HDLC Encapsulation Configuration and Troubleshooting

HDLC Encapsulation Configuration and Troubleshooting

This article explains how to configure HDLC Encapsulation on a point-to-point serial link, as well as some troubleshooting considerations.

WAN ENCAPSULATION PROTOCOLS

In each WAN connection, the data in the frames is encapsulated before crossing the WAN link. To ensure that the correct protocol is used, the corresponding layer 2 encapsulation type must be configured.
The protocol option depends on the WAN technology and the communication equipment. In Image 1, the most common WAN protocols and where they are used are shown.

WAN PROTOCOL TYPES

The following are brief descriptions of each type of WAN protocol:

• HDLC : is the default encapsulation type in point-to-point connections, dedicated links and circuit switched connections when the link uses two Cisco devices. Now, HDLC is the basis for synchronous PPP that many servers use to connect to a WAN, usually the Internet.
• PPP : provides router-to-router and host-to-network connections through synchronous and asynchronous circuits. PPP works with several network layer protocols, such as IPv4 and IPv6. It uses the HDLC encapsulation protocol, but it also has built-in security mechanisms such as PAP and CHAP.
• Serial Line Internet Protocol (SLIP) : it is a standard protocol for serial point-to-point connections via TCP / IP. PPP largely replaced the SLIP protocol.
• Balanced Link Access Procedure (LAPB) X.25 : is a ITU-T standard that defines how connections between a DTE and a DCE are maintained for remote terminal access and computer communications in public data networks .
• Frame Relay : is a switched data link layer protocol and an industry standard that handles several virtual circuits. Frame Relay eliminates some of the prolonged processes (such as error correction and flow control) used in X.25.
• ATM : is the international cell relay standard in which devices send various types of services (such as voice, video or data) in fixed-length cells (53 bytes). Fixed-length cells allow processing to be carried out on the hardware, which reduces traffic delays.

HDLC ENCAPSULATION

HDLC is a bit-oriented synchronous data link layer protocol developed by the International Organization for Standardization (ISO). The current standard for HDLC is ISO 13239 .
HDLC was developed from the synchronous data link control (SDLC) standard proposed in the 1970s. In addition, it  provides connection-oriented and offline service .
HDLC uses synchronous serial transmission, which provides error-free communication between two points. It also defines a layer 2 frame structure that allows flow and error control through the use of acknowledgments. Each frame has the same format either a data frame or a control frame.
When frames are transmitted by synchronous or asynchronous links, those links have no mechanism to mark either the beginning or the end of the frames. For this reason, HDLC uses a frame delimiter, or indicator, to mark the beginning and end of each frame.
Cisco developed an extension of the HDLC protocol to resolve the inability to provide multiprotocol compatibility. While Cisco HDLC (also known as cHDLC) is an exclusive protocol, Cisco allowed many other network equipment providers to implement it. Cisco HDLC frames contain a field to identify the network protocol that is encapsulated. In Image 2, the HDLC standard is compared with Cisco HDLC.

TYPES OF HDLC FRAMES

HDLC defines three types of frames, each with a different control field format.

• MARKER / INDICATOR

The Indicator field starts and ends error verification. The frame always begins and ends with an 8-bit Indicator field. The bit pattern is 01111110.
Because there is a probability that this pattern occurs in the actual data, the sending HDLC system always inserts a 0 bit after every five consecutive 1 in the data field, so that in practice, the sequence of indicators only It can occur at the ends of the plot. The receiving system eliminates the bits entered. When frames are transmitted consecutively, the end indicator of the first frame is used as the start indicator of the next frame.

• DIRECTION

The Address field contains the HDLC address of the secondary station. This address may contain a specific address, a group address or a broadcast address. A primary address is an origin or a communication destination, which eliminates the need to include the address of the main station.

• CONTROL

The Control field uses three different formats, depending on the type of HDLC frame used:

• Information frame (I) : frames I carry upper layer information and certain control information. This frame sends and receives sequence numbers, and the final polling bit (P / F) performs flow and error control.

The send sequence number refers to the frame number that must be sent next. The reception sequence number provides the frame number that is received next.
Both the sender and the receiver maintain send and receive sequence numbers.
The main stations use the P / F bit to inform the secondary ones if they require an immediate response. The secondary stations use the P / F bit to inform the main ones if the current frame is the last one in their current response.

• Supervisory frame (S) : S frames provide control information. In addition, they can request and suspend the transmission, report on the status and confirm receipt of the I frames. The S frames do not have an information field.
• Unnumbered frame (U) : U frames support control functions and are not sequential. Depending on the function of the U frame, the control field is 1 byte or 2 bytes. Some U frames have an information field.

PROTOCOL*

Only used in Cisco HDLC. This field specifies the type of protocol encapsulated within the frame (eg, 0x0800 for IP).

DATA

The data field contains a route information unit (PIU) or exchange identification information (XID).

FRAME CHECK SEQUENCE (FCS, FRAME CHECK SEQUENCE)

The FCS precedes the delimiter of the end indicator and is generally a remainder of the cyclic redundancy check (CRC) calculation. The CRC calculation is performed again at the receiver. If the result differs from the value in the original frame, an error is assumed to exist.

HDLC ENCAPSULATION CONFIGURATION

Cisco HDLC is the default encapsulation method used by Cisco devices on synchronous serial lines.
Use Cisco HDLC as a point-to-point protocol on leased lines between two Cisco devices. If you connect non-Cisco devices, use synchronous PPP .
If the default encapsulation method was modified, use the encapsulation hdlc command in privileged EXEC mode to re-enable HDLC.
Router (config) # interface s0 / 0/0 
Router (config-if) # encapsulation hdlc

TROUBLESHOOTING A SERIAL INTERFACE

The result of the show interfaces serial command shows specific information of the serial interfaces.
When HDLC is configured, HDLC encapsulation should appear in the result, as highlighted in Image.

Serial 0/0/0 is up, Line Protocol is up indicates that the line is active and functioning; encapsulation HDLC indicates that the default serial encapsulation (HDLC) is enabled.
The show controllers command is another important diagnostic tool for troubleshooting serial lines, as shown in Image.

The result indicates the status of the interface channels and whether a cable is connected to the interface or not. In the illustration, the serial interface 0/0/0 has a DCE V.35 cable connected.
The syntax of the commands varies, depending on the platform. Cisco 7000 series routers use a cBus controller card to connect serial links. With these routers, use the show controllers cbus command .
If the result of the electrical interface appears as UNKNOWN instead of V.35, EIA / TIA-449 or some other type of electrical interface, it is likely that the problem is a badly connected cable. It is also possible that there is a problem with the internal wiring of the card.
If the electrical interface is unknown, the corresponding result for the show interfaces serial command shows that the interface and the line protocol are inactive.