after the specified time has elapsed.

To configure the clear loop detected timer for the LCP component of a PPP session, include the loopback-clear-timer statement, and specify the number of seconds.

You can include this statement at the following hierarchy levels:

  • [edit interfaces interface-name unit logical-unit-number ppp-options]

  • [edit logical-systems logical-system-name interfaces interface-name unit logical-unit-number ppp-options]

To monitor the configuration, issue the show interfaces interface-name extensive command.

How to Compress PPP Fields

Understanding PPP Field Compression

For interfaces with PPP, PPP CCC, or PPP TCC encapsulation, you can configure compression of the Data Link Layer address, control, and protocol fields, as defined in RFC 1661, The Point-to-Point Protocol (PPP). By default, the address, control, and protocol fields are not compressed. Compressing these fields conserves bandwidth by transmitting less data.

Considerations:

  • The PPP session restarts when you configure or modify compression options.

  • The address, control, and protocol fields cannot be compressed in Link Control Protocol (LCP) packets.

Configure PPP Address and Control Field Compression

Use address and control field compression (ACFC) to conserve bandwidth by transmitting less data. By default, the address and control fields are not compressed. This means PPP-encapsulated packets are transmitted with two one-byte fields (0xff and 0x03). If you configure ACFC and ACFC is successfully negotiated with the local router's peer, the local router transmits packets without these two bytes.

On M320, M120, and T Series routers, ACFC is not supported for any ISO family protocols. Do not include the acfc statement at the [edit interfaces interface-name ppp-options compression] hierarchy level when you include the family iso statement at the [edit interfaces interface-name unit logical-unit-number] hierarchy level.

To configure ACFC:

  1. In configuration mode, go to the [edit interfaces interface-name ppp-options] hierarchy level.
  2. Include the compression statement at the [edit interfaces interface-name ppp-options] hierarchy level, and specify acfc.

Configure PPP Protocol Field Compression

For all protocols with identifiers in the range 0x0000 through 0x00ff, you can configure the router to compress the protocol field to one byte. This is known as Protocol Field Compression (PFC).

By default, the protocol field is not compressed. This means PPP-encapsulated packets are transmitted with a two-byte protocol field. For example, IPv4 packets are transmitted with the protocol field set to 0x0021, and MPLS packets are transmitted with the protocol field set to 0x0281.

To configure PFC:

  1. In configuration mode, go to the [edit interfaces interface-name ppp-options] hierarchy level.
  2. Include the compression statement at the [edit interfaces interface-name ppp-options] hierarchy level, and specify pfc.

Monitor PPP Field Compression

If ACFC and PFC are successfully negotiated, the local router sends packets with compressed protocol fields. To monitor whether negotiation was successful, issue the show interfaces interface-name command. Configured options are displayed in the Link flags field for the physical interface. Successfully negotiated options are displayed in the flags field for the logical interface. In this example, both ACFC and PFC are configured, but neither compression feature has been successfully negotiated.

This configuration causes the local router to try to negotiate ACFC and PFC with its peer. When you include the compression statement in the configuration, the PPP session restarts, and the local router sends the ACFC and PFC options in the LCP Configure-Request packet. The ACFC and PFC options inform the local router's peer that the local router can receive packets with compression.

If the peer indicates that it, too, can receive packets with compression, then ACFC and PFC are negotiated. If ACFC is successfully negotiated, the local router can receive packets with or without the address and control bytes included. If PFC is successfully negotiated, the local router can receive packets with either 2-byte (uncompressed) or 1-byte (compressed) protocol fields.

Monitoring a PPP Session

You can monitor PPP packet exchanges. When monitoring is enabled, packets exchanged during a session are logged by default to /var/log/pppd, or to the file specified in the traceoptions statement.

To monitor a PPP session:

  1. In configuration mode, go to the [edit protocols ppp] hierarchy level.
  2. Include the monitor-session statement.

When monitoring is configured, the operational mode commands show ppp summary and show ppp interface display a Monitored flag in the Session flags column or line.

Tracing Operations of the pppd Process

You can trace the operations of the device’s pppd process. To trace the device’s pppd process:

  1. In configuration mode, go to the [edit protocols ppp] hierarchy level.
  2. Include the traceoptions statement. To specify more than one tracing operation, include multiple flag statements.