Configuring PPP

This section looks at configuring PPP. First, you will enable PPP for asynchronous ports using modems, followed by verification and troubleshooting. Then, the "Scenarios" section discusses how to implement some of PPP's more-advanced features, including authentication, PPP compression, and PPP multilink.

NOTE

PPP over ISDN is covered in more detail in Chapter 6.

Initial PPP Configuration

This section offers a brief list of the steps necessary to configure PPP for asynchronous interfaces. These steps are described in greater detail in the "Scenarios" section. They are as follows:

Step 1. Attach and configure the modem. Then configure the router's asynchronous port. This step is not covered here because you already learned it in Chapter 3, "Modem Connections and Operation Overview." Review it if you feel the need.

Step 2. Configure PPP's asynchronous interface, including PPP encapsulation and authentication methods.

Step 3. Configure network layer addresses, and enable routing.

Step 4. Configure the asynchronous interface for dial-on-demand routing.

Verification and Troubleshooting

For troubleshooting purposes, it is important that you know and understand the different packet types available with LCP. This will assist you in reading the debug output. Table 5-2 describes the packet types you are most likely to encounter.

Table 5-2. LCP Packet Types
Packet Type
Debug Output
Description
Configure-Request
CONFREQ
Notifies the receiving peer of the local configuration parameters.
Configure-Ack
CONFACK
Acknowledges the receipt of CONFREQ packet. Sends back all the options specified in the CONFREQ received from the peer.
Configure-Nak
CONFNAK
Responds to the unacceptable CONFREQ options with a negative acknowledgment. Includes the unacceptable options, marked as such, along with any values that are OK.

Upon receiving the CONFNAK packet, the initiating peer has the option of sending a new CONFREQ with the changed NAKed values or to have those values omitted altogether.
Configure-Reject
CONFREJ
Refuses an option included in the received CONFREQ.

Upon receipt of the CONFREJ packet, the transmitting peer needs to retransmit the CONFREQ, this time without the rejected options.
Terminate-Request
TERMREQ
Requests an existing connection termination.
Terminate-Ack
TERMACK
Acknowledges the receipt of a TERMREQ packet.
Echo-Request
Echo-Request
Confirms connectivity and detects loopback.
Echo-Reply
Echo-Reply
Replies to an Echo-Request packet.

Table 5-2. LCP Packet Types
Packet Type	Debug Output	Description
Configure-Request	CONFREQ	Notifies the receiving peer of the local configuration parameters.
Configure-Ack	CONFACK	Acknowledges the receipt of CONFREQ packet. Sends back all the options specified in the CONFREQ received from the peer.
Configure-Nak	CONFNAK	Responds to the unacceptable CONFREQ options with a negative acknowledgment. Includes the unacceptable options, marked as such, along with any values that are OK. Upon receiving the CONFNAK packet, the initiating peer has the option of sending a new CONFREQ with the changed NAKed values or to have those values omitted altogether.
Configure-Reject	CONFREJ	Refuses an option included in the received CONFREQ. Upon receipt of the CONFREJ packet, the transmitting peer needs to retransmit the CONFREQ, this time without the rejected options.
Terminate-Request	TERMREQ	Requests an existing connection termination.
Terminate-Ack	TERMACK	Acknowledges the receipt of a TERMREQ packet.
Echo-Request	Echo-Request	Confirms connectivity and detects loopback.
Echo-Reply	Echo-Reply	Replies to an Echo-Request packet.

Table 5-3 presents several options that can be included in the CONFREQ packet.

Table 5-3. CONFREQ Packet Options
Packet Option
Description
Maximum-Receive-Unit
Identifies the maximum receive unit (MRU) to the peer.

Note that Cisco IOS ignores the peer's request to increase the MRU above the maximum of the interface MTU. Also, if the peer suggests an MRU that is less than the Cisco IOS interface MTU, the CONFREQ is CONFNAKed, indicating the Cisco IOS interface MTU. It is not recommended for a dialup peer to dynamically adjust the interface MTU to match the negotiated MRU/MRRU. Ideally, the MRU/MTU settings should be identical on both peers.
Multilink-MRRU
Indicates that the local device supports MPPP.
Authentication-Protocol
Advertises the desired authentication protocol.
Magic-Number
Identifies a random number to detect a loopback.
Compression
Announces PPP compression.
Callback
Requests a callback from the peer.

Table 5-3. CONFREQ Packet Options
Packet Option	Description
Maximum-Receive-Unit	Identifies the maximum receive unit (MRU) to the peer. Note that Cisco IOS ignores the peer's request to increase the MRU above the maximum of the interface MTU. Also, if the peer suggests an MRU that is less than the Cisco IOS interface MTU, the CONFREQ is CONFNAKed, indicating the Cisco IOS interface MTU. It is not recommended for a dialup peer to dynamically adjust the interface MTU to match the negotiated MRU/MRRU. Ideally, the MRU/MTU settings should be identical on both peers.
Multilink-MRRU	Indicates that the local device supports MPPP.
Authentication-Protocol	Advertises the desired authentication protocol.
Magic-Number	Identifies a random number to detect a loopback.
Compression	Announces PPP compression.
Callback	Requests a callback from the peer.

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