Ccna routing and Switching Complete Study Guide

Encapsulation HDLC, loopback not set

  Keepalive set (10 sec)

 

Corp#

sh run | begin interface Serial0/0

interface Serial0/0

 ip address 172.16.10.1 255.255.255.252

Note that all I did was add an IP address before I then enabled the interface—pretty 

simple! Now, as long as the SF router is running the default serial encapsulation, this link 

will come up. Notice in the preceding output that the 

show interface

 command does show 

the encapsulation type of HDLC, but the output of 

show running-config

 does not. This 

is important—remember that if you don’t see an encapsulation type listed under a serial 

interface in the active configuration file, you know it’s running the default encapsulation of 

HDLC.

So let’s say you have only one Cisco router and you need to connect to a non-Cisco 

router because your other Cisco router is on order or something. What would you do? You 

couldn’t use the default HDLC serial encapsulation because it wouldn’t work. Instead, you 

would need to go with an option like PPP, an ISO-standard way of identifying the upper-

layer protocols. Now is a great time to get into more detail about PPP as well as how to 

connect to routers using the PPP encapsulation. You can check out RFC 1661 for more 

information on the origins and standards of PPP.

Point-to-Point Protocol (PPP)

Point-to-Point Protocol (PPP) is a Data Link layer protocol that can be used over either 

asynchronous serial (dial-up) or synchronous serial media. It relies on Link Control 

Protocol (LCP) to build and maintain data-link connections. Network Control Protocol 

(NCP) enables multiple Network layer protocols (routed protocols) to be used on a point-

to-point connection.

Because HDLC is the default serial encapsulation on Cisco serial links and it works great, 

why in the world would you choose to use PPP? Well, the basic purpose of PPP is to trans-

port layer 3 packets across a Data Link layer point-to-point link, and it’s nonproprietary. So 

unless you have all Cisco routers, you need PPP on your serial interfaces because the HDLC 

encapsulation is Cisco proprietary, remember? Plus, since PPP can encapsulate several layer 

3 routed protocols and provide authentication, dynamic addressing, and callback, PPP could 

actually be the best encapsulation solution for you over HDLC anyway.

Figure 21.12 shows the PPP protocol stack compared to the OSI reference model.

---

Point-to-Point Protocol (PPP) 

899

f i g u r e   21.12     Point-to-Point Protocol stack

OSI layer

3

2

1

Upper-layer protocols

(such as IP and IPv6)

Network Control Protocol (NCP)

(specific to each Network layer protocol)

Link Control Protocol (LCP)

High-Level Data Link Control (HDLC)

Physical layer

(such as EIA/TIA-232, V.24, V.35, ISDN)

PPP contains four main components: