PPP-LEX, PPP LAN extension protocol

Description Glossary RFCs Publications Obsolete RFCs


Protocol suite: PPP.
Type:PPP network control protocol.
PPP Protocol:0x8041.

Used for establishing and configuring the LAN Extension Interface Protocol over a PPP link.

RFC 1841.

To accommodate this LAN extension interface architecture, a new Network Control Protocol (NCP) for PPP exists. This NCP is called PPP-LEX. The basic functionality of PPP-LEX is to encapsulate LAN extension interface control and data packets. The IETF has assigned two new protocol types for these functions, as shown:

Protocol typeFunction
0x8041Encapsulates control packets.
0x0041Encapsulates data packets (MAC frames).

PPP is a natural choice for a LAN extension interface protocol because it allows for negotiating a specific control protocol and options at connection time. This means that network administrators do not have to statically configure the router interface for remote interfaces. Rather, remote interfaces negotiate the link at connection time.

The LAN extension interface protocol employs PPP link operation as described in RFC 1331, which reads as follows:

In order to establish communications of a point-to-point link, each end of the PPP link must first send [Link Control Protocol] LCP packets to configure and test the data link. After the link has been established,the peer may be authenticated. Then PPP must send NCP packets to choose and configure one or more network layer protocols. Once each of the chosen network-layer protocols has been configured, datagrams from each network layer protocol can be sent over the link.

The link will remain configured for communications until explicit LCP or NCP packets close the link down, or until some external event occurs (an inactivity timer expires or network administrator intervention).

The LAN extension interface unit and the host router exchange PPP-LCP packets at connection time to dynamically configure and test the WAN serial link. Once the link reaches an "opened" state, the LAN extension interface unit and host router exchange PPP-LEX NCP packets to configure the LAN extension interface protocol. Once it is configured, the NCP (PPP-LEX) reaches an "opened" state, and PPP carries the PPP-LEX control and data packets across the serial link. At this point, link traffic is a any combination of LCPs, PPP-LEX NCPs, PPP-LEX control packets, and PPP-LEX data packets.

Packet format:


LAN Extension Interface Unit.
(RFC 1841) A hardware device installed at remote sites (such as a home office or small branch office) that connects a LAN across a WAN link to a router at a central site.


[RFC 1841] PPP Network Control Protocol for LAN Extension.

Description Glossary RFCs Publications Obsolete RFCs