Serial Interfaces

Serial interfaces are interfaces that connect to a device like a CSU/DSU, which in turn connects to a leased line to complete a point-to-point connection.

Serial Encapsulation

The three primary encapsulation types for a serial interface are PPP, HDLC, and Frame Relay. Other encapsulation types include X.25, SMDS, and ATM DXI. To see which encapsulation is being used on a given interface, use the show interface command. Here are brief descriptions of the different encapsulation types:


Point-to-Point Protocol (PPP) encapsulation is initially specified in RFC 1331 and 1332 (and many others). Echo requests are used as keepalives; use no keepalives to disable this feature. PPP is popular and supported by all router vendors. If you are creating a serial link with two different types of routers, you will need to use PPP for the two routers to communicate.


HDLC encapsulation provides synchronous frames and error detection without windowing or retransmission. HDLC is the default encapsulation method for a serial interface, but it is proprietary to Cisco. If you build a serial link with a Cisco router at one end and another type of router at the other end, HDLC is not an option. You will have to use PPP.

If you can use HDLC, you will find that it is simpler to configure and slightly faster than PPP, because it is less general.

Frame Relay

In Frame Relay, your packets are handled by a switched network that provides virtual circuits between you and the sites with ...

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