PPP Configuration Options


In the previous section, you were introduced to LCP options you can configure to meet specific WAN connection requirements. PPP may include the following LCP options:
Authentication - Peer routers exchange authentication messages. Two authentication choices are Password Authentication Protocol (PAP) and Challenge Handshake Authentication Protocol (CHAP). Authentication is explained in the next section.
Compression - Increases the effective throughput on PPP connections by reducing the amount of data in the frame that must travel across the link. The protocol decompresses the frame at its destination. Two compression protocols available in Cisco routers are Stacker and Predictor.
Error detection - Identifies fault conditions. The Quality and Magic Number options help ensure a reliable, loop-free data link. The Magic Number field helps in detecting links that are in a looped-back condition. Until the Magic-Number Configuration Option has been successfully negotiated, the Magic-Number must be transmitted as zero. Magic numbers are generated randomly at each end of the connection.
Multilink - Cisco IOS Release 11.1 and later supports multilink PPP. This alternative provides load balancing over the router interfaces that PPP uses. Multilink PPP (also referred to as MP, MPPP, MLP, or Multilink) provides a method for spreading traffic across multiple physical WAN links while providing packet fragmentation and reassembly, proper sequencing, multivendor interoperability, and load balancing on inbound and outbound traffic. Multilink is not covered in this course.
PPP Callback - To enhance security, Cisco IOS Release 11.1 and later offers callback over PPP. With this LCP option, a Cisco router can act as a callback client or a callback server. The client makes the initial call, requests that the server call it back, and terminates its initial call. The callback router answers the initial call and makes the return call to the client based on its configuration statements. The command is ppp callback [accept | request].
When options are configured, a corresponding field value is inserted into the LCP option field.
PPP Configuration Commands
Before you actually configure PPP on a serial interface, we will look at the commands and the syntax of these commands as shown in the figure. This series of examples shows you how to configure PPP and some of the options.
Example 1: Enabling PPP on an Interface
To set PPP as the encapsulation method used by a serial or ISDN interface, use the encapsulation ppp interface configuration command.
The following example enables PPP encapsulation on serial interface 0/0:
R3#configure terminal
R3(config)#interface serial 0/0
R3(config-if)#encapsulation ppp
The encapsulation ppp command has no arguments, however, you must first configure the router with an IP routing protocol to use PPP encapsulation. You should recall that if you do not configure PPP on a Cisco router, the default encapsulation for serial interfaces is HLDC.
Example 2: Compression
You can configure point-to-point software compression on serial interfaces after you have enabled PPP encapsulation. Because this option invokes a software compression process, it can affect system performance. If the traffic already consists of compressed files (.zip, .tar, or .mpeg, for example), do not use this option. The figure shows the command syntax for the compress command.
To configure compression over PPP, enter the following commands:
R3(config)#interface serial 0/0
R3(config-if)#encapsulation ppp
R3(config-if)#compress [predictor | stac]
Example 3: Link Quality Monitoring
Recall from our discussion on LCP phases that LCP provides an optional link quality determination phase. In this phase, LCP tests the link to determine whether the link quality is sufficient to use Layer 3 protocols. The command ppp qualitypercentage ensures that the link meets the quality requirement you set; otherwise, the link closes down.
The percentages are calculated for both incoming and outgoing directions. The outgoing quality is calculated by comparing the total number of packets and bytes sent to the total number of packets and bytes received by the destination node. The incoming quality is calculated by comparing the total number of packets and bytes received to the total number of packets and bytes sent by the destination node.
If the link quality percentage is not maintained, the link is deemed to be of poor quality and is taken down. Link Quality Monitoring (LQM) implements a time lag so that the link does not bounce up and down.
This example configuration monitors the data dropped on the link and avoids frame looping:
R3(config)#interface serial 0/0
R3(config-if)#encapsulation ppp
R3(config-if)#ppp quality 80
Use the no ppp quality command to disable LQM.
Example 4: Load Balancing Across Links
Multilink PPP (also referred to as MP, MPPP, MLP, or Multilink) provides a method for spreading traffic across multiple physical WAN links while providing packet fragmentation and reassembly, proper sequencing, multivendor interoperability, and load balancing on inbound and outbound traffic.
MPPP allows packets to be fragmented and sends these fragments simultaneously over multiple point-to-point links to the same remote address. The multiple physical links come up in response to a user-defined load threshold. MPPP can measure the load on just inbound traffic, or on just outbound traffic, but not on the combined load of both inbound and outbound traffic.
The following commands perform load balancing across multiple links:
Router(config)#interface serial 0/0
Router(config-if)#encapsulation ppp
Router(config-if)#ppp multilink
The multilink command has no arguments. To disable PPP multilink, use the no ppp multilink command.
Verifying PPP Encapsulation Configuration
Use the show interfaces serial command to verify proper configuration of HDLC or PPP encapsulation. The command output in the figure shows a PPP configuration.
When you configure HDLC, the output of the show interfaces serial command should show "encapsulation HDLC". When you configure PPP, you can check its LCP and NCP states.
Troubleshooting the Serial Encapsulation Configuration
By now you are aware that debug command is used for troubleshooting and is accessed from privileged exec mode of the command line interface. Debug displays information about various router operations and the related traffic generated or received by the router, as well as any error messages. It is a very useful and informative tool, but you must always remember that Cisco IOS treats debug as a high priority task. It can consume a significant amount of resources, and the router is forced to process-switch the packets being debugged. Debug must not be used as a monitoring tool-it is meant to be used for a short period of time for troubleshooting. When troubleshooting a serial connection, you use the same approach as you have used in other configuration tasks.
Use the debug ppp command to display information about the operation of PPP. The figure shows the command syntax. The no form of this command disables debugging output.
Output of the debug ppp packet Command
A good command to use when troubleshooting serial interface encapsulation is the debug ppp packet command. The example in the figure is output from the debug ppp packet command as seen from the Link Quality Monitor (LQM) side of the connection. This display example depicts packet exchanges under normal PPP operation. This is only a partial listing, but enough to get you ready for the practice lab.
Look at each line in the output and match it to the meaning of the field. Use the following to guide your examination of the output.
PPP - PPP debugging output.
Serial2 - Interface number associated with this debugging information.
(o), O - The detected packet is an output packet.
(i), I - The detected packet is an input packet.
lcp_slqr() - Procedure name; running LQM, send a Link Quality Report (LQR).
lcp_rlqr() - Procedure name; running LQM, received an LQR.
input (C021) - Router received a packet of the specified packet type (in hexadecimal). A value of C025 indicates packet of type LQM.
state = OPEN - PPP state; normal state is OPEN.
magic = D21B4 - Magic Number for indicated node; when output is indicated, this is the Magic Number of the node on which debugging is enabled. The actual Magic Number depends on whether the packet detected is indicated as I or O.
datagramsize = 52 - Packet length including header.
code = ECHOREQ(9) - Identifies the type of packet received in both string and hexadecimal form.
len = 48 - Packet length without header.
id = 3 - ID number per Link Control Protocol (LCP) packet format.
pkt type 0xC025 - Packet type in hexadecimal; typical packet types are C025 for LQM and C021 for LCP.
LCP ECHOREQ (9) - Echo Request; value in parentheses is the hexadecimal representation of the LCP type.
LCP ECHOREP (A) - Echo Reply; value in parentheses is the hexadecimal representation of the LCP type.
Output of the debug ppp negotiation Command
The figure shows the output of the debug ppp negotiation command in a normal negotiation, where both sides agree on network control program (NCP) parameters. In this case, protocol type IP is proposed and acknowledged. Taking the output a line or two at a time:
The first two lines indicate that the router is trying to bring up the LCP and will use the indicated negotiation options (Quality Protocol and Magic Number). The value fields are the values of the options themselves. C025/3E8 translates to Quality Protocol LQM. 3E8 is the reporting period (in hundredths of a second). 3D56CAC is the value of the Magic Number for the router.
ppp: sending CONFREQ, type = 4 (CI_QUALITYTYPE), value = C025/3E8
ppp: sending CONFREQ, type = 5 (CI_MAGICNUMBER), value = 3D56CAC
The next two lines indicate that the other side negotiated for options 4 and 5 and that it requested and acknowledged both. If the responding end does not support the options, the responding node sends a CONFREJ. If the responding end does not accept the value of the option, it sends a CONFNAK with the value field modified.
ppp: received config for type = 4 (QUALITYTYPE) acked
ppp: received config for type = 5 (MAGICNUMBER) value = 3D567F8 acked (ok)
The next three lines indicate that the router received a CONFACK from the responding side and displays accepted option values. Use the rcvd id field to verify that the CONFREQ and CONFACK have the same id field.
PPP Serial4: state = ACKSENT fsm_rconfack(C021): rcvd id 5
ppp: config ACK received, type = 4 (CI_QUALITYTYPE), value = C025

ppp: config ACK received, type = 5 (CI_MAGICNUMBER), value = 3D56CAC
The next line indicates that the router has IP routing enabled on this interface and that the IPCP NCP negotiated successfully.
ppp: ipcp_reqci: returning CONFACK
(ok)
Output of the debug ppp error Command
You can use the debug ppp error command to display protocol errors and error statistics associated with PPP connection negotiation and operation. These messages might appear when the Quality Protocol option is enabled on an interface that is already running PPP. The figure shows an example.
Look at each line in the output and match it to the meaning of the field. Use the following to guide your examination of the utput.
PPP - PPP debugging output.
Serial3(i) - Interface number associated with this debugging information; indicates that this is an input packet.
rlqr receive failure - Receiver does not accept the request to negotiate the Quality Protocol option.
myrcvdiffp = 159 - Number of packets received over the time period specified.
peerxmitdiffp = 41091 - Number of packets sent by the remote node over this period.
myrcvdiffo = 2183 - Number of octets received over this period.
peerxmitdiffo = 1714439 - Number of octets sent by the remote node over this period.
threshold = 25 - Maximum error percentage acceptable on this interface. You calculate this percentage using the threshold value entered in the ppp quality percentage interface configuration command. A value of 100 minus number is the maximum error percentage. In this case, a number of 75 was entered. This means that the local router must maintain a minimum 75 percent non-error percentage, or the PPP link closes down.
OutLQRs = 1 - Current send LQR sequence number of the local router.
LastOutLQRs = 1 - Last sequence number that the remote node side has seen from the local node.
This is all about PPP

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