Specifications

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Cisco AS5800 Operations, Administration, Maintenance, and Provisioning Guide
DOC-7810814=
Chapter 3 Operations
Configuring PPP and Authentication
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 3-11 shows an example LCP negotiation process for Peer 1 using the fictional options W, X, Y,
Z. Additionally, Figure 3-11 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 3-11 LCP Layer Negotiations
Figure 3-11 shows that:
• 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.)
• Peer 2 does not understand option Z so it sends a Configure-Reject containing option Z.
• Peer 1 sends a new Configure-Request packet requesting option W, option X set to 100, and option
Y set to 0.
• Peer 2 prefers that option X be set to 200 so it sends a Configure-Nack containing option X and its
preferred value.
• Peer 1 sends a new Configure-Request packet requesting option W, option X set to 200, and option
Y set to 0.
• 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.
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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