Configuring the PPP Protocol Field Compression

For interfaces with PPP, PPP CCC, or PPP TCC encapsulation, you can configure protocol field compression. 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.

For all protocols with identifiers in the range 0x0000 through 0x00ff, you can configure the router to compress the protocol field to one byte, as defined in RFC 1661, The Point-to-Point Protocol (PPP). Protocol field compression (PFC) allows you to conserve bandwidth by transmitting less data.

NOTE: The protocol field cannot be compressed in Link Control Protocol (LCP) packets. The PPP session restarts when you configure or modify compression options.

To configure PFC, include the compression statement at the [edit interfaces interface-name ppp-options] hierarchy level, and specify pfc:

[edit interfaces interface-name ppp-options]

compression pfc;

This configuration causes the local router to try to negotiate PFC with its peer. If PFC is successfully negotiated, the local router sends packets with compressed protocol fields. When you include the compression pfc statement in the configuration, the PPP session restarts, and the local router sends the PFC option in the LCP Configure-Request packet. The PFC option informs 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 PFC is negotiated. If PFC is successfully negotiated, the local router can receive packets with either two-byte (uncompressed) or one-byte (compressed) protocol fields.

To monitor the configuration, 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 only ACFC has been successfully negotiated.

user@router# run show interfaces so-0/1/1 

Physical interface: so-0/1/1, Enabled, Physical link is Up

  Interface index: 133, SNMP ifIndex: 27

  Link-level type: PPP, MTU: 4474, Clocking: Internal, SONET mode, Speed: OC3, 
Loopback: None, FCS: 16,

  Payload scrambler: Enabled

  Device flags   : Present Running

  Interface flags: Point-To-Point SNMP-Traps 16384

  Link flags     : No-Keepalives ACFC PFC

  LCP state: Opened

  NCP state: inet: Opened, inet6: Not-configured, iso: Not-configured, mpls: 
Not-configured

  CHAP state: Not-configured

  CoS queues     : 4 supported

  Last flapped   : 2004-12-29 10:49:32 PST (00:18:35 ago)

  Input rate     : 0 bps (0 pps)

  Output rate    : 0 bps (0 pps)

  SONET alarms   : None

  SONET defects  : None

  Logical interface so-0/1/1.0 (Index 68) (SNMP ifIndex 169) 

    Flags: Point-To-Point SNMP-Traps ACFC Encapsulation: PPP

    Protocol inet, MTU: 4470

      Flags: None

      Addresses, Flags: Is-Preferred Is-Primary

        Destination: 3.3.3/24, Local: 3.3.3.2, Broadcast: 3.3.3.255