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
Explanation:
PPP is a Layer 2 protocol encasulation type that supports both synchronous and asynchronous circuits and
provides built-in security mechanaims. 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 absense 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.
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)#interface Serial0
Router1(config-if)#encapsulation ppp
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 ppp
encapsulation hdlc
encapsulation frame relay
All other options are invalid commands.
Objective:WAN Technologies
Sub-Objective:
Configure and verify PPP and MLPPP on WAN interfaces using local authenticationInternetworking Technology Handbook > WAN Technologies > Point-to-Point Protocol
