Point-to-Point Protocol (PPP) Chapter 2 explained how to configure an

asynchronous line for a dialup connection. In this chapter, you will learn how to configure that dialup connection to connect to the network using point-to-point protocol (PPP) at the data link layer.

It is based on open standards and includes a variety of essential features, making it the de facto standard for dialup and dedicated WAN connections.

Point-to-Point Protocol (PPP) In order for any layer 3 protocol to traverse

the WAN over a dialup or dedicated link, it must be encapsulated by a data-link layer protocol.

PPP, the Serial Line Internet Protocol (SLIP), and the AppleTalk Remote Access Protocol (ARAP) work at the data-link layer (Layer 2) to encapsulate routed protocols like TCP/IP, Novell IPX and Appletalk.

Point-to-Point Protocol (PPP) The rest of this chapter focuses only on

encapsulating TCP/IP. Today, there are essentially two data-

link layer protocols used to encapsulate TCP/IP:

Point-to-Point Protocol (PPP) SLIP - SLIP is a standard protocol for point-

to-point serial connections, using TCP/IP. SLIP was a predecessor of PPP.

PPP - PPP provides router-to-router and host-to-network connections over synchronous and asynchronous circuits, which can be either dialup or leased lines.

PPP PPP is a more recent standard than

SLIP, and is almost always the preferred data-link layer encapsulation on dialup asynchronous links. On a Cisco router, SLIP is the default encapsulation on an asynchronous dialup interface, so you must manually configure the encapsulation in order to use PPP.

PPP SLIP is essentially limited to use with IP,

whereas PPP can be used for other network-layer protocols such as IPX and AppleTalk. Moreover, PPP supports essential features such as dynamic address allocation, PAP authentication, CHAP authentication, and Multilink PPP. SLIP does not support these features.

PPP Overview High-Level Data Link Control (HDLC) is the

default encapsulation for ISDN and serial interfaces on a Cisco router.

Cisco's HDLC is not necessarily compatible with other vendors' HDLC implementations. PPP implementations follow open standards and are almost always compatible. Thus, PPP is the protocol of choice when configuring serial links in a multivendor environment.

PPP Overview It is important to note that PPP actually

uses HDLC as a basis for encapsulating datagrams. However, PPP is more expansible than HDLC because it adds extensions (features) to the link layer.

PPP Overview PPP can negotiate link options dynamically

and can support multiple Layer 3 protocols (IP, IPX, AppleTalk, etc.). PPP accomplishes these two tasks by encapsulating Layer 3 datagrams with a specialized frame. PPP's frame format is based on the HDLC frame format.

PPP defines the Link Control Protocol (LCP). The job of LCP is to establish, configure, and test the data-link connection.

PPP Overview When hosts negotiate a PPP

connection, they exchange LCP packets. These packets allow link partners to dynamically negotiate link options, including authentication, compression, and MLP.

PPP Overview Once the LCP establishes the Layer 2

connection, the Network Control Protocol (NCP) takes over. Link partners exchange NCP packets to establish and configure different network-layer protocols including IP, IPX, and AppleTalk. Each Layer 3 protocol has its own NCP. For example, IP's NCP is IPCP; IPX's NCP is IPXCP, and Appletalk's NCP is ATALKCP.

PPP Overview The NCP can build up and tear down multiple

Layer 3 protocol sessions over a single data link. This capability is called protocol multiplexing. When a host requests that the connection be terminated, the NCP tears down the Layer 3 sessions and then the LCP tears down the data link.

PPP's components operate at Layer 2 of the OSI model.

Configuring PPP You enable PPP encapsulation on an

interface by using the following command: Router(config-if)#encapsulation ppp

Thus, if you want dial-in hosts on terminal line 2 to use PPP, you would enter the following commands:

RTA(config)#interface async 2RTA(config-if)#encapsulation ppp

Configuring PPP Note that the encapsulation

command is issued in interface configuration mode, not line configuration mode.

Configuring PPP When a remote host dials into an access

server's asynchronous interface, it can start an EXEC session with the router. This feature allows remote users to login to the router and issue commands as if the user was connected to the console port. No IP addressing or PPP encapsulation is needed for this type of connection. Data is sent as asynchronous characters.

Configuring PPP Alternately, a remote host can dial in to an

access server and send a Layer 3 protocol packet encapsulated by PPP, SLIP, or ARA. This type of connection allows the remote user to access network resources such as file servers and mail servers.

You can also configure the router's asynchronous interface to automatically select between PPP data sessions and EXEC sessions.

Configuring PPP Generally, you will want to restrict the ability

of remote users to start EXEC sessions with your router. Typical end users do not require access to the router's interface. Instead, they need a Layer 3 protocol (IP, etc.) connection to the corporate network or the Internet. In most cases, you should force the asynchronous interface to use PPP and not allow an EXEC connection.

Configuring interactive PPP sessions With the PPP autoselect feature, you can

configure an access server's terminal line to provide either a PPP session or an EXEC session based on input from the remote host.

Essentially, this feature allows the remote host to determine the session type. The access server automatically detects which type of session is being requested, and responds accordingly.

Configuring interactive PPP sessions Enabling this feature requires two steps. First,

you must configure the asynchronous interface(s) with the async mode interactive command in interface configuration mode.

This command configures the router so that it allows the remote host to choose either a PPP session or an EXEC session.

Configuring interactive PPP sessions The following example shows how to configure

interface async 1: RTA(config)#interface async 1RTA(config-if)#encapsulation pppRTA(config-if)#async mode interactive.

Second, you must configure the corresponding terminal line(s) with the autoselect ppp command in line configuration mode.

Configuring interactive PPP sessions To complete the example configuration,

you would enter the following commands:

RTA(config)#line 1 RTA(config-line)#autoselect ppp during-login

Configuring interactive PPP sessions The autoselect command permits the

access server to allow an appropriate process to start automatically when a starting character is received. If the start character is a return character, then the access server starts an EXEC session. So, users who want to begin an EXEC session typically must press the Return key after establishing a dialup connection.

Configuring interactive PPP sessions On the other hand, if the access server

recognizes the start character as PPP, SLIP, or ARAP, it will begin a session for whichever protocol it detects.

So, if an end user is using a program that sends a PPP frame, the access server will automatically start a PPP session.

Configuring interactive PPP sessions The during-login option of the autoselect command causes the username/password prompt to display in the remote hosts' terminal window without the user having to press the Return key. This command is not required.

After a host has established an EXEC session, the remote user can switch to a PPP session at any time by issuing the ppp command at the router prompt.

Dedicated PPP Sessions In most cases, you will want to configure your

access server's asynchronous lines in dedicated mode so that users are forced into using PPP. In dedicated mode, an interface is automatically configured for PPP connections. There is no user prompt or EXEC level, and no end-user commands are required to initiate remote-node connections.

Dedicated PPP Sessions To ensure that the dial-in user must run PPP on

the specified line, use the async mode dedicated command:

Router(config-if)#async mode dedicated.

To summarize: Interactive mode places the interface in interactive mode: allows an EXEC session. Dedicated Mode places the interface into dedicated PPP/SLIP mode.

Configuring the interface addressing method Most dialup PPP sessions are established for

the purpose of sending and receiving TCP/IP packets. Asynchronous PPP connections allow remote users to dial up and access the corporate IP network or the Internet. In order for remote nodes to be able to participate in a TCP/IP network, they must have an IP address. This means that the remote node's link partner, the router's asynchronous interface, must have an IP address as well.

Configuring the interface addressing method To assign an IP address to an access server's

asynchronous interface, use the ip address command (which is the same command used to assign addresses to Ethernet or Serial interfaces).

The following example configures the IP address of interface async 1:

RTA(config)#interface async 1RTA(config-if)#ip address 10.1.1.1 255.255.255.

Configuring the interface addressing method Because access servers can have literally

hundreds of asynchronous interfaces, and because all of them are not likely to be in use at the same time, you may wish to conserve IP addresses by using the IP unnumbered feature.

Multiple async interfaces on the same router can share the same IP address, including an address assigned by the IP unnumbered feature.

Configuring the interface addressing method The IP unnumbered feature can only be

used with point-to-point configurations. The syntax for the ip unnumbered command is:

Router(config-if)#ip unnumbered type number.

Configuring the interface addressing method With this command, you must specify

the type and number of the interface to borrow the IP address from (Ethernet 0, Loopback 0, etc.). A loopback interface is a virtual interface that never goes down and is therefore an ideal line to use as the reference with the ip unnumbered command.

Configuring the interface addressing method The following commands illustrate how to

configure an asynchronous interface for IP unnumbered using a loopback interface:

RTA(config)#interface loopback 0RTA(config-if)#ip address 10.1.1.1 255.255.255.0RTA(config-if)#exitRTA(config)#interface async 1RTA(config-if)#ip unnumbered loopback 0.

Configuring the interface addressing method Addressing the access server's

asynchronous interface is only half of the IP configuration equation. You must also implement a mechanism for assigning IP addresses to remote dial-in users.

Configuring the interface addressing method PPP allows for the automatic assignment of

IP addresses using a specific address, an address from a pool defined on the router, or Dynamic Host Configuration Protocol (DHCP). Alternately, you can configure the access server to allow the remote host to choose its own address.

To assign a default (predefined) IP address to the remote dial-in host, use the peer default ip address command.

Configuring the interface addressing method Additionally, the pool and dhcp arguments

allow address allocation from a local pool of addresses or a DHCP server.

Router(config-if)#peer default ip address {address | pool pool name | dhcp}.

The following example shows how to configure a group of asynchronous interfaces (rotary group) to assign IP addresses from a locally defined pool.

Configuring the interface addressing method RTA(config)#IP local pool DIAL-IN

10.1.1.2 10.1.1.254. RTA(config)#Interface group-async 1. RTA(config-if)# peer default ip

address pool DIAL-IN.

Configuring the interface addressing method Note that the pool option to the peer default ip address command require a global command to create the pool of addresses; for example, ip local pool pool-name starting-address end-address.

Configuring the interface addressing method If you have configured an asynchronous

interface for interactive mode, you have the option to allow the IP address to be assigned dynamically by the caller. After the remote user enters the ppp EXEC command, the access server will prompt the user for an IP address or logical host name.

Configuring the interface addressing method To enable this dynamic addressing

feature, use the async dynamic address command in interface configuration mode, as shown:

Router(config-if)#async dynamic address.

PPP LCP Options PPP offers a rich set of features that are

configured by LCP during link establishment. The configuration features negotiated through the LCP are:

Authentication, with PAP or CHAP, is used as a security measure with PPP. Authentication allows the dial-up target to identify that any given dialup client is a valid client with a preassigned username and password. 

PPP LCP Options Callback is a PPP option used to

provide call and dialup billing consolidation. PPP callback was first supported in Cisco IOS Release 11.0(3). 

PPP LCP Options Compression reduces the size of data

frames to be transmitted over a network link which reduces the time required to transmit the frame across the network. PPP compression was first supported in Cisco IOS Release 10.3. Cisco routers support Stacker, Predictor, and Microsoft Point to Point Compression (MPPC). .

PPP LCP Options Multilink PPP (MLP) feature provides load

balancing functionality over multiple WAN links, while providing multivendor interoperability, proper sequencing, and load calculation on both inbound and outbound traffic. Datagrams are split, sequenced, transmitted across multiple links, and then recombined at the destination. The multiple links together are called a bundle.

PPP LCP Options Multilink is especially useful with ISDN

BRI configurations, in which both B channels can be used to achieve 128-kbps throughput. MLP also works with modems to provide additional bandwidth. This protocol was first supported in Cisco IOS Release 11.0(3).

PAP and CHAP Generally, you should always configure

asynchronous lines to require authentication. With PPP, you have the option to require that callers authenticate using one of two authentication protocols, PAP or CHAP. If you are using PPP over a point-to-point leased line, authentication is unnecessary, and should not be configured.

PAP and CHAP PAP is not as good as CHAP

CHAP is considered a superior authentication protocol and should be used when possible. When is it appropriate to run PAP? You may find that hosts running legacy software may not support CHAP, in which case PAP is your only authentication option.

PAP and CHAP When using PAP, the remote host is in

control of the frequency and timing of login requests. This is undesirable, as the access server must respond to all login requests, even the repeated attempts of a hacker to guess, or "brute force," a username/password combination. PAP also sends passwords as clear text over the media, which means a strategically placed packet sniffer could capture and easily decode the password.

PAP and CHAP On the other hand, access servers that

are configured with CHAP are in control of login attempts.

The server must send a challenge packet. The challenge packet consists of an ID, a random number, and the host name of the local router.

PAP and CHAP The CHAP protocol also allows servers

to request that the remote host re-authenticate at any time, which provides another dimension of security. This feature is not typically used with Cisco routers.

Configuring PAP Example Enable PPP encapsulation and PAP

authentication with the following commands: Router(config-if)#encapsulation pppRouter(config-if)#ppp authentication pap

Configuring PAP Example You must also configure the router with a local

username/password database, or point it to a network host that has that information (such as a TACACS+ server). Without access to a username/password database, the router won't know which combinations are authorized and will deny all login attempts. You can configure a local username/password database by using the following command in global configuration mode:

Configuring PAP Example Router(config)#username username password password

Router(config)#username Romeo password Juliet

Configuring PAP Example In some cases, you must also configure a

router's asynchronous interface to place calls to other access servers. If you want to configure an interface to respond to a peer's request to authenticate with PAP, you must use the ppp pap sent-username command:

Router(config-if)#ppp pap sent-username username password password

Configuring PAP Example Note that in router RTA's configuration,

the ppp pap sent-username command is used to specify what username/password information to send in the event that it dials RTB and is asked to authenticate. Router RTB is also configured to send a username and password for PAP, if challenged.

Configuring PAP Example The name included with the username

and dialer map commands is case sensitive. If the remote host's name is RTA, and you create a username entry for rta instead, authentication will fail.

Configuring PAP Example To ensure that both systems in the example

can communicate properly, their asynchronous interfaces have been configured with the dialer map command that includes the remote router's name. By configuring each router with a dialer map statement, each system knows what to do with authentication issues because the systems have prior knowledge of each other's names.