Faults of Link Layer

I. Fault Description

The following faults occur at the link layer:

z The physical port is UP and the protocol is DOWN.

z The physical port is UP and the protocol is UP, but the opposite port cannot be

pinged.

z The physical port is UP and the protocol is UP, but many packets are lost when the

data traffic is large. The problem usually occurs at the Ethernet interface. Execute the command display port to confirm the faults.

II. Fault Analysis

The possible causes are as follows:

z The port is active, but the link negotiation fails.

The system prompts that the port (such as serial number) is up, but line protocol is down.

z The address mapping of the link layer or the operational mode is incorrect. z The format of frames at the link layer is incorrect.

III. Fault Location Procedure

To locate the fault, check whether

z The protocol is correct (such as PPP, X.25, and FR). z The operational mode is correct (DCE and DTE).

z The frame format of the link layer is correct (X.25, FR, and Ethernet).

z The address mapping of the link layer is correct (when X.25 and FR protocols are

encapsulated).

z The IP address is correct (X.25 and FR).

Note:

z PPP is a widely used WAN protocol. It supports synchronous/asynchronous transmission media

simultaneously and also supports the dialing mode. During daily life and work, the network connection modes such as dial-up access and DDN uses encapsulated PPP protocol.

z X.25 protocol is the interface protocol between the DTE and DCE. It describes how to establish virtual

circuits, transmit packets, set up links, transmit data, and disconnect links and virtual circuits between DTE and DCE. In the meantime, it performs error control, flow control, and condition measurement and provides users with some optional service functions and configuration functions.

z The frame relay (FR) is a simple and effective packet switching protocol.

IV. Troubleshooting

After fault location, you need to solve the problem according to different link protocols. If the PPP protocol is used, proceed as follows:

1) Check the setting of the link layer protocol.

The routers on both sides of the WAN can communicate with each other only when they are set with the same protocol.

When the PPP protocol is adopted and Password Authentication Protocol (PAP) or Challenge Handshake Authentication Protocol (CHAP) is adopted for password authentication, you need to confirm whether the passwords on both sides are correctly set. If they are incorrect, turn on the PPP debug switch.

#debug ppp packet #mon

After the Link Control Protocol (LCP) negotiation succeeds and the LCP enters the UP state, PAP or CHAP negotiation is performed. Then the LCP enters the DOWN state. After debugging, the state of the LCP is changed to the UP state.

V. Solution to Common Problems

Table A-3 lists solutions to some common problems.

Table A-3 Solutions to some common problems

Description Analysis Solution

The PPP protocol of Huawei Quidway 2501 router and that of CISCO router cannot be interconnected with each other.

The default protocol of CISCO router is high-speed digital link channel (HDLC), while the default protocol of Huawei router is PPP. When executing the command display int sN (N is serial port number which can be 0 or 1) on 2501, you find the state of the serial port is UP and that of the link protocol is DOWN.

Change the encapsulated protocol of the CISCO router to PPP (recommended solution) or change the encapsulated protocol at the Quidway side to HDLC using the command link-protocol ppp or link-protocol hdlc.

The two sides connected

encapsulate the X.25 (or the LAPB directly), but the protocol is in Off state all the time. Turn on the debug switch DEBUGGING X.25 PACKET frame, it cycles continuously.

This is because both sides are encapsulated in the same operational mode (DTE or DCE). Change the operational mode at one end.

The X.25 can set up virtual circuits, but frequent reset or clearance occurs during data transmission.

This is because the incorrect setting of flow control parameters. If it is the direct back to back connection, check the local transmit window, receive window, and the flow control parameters that are interconnected to the common packet network and must be consulted with the network management department.

During the configuration of X.25

address mapping, the system prompts that address mapping is repeated.

In X.25 address mapping, an IP address can only correspond to an X.121 address. When an IP address is configured with two different X.121 addresses, conflict occurs.

Execute the command undo x25 map ip to delete the previous address mapping. Re-configure the address mapping.

Description Analysis Solution

The two routers are interconnected

through DDN, and encapsulated with PPP protocol at both ends. Execute the command display to check the state of the port. The physical layer and the protocol layer are all in the UP state, but after the configuration the peer end cannot be pinged.

The PPP protocol belongs to the ISO protocol on the second layer. Thus you must locate the fault from the first layer. Execute the command display int sn (n is the serial port used) to check whether the bottom layer signals of DTR, DSR, RTS, CTS, and DCD are all in the UP state.

If they are not, it shows that the physical line between DTE and DCE is not connected properly.

When the system prompts that the serial port is in the UP state and there is no frame error, it shows the physical layer is normal. In this case, use the command display int sn to check if LCP and IPCP are in the OPEN state. If yes, it shows the PPP verification succeeds.

Observe the debugging information with the BEBUGGING PPP PACKET switch and find out that both of the routers are in REQUEST states during the router LCP negotiation at both sides. This shows that a fault may occur in the intermediate link. In this case, check the state of the intermediate transmission cable.