BCP, PPP Bridging Control Protocol

Description Glossary RFCs Publications Obsolete RFCs


Protocol suite: PPP.
Protocol type:PPP network control protocol.
PPP Protocol:0x8031.
Working groups: pppext, Point-to-Point Protocol Extensions.
Links: PPP Assigned numbers.

The BCP protocol is used for establishing and configuring Remote Bridging over PPP links.

RFC 3518:

BCP packets may not be exchanged until PPP has reached the Network-Layer Protocol phase. An implementation SHOULD be prepared to wait for Authentication and Link Quality Determination to finish before timing out waiting for a Configure-Ack or other response. It is suggested that an implementation give up only after user intervention or a configurable amount of time.

RFC 1638 pages 1, 2, 9 and 10:

Two basic algorithms are ambient in the industry for Bridging of Local Area Networks. The more common algorithm is called "Transparent Bridging", and has been standardized for Extended LAN configurations by IEEE 802.1. The other is called "Source Route Bridging", and is prevalent on IEEE 802.5 Token Ring LANs.

The IEEE has combined these two methods into a device called a Source Routing Transparent (SRT) bridge, which concurrently provides both Source Route and Transparent bridging.

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

Before any Bridged LAN Traffic or BPDUs may be communicated, PPP MUST reach the Network-Layer Protocol phase, and the Bridging Control Protocol MUST reach the Opened state.

PPP header BCP header Data :::

BCP header:

0001020304050607 0809101112131415 1617181920212223 2425262728293031
Code Identifier Length
Data :::

Code. 8 bits.
Specifies the function to be performed. All other Codes SHOULD be treated as unrecognized and SHOULD result in Code-Rejects.

0Vendor Specific. RFC 2153

Identifier. 8 bits.
Used to match requests and replies.

Length. 16 bits.
Size of the packet including the header.

Data. Variable length.
Zero or more bytes of data as indicated by the Length field. This field may contain one or more Options.

BCP Configuration Options:

0001020304050607 0809101112131415
Option Length
Data :::

Option. 8 bits.

14Bridge-Identification. RFC 3518
24Line-Identification. RFC 3518
33MAC-Support. RFC 3518
43Tinygram-Compression. RFC 3518
53LAN-Identification (obsoleted). RFC 3518
68MAC-Address. RFC 3518
72+Spanning-Tree-Protocol (old formatted). RFC 3518
83IEEE 802 Tagged Frame. RFC 3518
92Management Inline. RFC 3518
102Bridge Control Packet Indicator. RFC 3518

Length. 8 bits.

Data. Variable length.


A hardware device that connects two LANs together.

Remote Bridge.
(RFC 1638) Interconnected LANs via an intermediate medium such as a leased line.

Source Routing.
(RFC 1638) The originating system has the responsibility of indicating the path that the message should follow. Each bridge en route determines from this source route list whether it should accept the message and how to forward it.

Spanning Tree Protocol.
(RFC 1638) This protocol is executed between the bridges to detect and logically remove redundant paths from the network.

Transparent Bridge.
(RFC 1638) Essentially, the bridges in a network operate as isolated entities, largely unaware of each others' presence.


[RFC 3518] Point-to-Point Protocol (PPP) Bridging Control Protocol (BCP).


Obsolete RFCs:

[RFC 1638] PPP Bridging Control Protocol (BCP).

[RFC 2878] PPP Bridging Control Protocol (BCP).

Description Glossary RFCs Publications Obsolete RFCs