Which Cisco IOS command is used to configure encapsulation for a PPP serial link on a Cisco router?
- A: encapsulation ppp
- B: encapsulation ip ppp
- C: ip encapsulation ppp
- D: encapsulation ppp-synch
Correct Answer: A
PPP is a Layer 2 protocol encapsulation type that supports both synchronous and asynchronous circuits and provides built-in security mechanisms. The encapsulation ppp interface configuration mode command is used to configure encapsulation for a PPP (Point to Point Protocol) serial link on a Cisco router. PPP encapsulation provides for router-to-router and host-to-network connections over both synchronous and asynchronous circuits. Serial links are configured to use Cisco High Level Data Link Control (HDLC) encapsulation, by default, on Cisco routers. The Cisco version of HDLC is incompatible with the industry standard version used on other router brands because it contains a type field that identifies the underlying network protocol being encapsulated by HDLC. This is a beneficial feature of Cisco HDLC but makes it incompatible with other router brands.
For this reason, a Cisco router that is going to be connected to a non-Cisco router should be configured to use PPP instead of the default. The encapsulation ppp interface configuration mode command will do this. If you set one of the routers for PPP and leave the other router at the default encapsulation for a serial connection, the connection will fail due to incompatible encapsulation.
You would use the show run command to verify matching encapsulation types. In the partial output of the show run command for two routers shown below, it can be seen that although one of the routers has the encapsulation ppp command in its configuration, the other does not. The absence of the encapsulation ppp command means that the default HDLC is being used. This incompatibility will cause both routers to report a serial interface up, line protocol down condition since the connection is live, but the Layer 2 framing is misconfigured.
router1#show run router2#show run
<output omitted> <output omitted>
interface serial 0/0 interface serial 0/1
If authentication between the routers is also required, the authentication pap, authentication ms-chap, or authentication chap commands could be used to apply Password Authentication Protocol (PAP), Microsoft Challenge Authentication Protocol (MS-CHAP), or Challenge Authentication Protocol (CHAP) authentication to the connection, respectively.
A full configuration of a serial link for using PPP with authentication is as shown below:
Router1(config-if)#ppp authentication pap
Note above that the third line enables PAP authentication, which is not secure. Alternately, you can use CHAP authentication (which is secure) with the ppp authentication chap command. Regardless of which authentication mechanism you choose, these authentication commands will only be accepted on an interface where PPP encapsulation has been enabled, which rules out any non-serial interfaces.
The third type of encapsulation that can be configured on a serial WAN link is Frame Relay, which can be selected with the encapsulation frame relay command under the interface.
In summary, the three encapsulation types available for WAN serial links are PPP, HDLC, and Frame Relay. The command for each is as follows, executed under the interface configuration prompt:
encapsulation frame relay
All other options are invalid commands.