Specifications

5-9
Cisco AS5x00 Case Study for Basic IP Modem Services
11/24/1999
Section 5 Configuring PPP and Authentication
Testing Asynchronous PPP Connections
LCP negotiation is a series of LCP packets exchanged between PPP peers to negotiate a set of options
and option values when sending data. The LCP negotiation is actually two separate dialogs between two
PPP peers (Peer1 and Peer 2):
Peer 1 and Peer 2 do not have to use the same set of LCP options. When a PPP peer sends its initial
Configure-Request, the response is any of the following:

A Configure-Nack because one or more options have unacceptable values.

A Configure-Reject because one or more of the options are unknown or not negotiable.

A Configure-Ack because all of the options have acceptable values.
When a PPP peer receives a Configure-Nack or Configure-Reject in response to its Configure-Request,
it sends a new Configure-Request with modified options or option values. When a Configure-Ack is
received, the PPP peer is ready to send data.
Figure 5-4 shows an example LCP negotiation process for Peer 1 using the fictional options W, X, Y, Z.
Additionally, Figure 5-4 shows Peer 1 sending data to Peer 2 only. Separate LCP negotiation must be
configured so that Peer 2 can send data back to Peer 1. Very often, the LCP packets for both Peer 1 and
Peer 2 are intermixed during the connection process (that is, Peer 1 is configuring the way it sends data
at the same time as Peer 2.).
Figure 5-4 LCP Layer Negotiations
Figure 5-4 shows that:
1.
Peer 1 sends a Configure-Request requesting option W, option X set to 100, option Y set to 0, and
option Z. (Options W and Z are flag options.)
2.
Peer 2 does not understand option Z so it sends a Configure-Reject containing option Z.
3.
Peer 1 sends a new Configure-Request packet requesting option W, option X set to 100, and option
Y set to 0.
4.
Peer 2 prefers that option X be set to 200 so it sends a Configure-Nack containing option X and its
preferred value.
5.
Peer 1 sends a new Configure-Request packet requesting option W, option X set to 200, and option
Y set to 0.
6.
Peer 2 sends a Configure-Ack.
Each time Peer 1 sends a new Configure-Request, it changes the Identifier value in the LCP header so
that Configure-Requests can be matched with their responses.
28739
Configure-Request: W, X=100, Y=0, Z
1
Configure-Reject: Z
2
Configure-Request: W, X=100, Y=0
3
Configure-Nack: X=200
4
Configure-Request: W, X=200, Y=0
5
Configure-Ack: W, X=200, Y=0
6
Peer 2
Peer 1