The Bridging Control Protocol (BCP) is responsible for configuring the bridge protocol parameters on both ends of the point-to-point link. BCP uses the same packet exchange mechanism as the Link Control Protocol. BCP packets can not be exchanged until PPP has reached the Network-Layer Protocol phase. BCP packets received before this phase is reached are discarded.

The Point-to-Point Protocol (PPP) provides a standard method for transporting multi-protocol datagrams over point-to-point links. PPP defines an extensible Link Control Protocol (LCP) and proposes a family of Network Control Protocols (NCP) for establishing and configuring different network-layer protocols. This document defines the NCP for establishing and configuring Remote Bridging for PPP links.

BCP compares the configurations of two devices and seeks to negotiate an acceptable subset of their intersection, to enable correct interoperation even in the presence of minor configuration or implementation differences. In the event that a major misconfiguration is detected, the negotiation will not complete successfully, resulting in the link coming down or not coming up. It is possible that if a bridged link comes up with a rogue peer, network information may be learned from forwarded multicast traffic, or denial of service attacks may be created by closing loops that should be detected and isolated or by offering rogue load.

Such attacks are not isolated to BCP. Any PPP NCP is subject to attack when connecting to a foreign or compromised device. However, no situations arise which are not common to all NCPs; any NCP that comes up with a rogue peer is subject to snooping and other attacks. Therefore, it is recommended that links on which this may happen should be configured to use PPP authentication during the LCP start-up phase.

Protocol Structure

• Code - Decimal value which indicates the type of BCP packet.
• Identifier - Decimal value which aids in matching requests and replies.
• Length - Length of the BCP packet, including the Code, Identifier, Length and Data fields.
• Data - Variable length field which may contain one or more configuration options. The following is a list of BCP configuration options:
  • Bridge-Identification
  • Line-Identification
  • MAC-Support
  • Tinygram-Compression
  • MAC-Address
  • Spanning Tree Protocol (old formatted)
  • IEEE-802-Tagged-Frame
  • Management-Inline
  • Bridge-Control-Packet-Indicator

Related protocols: PPP, PPPoE, PPPoA, SLIP, CHAP, HDLC, LCP, NCP

Sponsor Source: BCP is defined by IETF (http://www.ietf.org) RFC 3518

Reference: http://www.javvin.com/protocol/rfc3518.pdf: Point-to-Point Protocol (PPP) Bridging Control Protocol (BCP).