Understanding and Configuring Device Link Detection Protocol (DLDP)

The Device Link Detection Protocol (DLDP) monitors the link status of optical fibers or copper twisted pairs such as super Category 5 twisted pairs. Upon detecting a unidirectional link on an interface, DLDP automatically shuts down or prompts users to manually shut down the interface to prevent network faults.

Requirements

There are no specific requirements for this document.

Components Used

This document is not restricted to specific software and hardware versions.

A Link Is Unidirectional Before DLDP Is Enabled

In Figure 1-1, DLDP is enabled between the optical fibers connecting two switches.

Figure 1-1 Intersected fibers

When DLDP is enabled, interfaces in Up state enter the Active state and send Advertisement packets with RSY tags to notify neighbors and request neighbor information. The following uses Interface 1 as an example to describe the detection process:

1. When receiving an Advertisement packet with the RSY tag from Interface 4, Interface 1 regards that it has detected a neighbor. Interface 1 starts the echo timer, sets up a neighbor entry, and starts the entry aging timer. Interface 1 then enters the Probe state and sends probe packets to detect Interface 4.
2. Interface 4 cannot receive the probe packets from Interface 1, so Interface 1 will not receive echo packets from Interface 4. When the echo timer on Interface 1 times out, Interface 1 enters the Disable state.

The detection process on other interfaces is similar to that on Interface 1. At last, the four interfaces enter the Disable state.

A Link Changes from Bidirectional to Unidirectional After DLDP Is Enabled

As shown in Figure 1-2, optical fibers connect switches.

Figure 1-2 Correct optical fiber connections when a single neighbor exists

When the Tx and Rx optical fibers are working properly, Switch A and Switch B establish a bidirectional relationship as follows:

1. When DLDP is enabled, Interface 1 in Up state enters the Active state and sends Advertisement packets with RSY tags to notify neighbors and request neighbor information.
2. When receiving an Advertisement packet with the RSY tag from Interface 1, Interface 2 regards that it has detected a neighbor. Interface 2 then starts the echo timer, sets up a neighbor entry, and starts the entry aging timer. Interface 2 enters the Probe state and sends a probe packet.
3. Upon receiving the probe packet, Interface 1 sets up a neighbor entry, enters the Probe state, and returns an echo packet to Interface 2.
4. When Interface 2 receives the echo packet, it finds that the neighbor entry exists, and the information about the neighbor carried by the echo packet is the same as that saved on the local device. Interface 2 marks the connection with this neighbor as bidirectional.
5. Interface 2 transitions from the Probe state to the Advertisement state, and periodically sends Advertisement packets. Interface 2 in Advertisement state resets the aging timer for a known neighbor each time a packet is received from the neighbor.

The process for sending Advertisement packets from Interface 2, and setting up a neighbor on Interface 1, is similar to steps 1 to 4 in the preceding explanation. Once these processes have been completed, Interface 1 and Interface 2 regard each other as neighbors with a bidirectional connection.

In Figure 1-3, the Rx optical fiber of Interface 2 has failed and cannot receive an optical signal. When this occurs, Interface 2 enters the Inactive state and stops sending and receiving packets. The Tx optical fiber of Interface 2 remains normal, so Interface 1 can receive signals and remain in Up state. Interface 1 cannot receive DLDPDUs from Interface 2 before the entry aging timer phases out.

Figure 1-3 Disconnection of one optical fiber when a single neighbor exists

The procedure for detecting unidirectional links varies depending on either of the configured DLDP working modes as follows:

• Normal mode: Interface 1 deletes the neighbor entry, enters the Active state, and sends an Advertisement packet with the RSY tag when the entry aging timer times out. After 5 seconds in Active state, Interface 1 enters the Advertisement state. Then Interface 1 retains in Advertisement state and has no neighbor. Interface 2 retains the Inactive state. In this case, DLDP cannot detect unidirectional links.
• Enhanced mode: Interface 1 starts the enhanced timer and echo timer and sends a probe packet to the neighbor when the entry aging timer times out. The Tx optical fiber of Interface 1 is disconnected, making Interface 1 unable to receive the echo packet from Interface 2. When the echo timer times out, Interface 1 enters the Disable state and sends a disable packet to the neighbor. In addition, Interface 1 deletes the neighbor entry and starts the RecoverProbe timer to check whether the Tx optical fiber is restored. Interface 2 retains the Inactive state.
  • In enhanced mode, Interface 2 is physically Down, but Interface 1 cannot detect the change. DLDP supports the fast Link-Down notification mechanism that can rapidly detect a fault on the link connecting Interface 1 and Interface 2 before the entry aging timer times out. Upon detecting that Interface 2 is Down, the physical layer sends a Link-Down notification packet to Interface 1. When receiving the Link-Down notification packet, Interface 1 enters the Disable state.
  • The fast Link-Down notification mechanism applies only to the enhanced mode.

The Device Link Detection Protocol (DLDP) allows a device to identify the remote device and check connectivity of the unidirectional link by exchanging DLDP Data Units (DLDPDUs) with the remote device. DLDP defines the following states: Initial, Inactive, Active, Advertisement, Probe, Disable, and DelayDown.

Figure 1-4 State transition during DLDP packet exchange

DLDP has two working modes: normal mode or enhanced mode. These modes can be selected when a neighbor entry expires, or when a unidirectional link is detected.

DLDP processes a received DLDP packet as follows:

1. Authenticates the DLDP packet and discards the DLDP packet if it fails to be authenticated.
2. Discards the DLDP packet if the interval for sending Advertisement packets in the DLDP packet is different from that on the local device.
3. Processes the DLDP packet if the packet is authenticated and contains the same Advertisement interval as that configured on the local device.

Pre-configuration Tasks

Before configuring DLDP, ensure that the interfaces on both ends work in non-auto-negotiation mode. When directly connected interfaces at both ends are configured with auto-negotiation and DLDP, if one unidirectional link fails, the two interfaces go Down due to auto-negotiation configuration.

Enabling DLDP

Unidirectional links can be detected only when the devices on both ends of optical fibers or copper twisted pairs support DLDP functions.

DLDP can be configured on Layer 2 and Layer 3 Ethernet interfaces. By default, an Ethernet interface works in Layer 2 mode. Before configuring DLDP on a Layer 3 Ethernet interface, switch the Ethernet interface to Layer 3 mode.

Procedure

1. Run system-view

   The system view is displayed.

2. Run dldp enable

   DLDP is enabled globally.

   By default, DLDP is disabled globally and on each interface.

3. Run interface interface-type interface-number

   The interface view is displayed.

   NOTE:

   DLDP cannot be configured on logical interfaces.

4. Run dldp enable

   DLDP is enabled on the interfaces.

(Optional) Configuring the Working Mode of DLDP

If DLDP works in normal mode, the system can identify only unidirectional links caused by intersected fibers.

If DLDP works in enhanced mode, the system can identify unidirectional links caused by intersected fibers or fiber disconnection. To detect unidirectional links caused by disconnection of one optical fiber, manually set the rate and full duplex mode of the connected interfaces. If you do not set the rate and full duplex mode of the connected interfaces, DLDP does not take effect even if it is enabled.

Procedure

1. Run system-view

   The system view is displayed.

2. Run dldp work-mode { enhance | normal }

   The working mode of DLDP is configured.

   By default, the working mode of DLDP is enhance.

Verifying the DLDP Configuration

Run the display dldp [ interface interface-type interface-number ] command to verify the DLDP configuration and neighbor information entries.

• DLDP Configuration Commands
• DLDP Configuration