1. 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.

2. Point-to-Point Protocol (PPP)
This chapter focuses only on encapsulating TCP/IP.
Today, there are essentially two data-link layer protocols used to encapsulate TCP/IP:

3. 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.

4. 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.

5. 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.

6. 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.

7. 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.

8. 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.

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

10. 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 multilink PPP - MLP (for load balancing).

11. 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.

12. 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.

13. 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 pre-assigned username and password. 

14. 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). 

15. 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 Cisco routers support Stacker, Predictor, and Microsoft Point to Point Compression (MPPC). .

16. 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.

17. 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).

18. 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. (because you control the lines, as opposed to tunneling through the internet.)

19. 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.

20. 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.

21. 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.

22. 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.

23. Configuring PAP Example
Enable PPP encapsulation and PAP authentication with the following commands: Router(config-if)#encapsulation ppp Router(config-if)#ppp authentication pap
STOP HERE PLEASE

24. 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:

25. Configuring PAP Example
Router(config)#username username password password
Router(config)#username Romeo password Juliet

26. 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

28. 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.

29. 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.

30. 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.

31. 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 2 RTA(config-if)#encapsulation ppp

32. Configuring PPP
Note that the encapsulation command is issued in interface configuration mode, not line configuration mode.

33. 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.

35. 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.

36. 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.

37. 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.

38. 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.

39. Configuring interactive PPP sessions
The following example shows how to configure interface async 1:
RTA(config)#interface async 1 RTA(config-if)#encapsulation ppp RTA(config-if)#async mode interactive.
Second, you must configure the corresponding terminal line(s) with the autoselect ppp command in line configuration mode.

40. 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

41. 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.

42. 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.

44. 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.

46. 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.

47. 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.

48. 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.

49. 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 1 RTA(config-if)#ip address

50. 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.

51. 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.

52. 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.

53. 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 0 RTA(config-if)#ip address RTA(config-if)#exit RTA(config)#interface async 1 RTA(config-if)#ip unnumbered loopback 0.

54. 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.

55. 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.

56. 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.

57. Configuring the interface addressing method
RTA(config)#IP local pool DIAL-IN
RTA(config)#Interface group-async 1.
RTA(config-if)# peer default ip address pool DIAL-IN.

58. 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.

59. 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.

60. 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.