Troubleshoot Layer 2 protocols

Exam: Cisco 200-301 - Cisco Certified Network Associate (CCNA)

The CCNP exam number 300-135 TSHOOT, deals with the topic of troubleshooting and maintaining the Cisco IP networks mostly. In this chapter we will discuss all that you may need to know about the troubleshoot layer 2 protocols. This is an important chapter from the exam point of view so try to prepare it well.

First of all let’s try to understand what is a layer 2 protocol, well it is the data link layer. It is a part of the link layer. It is a protocol layer that will transfer data between the nearby network nodes that can be found in a WAN (wide area network) or even in local area network segment for that matter.

The data link layer provides the functional means that are needed in order to transfer the data between the network entities. It may also help to detect and correct the errors that may occur in the physical layer of the protocol.

Some examples of the data link layer would be the ppp (point to point protocol), the Ethernet and ADCCP. The data link layer will have two sub layers and these are MAC (media access control) and LLC (logical link control).

The data link layers include the mechanism that can help to recover and detect the transmission errors. The simplest error detection code in a data link layer is parity bit. The data link layers clearly specify how the frame collisions can be detected and recovered from. As a result they also provide ways on how the collisions can be avoided too.


CDP is Cisco discovery protocol. It is a data link layer protocol that was exclusively designed by Cisco. It can help to share information about other Cisco equipments that are connected directly. The CDP is also used in demand routing. This is a common routing method that is used in CDP. The Cisco devices will send the CDP announcements to a number of multicast destination addresses.

It is a default setting by which the CDP announcements are sent in every 60 seconds. Every Cisco device that supports the CDP will store the information that they receive in a table that can be seen using the command "show cdp neighbours command". This table can be accessed through SNAP (sub network access protocol). Each time an announcement is received the CDP information table is refreshed.

The hold time gives the lifetime of the entry into the cdp table. If there is no announcement that is received from the device for a time that exceeds the hold time then the information is considered as discarded. The information will generally contain the hostname, the ip address, the version of the operating system, VTP domain, native LAN, device type and model. The sequence of the information may vary as per the device.

The common troubleshooting that happens with CDP is that when large amount of cdp neighbour announcements are sent there is a possibility that it will consume all the memory in the device. To handle this situation please report to the cisco bug id CSCdu09909. This facility is available to the Cisco registered clients only. The attacker and the target device must be in the same segment for this trigger to work. You can also disable the CDP so that you can protect the devices from this vulnerable situation.


LLDP is link layer discovery protocol.The LLDP can support to set the attributes that are needed to discover the neighbour devices. The LLDP supported devices can use TLV to send and receive information. This protocol generally runs over the data link protocol.

This is exactly why it allows two network layer protocols that are running on two different systems to learn about each otner. The LLDP-MED (media end point devices) can operate between different classes of network like IP phones, network connectivity devices and conference bridges. The LLDP- MED network will support the endpoints that are listed below.

  1. Generic (class 1) - IP communication controllers.
  2. Media (class 2) -include media gateways and conference bridges.
  3. Communication device (class 3) - includes soft phone and Ip phone.

There are many advantages of the LLDP-MED includes fast start capability, provides MIB support, it follows an open standard and it can also support plug and play installation.

There are many trouble shooting mechanisms that you can find in LLDP- MED and these are duplex network policy and speed. They use network management tools for trouble shooting. The LLDP can also be disabled if it is causing a lot of problems.


UDLD is the short form of unidirectional link detection. The UDLD will monitor the physical configuration of links and devices that support the UDLD. When a unidirectional link is detected the UDLD will put the ports that are affected into errdisabled state and will alert the users. The UDLD can operate in both aggressive and in normal mode.

The UDLD can avoid alot of problems like incorrect cabling of fibre strands that are unbundled, excessive flooding of packets and loss of traffic without any notice.

UDLD is a layer 2 protocol but it can also work with the layer 1 protocols. The UDLD is generally disabled on the nonfiber LAN ports. This is done so that unnecessary traffic can be controlled. The UDLD aggressive mode is also disabled. It is only used between network devices that can support this mode.

The UDLD will try to re-establish the connection with the neighbour for eight times. After that the port will automatically disable. You must remember that both aggressive and normal UDLD will support fast UDLD. Fast UDLD is a pre port configuration that supports the UDLD messages that have a time interval of 200 to 1000 milliseconds.

You can use the UDLD error reporting to understand if there is any problem in the proper functioning of the UDLD. As per the reports given by the UDLD error reporting you can take action that is required. Router (config) # udld fast - hello error reporting is the command that is used to enable fast UDLD error reporting.

We hope that this chapter on troubleshoot Layer 2 protocols will help you to get a better score in this section. The exam is a closed book one so the better and more you prepare these topics the better will be your chances to get a good score.

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