Information Technology Reference
In-Depth Information
Step
Purpose
Command
4
Specify the remote router CHAP
authentication name
dialer remote-name
username
5
Specify the remote destination to call
and the map class that defines
characteristics for calls to this
destination
dialer string
dial-string
class
class-name
6
Specify the dialing pool to use for
calls to this destination
dialer pool
number
7
Assign the dialer interface to a dialer
group
dialer-group
group-number
8
Specify an access list by list number
or by protocol and list number to
define the “interesting” packets that
can trigger a call
dialer-list
dialer-group
protocol
protocol-name
{permit | deny | list
access-list-number
}
PPP Operations
The Point-to-Point Protocol (PPP) is by far the most common link-layer transport protocol, having
completely usurped SLIP as the protocol of choice for dial (and, in many cases, nondial) synchronous
and asynchronous serial connections. PPP was originally defined in 1989 by Request For Comments
1134 (RFC 1134), which has since been made obsolete by a series of RFCs culminating (as of this
writing) in RFC 1661. Numerous RFCs also define elements of the protocol, such as RFC 1990 (“The
PPP Multilink Protocol”), RFC 2125 (“The PPP Bandwidth Allocation Protocol”), and many others. An
online repository of RFCs can be found at ftp://ftp.cisco.com/pub/rfc/RFC/.
Perhaps the best definition of just what PPP is can be found in RFC 1661, which states:
•
The Point-to-Point Protocol (PPP) provides a standard method for transporting multiprotocol
datagrams over point-to-point links. PPP is comprised of three main components:
1.
A method for encapsulating multiprotocol datagrams
2.
A Link Control Protocol (LCP) for establishing, configuring, and testing the data-link connection
3.
A family of Network Control Protocols (NCPs) for establishing and configuring different network
layer protocols
Phases of PPP Negotiation
PPP negotiation consists of three phases: Link Control Protocol (LCP), authentication, and Network
Control Protocol (NCP). Each proceeds in order following the establishment of the async or ISDN
connection.
Link Control Protocol
PPP does not follow a client/server model—all connections are peer-to-peer so that, although there is a
caller and a receiver, both ends of the point-to-point connection must agree on the negotiated protocols
and parameters.
