Patton electronic 1082/I

3.0 PPP OPERATIONAL BACKGROUND

PPP is a protocol used for multi-plexed transport over a point-to-point

link. PPP operates on all full duplex media, and is a symmetric peer-to-

peer protocol, which can be broken into three main components:

•A standard method to encapsulate datagrams over serial links

•A Link Control Protocol (LCP) to establish, conﬁgure, and test the

data-link connection

•A family of Network Control Protocols (NCPs) to establish and conﬁg-

ure different network layer protocols

In order to establish communications over a point-to-point link, each end

of the PPP link must ﬁrst announce its capabilities and agree on the

parameters of the link’s operation. This exchange is facilitated through

LCP Conﬁgure-Request packets.

Once the link has been established and optional facilities have been

negotiated, PPP will attempt to establish a network protocol. PPP will

use Network Control Protocol (NCP) to choose and conﬁgure one or

more network layer protocols. Once each of the network layer protocols

have been conﬁgured, datagrams from the established network layer

protocol can be sent over the link. The link will remain conﬁgured for

these communications until explicit LCP or NCP packets close the link

down, or until some external event occurs.

The PPP Bridging Control Protocol (BCP), deﬁned in RFC 1638, conﬁg-

ures and enables/disables the bridge protocol on both ends of the point-

to-point link. BCP uses the same packet exchange mechanism as the

Link Control Protocol (LCP). BCP is a Network Control Protocol of PPP,

bridge packets may not be exchanged until PPP has reached the net-

work layer protocol phase.

3.1 APPLICATIONS

In situations where a routed network requires connectivity to a remote Ether-

net network, the interface on a router can be conﬁgured as a PPP IP Half

Bridge. The serial line to the remote bridge functions as a Virtual Ethernet

interface, effectively extending the routers serial port connection to the

remote network. The bridge device sends bridge packets (BPDU’s) to the

router's serial interface. The router will receive the layer three address infor-

mation and will forward these packets based on its IP address.