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In order to exchange traffic over a WAN link, the packets must be encapsulated into a Layer 2 frame while it is crossing the WAN link. You need to configure the correct Layer 2 encapsulation type to ensure that the correct protocol is used. Some encapsulation types are listed on the
Figure.
Encapsulation must be
configured on the router when configuring the interface. In an ISDN
environment, the Point-to-Point Protocol (PPP) is the Layer 2
encapsulation of the B channel. LAPD is the encapsulation for the D
channel.
Either the
proprietary Cisco or Internet Engineering Task Force (IETF) (defined
in RFC 1490) encapsulations are the Layer 2 encapsulations for Frame
Relay. Link Access Procedure, Balanced (LAPB) is the X.25
encapsulation.
The choice of encapsulation protocol depends
on the WAN technology and the communicating equipment. Typical WAN
protocols include the following:
- PPP -- PPP is a standards-based
protocol for router-to-router and host-to-network connections
over synchronous and asynchronous circuits.
- Serial Line Internet Protocol (SLIP)
-- SLIP is an older standard protocol for point-to-point serial
connections using a variation of TCP/IP. SLIP is the predecessor
of PPP.
- High-Level Data Link Control (HDLC)
-- On a Cisco router, HDLC is the default encapsulation type on
serial interfaces. As a bit-oriented synchronous data link layer
protocol, HDLC specifies a data-encapsulation method on
synchronous serial links using frame characters and checksums.
HDLC implementations are proprietary, so Cisco's HDLC is
typically used only when connecting two Cisco devices. When
connecting routers from different vendors, PPP (which is
standards-based) is used instead.
- X.25/LAPB -- X.25 is an ITU-T
standard that defines the way connections between DTE and DCE
devices are maintained for remote terminal access and computer
communications in public data networks. It uses LAPB to provide
high reliability.
- Frame Relay -- Frame Relay is a
high-performance, packet-switched, WAN protocol that can be used
over a variety of network interfaces. It is a very streamlined
protocol that does not have the windowing and reliability of
X.25.
- Asynchronous Transfer Mode (ATM) --
ATM is an international standard for cell relay, in which
multiple service types (such as voice, video, or data) are
conveyed in fixed-length (53-byte) cells. Fixed-length cells
allow processing to occur in hardware, thereby reducing transit
delays. ATM is designed to take advantage of high-speed
transmission media such as E3, Synchronous Optical Network (SONET),
and T3.
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