Table of Contents

1 DLDP Configuration· 1-1

Overview· 1-1

Background· 1-1

DLDP Fundamentals· 1-2

DLDP Configuration Task List 1-8

Enabling DLDP· 1-9

Setting DLDP Mode· 1-9

Setting the Interval for Sending Advertisement Packets· 1-10

Setting the DelayDown Timer 1-10

Setting the Port Shutdown Mode· 1-11

Configuring DLDP Authentication· 1-11

Resetting DLDP State· 1-12

Resetting DLDP State in System View· 1-12

Resetting DLDP State in Port view/Port Group View· 1-12

Displaying and Maintaining DLDP· 1-13

DLDP Configuration Example· 1-13

Troubleshooting· 1-15

 

When performing DLDP configuration, go to these sections for information you are interested in:

l          Overview

l          DLDP Configuration Task List

l          Enabling DLDP

l          Setting DLDP Mode

l          Setting the Interval for Sending Advertisement Packets

l          Setting the DelayDown Timer

l          Setting the Port Shutdown Mode

l          Configuring DLDP Authentication

l          Resetting DLDP State

l          Displaying and Maintaining DLDP

l          DLDP Configuration Example

l          Troubleshooting

Overview

Background

Sometimes, unidirectional links may appear in networks. On a unidirectional link, one end can receive packets from the other end but the other end cannot. Unidirectional links result in problems such as loops in an STP-enabled network.

As for fiber links, two kinds of unidirectional links exist. One occurs when fibers are cross-connected, as shown in Figure 1-1. The other occurs when one end of a fiber is not connected or one fiber of a fiber pair gets disconnected, as illustrated by the hollow arrows in Figure 1-2.

Figure 1-1 Unidirectional fiber link: cross-connected fibers

 

Figure 1-2 Unidirectional fiber link: a fiber not connected or disconnected

 

Device Link Detection Protocol (DLDP) can detect the link status of a fiber cable or twisted pair. On detecting a unidirectional link, DLDP can shut down the related port automatically or prompt users to take measures as configured to avoid network problems.

As a data link layer protocol, DLDP cooperates with physical layer protocols to monitor the link status of a device. While the auto-negotiation mechanism provided by the physical layer detects physical signals and faults, DLDP performs operations such as identifying peer devices, detecting unidirectional links, and shutting down unreachable ports. The cooperation of physical layer protocols and DLDP ensures that physical/logical unidirectional links be detected and shut down. For a link with the devices on the both sides of it operating properly, DLDP checks to see if the cable is connected correctly and if packets can be exchanged between the two devices. Note that DLDP is not implemented through auto-negotiation.

DLDP Fundamentals

DLDP link states

A device is in one of these DLDP link states: Initial, Inactive, Active, Advertisement, Probe, Disable, and DelayDown, as described in Table 1-1.

Table 1-1 DLDP link states

State	Indicates…
Initial	DLDP is disabled.
Inactive	DLDP is enabled but the link is down.
Active	DLDP is enabled and the link is up, or the neighbor entries have been cleared.
Advertisement	All neighbors are bi-directionally reachable or DLDP has been in active state for more than five seconds. This is a relatively state where no bidirectional link has been detected.
Probe	DLDP enters this state if it receives a packet from an unknown neighbor. In this state, DLDP sends packets to check whether the link is unidirectional. As soon as DLDP transits to this state, a probe timer starts and an echo timeout timer starts for each neighbor to be probed.
Disable	A port enters this state when: l      A unidirectional link is detected. l      The contact with the neighbor in enhanced mode gets lost. In this state, the port does not receive or send packets other than DLDPDUs.
DelayDown	A port in the Active, Advertisement, or Probe DLDP link state transits to this state rather than removes the corresponding neighbor entry and transits to the Inactive state when it detects a port-down event. When a port transits to this state, the DelayDown timer is triggered.

 

DLDP timers

Table 1-2 DLDP timers

DLDP timer	Description
Active timer	Determines the Interval for sending Advertisement packets with RSY tags, which defaults to 1 second. That is, a device in the active DLDP link state sends one Advertisement packet with RSY tags every second by default. The maximum number of advertisement packets with RSY tags that can be sent successively is 5.
Advertisement timer	Determines the interval to send advertisement packets, which defaults to 5 seconds.
Probe timer	Determines the interval to send Probe packets, which defaults to 0.5 seconds. That is, a device in the probe state sends two Probe packets every second by default. The maximum number of Probe packets that can be sent successively is 10.
Echo timer	This timer is set to 10 seconds and is triggered when a device transits to the Probe state or an enhanced detect is launched. When the Echo timer expires and no Echo packet has been received from a neighbor device, the state of the link is set to unidirectional and the device transits to the Disable state. In this case, the device sends Disable packets, prompts the user to shut down the port or shuts down the port automatically (depending on the DLDP down mode configured), and removes the corresponding neighbor entries.
Entry timer	When a new neighbor joins, a neighbor entry is created and the corresponding entry timer is triggered. When a DLDP packet is received, the device updates the corresponding neighbor entry and the entry aging timer. In the normal mode, if no packet is received from a neighbor when the corresponding entry aging timer expires, DLDP sends advertisement packets with RSY tags and removes the neighbor entry. In the enhanced mode, if no packet is received from a neighbor when the Entry timer expires, DLDP triggers the enhanced timer. The setting of an Entry timer is three times that of the Advertisement timer.
Enhanced timer	In the enhanced mode, this timer is triggered if no packet is received from a neighbor when the entry aging timer expires. Enhanced timer is set to 1 second. After the Enhanced timer is triggered, the device sends up to eight probe packets to the neighbor at a frequency of one packet per second.
DelayDown timer	A device in the Active, Advertisement, or Probe DLDP link state transits to DelayDown state rather than removes the corresponding neighbor entry and transits to the Inactive state when it detects a port-down event. When a device transits to this state, the DelayDown timer is triggered. A device in DelayDown state only responds to port-up events. A device in the DelayDown state resumes its original DLDP state if it detects a port-up event before the DelayDown timer expires. Otherwise, it removes the corresponding DLDP neighbor information and transits to the Inactive state.
RecoverProbe timer	This timer is set to 2 seconds. That is, a port in the Disable state sends one RecoverProbe packet every two seconds to detect whether a unidirectional link has restored.

 

DLDP mode

DLDP can operate in two modes: normal mode and enhanced mode, as described below.

l          In normal DLDP mode, when an entry timer expires, the device removes the corresponding neighbor entry and sends an Advertisement packet with RSY tag.

l          In enhanced DLDP mode, when an entry timer expires, the Enhanced timer is triggered and the device sends up to eight Probe packets at a frequency of one packet per second to test the neighbor. If no Echo packet is received from the neighbor when the Echo timer expires, the device transits to the Disable state.

Table 1-3 DLDP mode and neighbor entry aging

DLDP mode	Detecting a neighbor after the corresponding neighbor entry ages out	Removing the neighbor entry immediately after the Entry timer expires	Triggering the Enhanced timer after an Entry timer expires
Normal DLDP mode	No	Yes	No
Enhanced DLDP mode	Yes	No	Yes

 

The enhanced DLDP mode is designed for addressing black holes. It prevents the cases where one end of a link is up and the other is down. If you configure the speed and the duplex mode by force on a device, the situation shown in Figure 1-3 may occur, where Port B is actually down but the state of Port B cannot be detected by common data link protocols, so Port A is still up. In enhanced DLDP mode, however, Port A tests Port B after the Entry timer concerning Port B expires. Port A then transits to the Disable state if it receives no Echo packet from Port A when the Echo timer expires. As Port B is physically down, it is in the Inactive DLDP state.

Figure 1-3 A case for Enhanced DLDP mode

 

 

DLDP authentication mode

You can prevent network attacks and illegal detect through DLDP authentication. Three DLDP authentication modes exist, as described below.

l          Non-authentication. In this mode, the sending side sets the Authentication field and the Authentication type field of DLDP packets to 0. The receiving side checks the values of the two fields of received DLDP packets and drops the packets with the two fields conflicting with the corresponding local configuration.

l          Plain text authentication. In this mode, before sending a DLDP packet, the sending side sets the Authentication field to the password configured in plain text and sets the Authentication type field to 1. The receiving side checks the values of the two fields of received DLDP packets and drops the packets with the two fields conflicting with the corresponding local configuration.

l          MD5 authentication. In this mode, before sending a packet, the sending side encrypts the user configured password using MD5 algorithm, assigns the digest to the Authentication field, and sets the Authentication type field to 2. The receiving side checks the values of the two fields of received DLDP packets and drops the packets with the two fields conflicting with the corresponding local configuration.

DLDP implementation

1)        On a DLDP-enabled link that is in up state, DLDP sends DLDP packets to the peer device and processes the DLDP packets received from the peer device. DLDP packets sent vary with DLDP states. Table 1-4 lists DLDP states and the corresponding packets.

Table 1-4 DLDP packet types and DLDP states

DLDP state	Type of DLDP packets sent
Active	Advertisement packet with RSY tag
Advertisement	Normal Advertisement packet
Probe	Probe packet
Disable	Disable packet and RecoverProbe packet

 

 

2)        A received DLDP packet is processed as follows.

l          In any of the three authentication modes, the packet is dropped if it fails to pass the authentication.

l          The packet is dropped if the setting of the interval for sending Advertisement packets it carries conflicts with the corresponding local setting.

l          Other processes.

Table 1-5 Procedures for processing different types of DLDP packets

Packet type	Processing procedure
Advertisement packet with RSY tag	Retrieving the neighbor information.	If the corresponding neighbor entry does not exist, creates the neighbor entry, triggers the Entry timer, and transits to Probe state.
		If the corresponding neighbor entry already exists, resets the Entry timer and transits to Probe state.
Normal Advertisement packet	Retrieves the neighbor information.	If the corresponding neighbor entry does not exist, creates the neighbor entry, triggers the Entry timer, and transits to Probe state.
		If the corresponding neighbor entry already exists, resets the Entry timer.
Flush packet	Determines whether or not the local port is in Disable state.	If yes, no process is performed.
		If not, removes the corresponding neighbor entry (if any).
Probe packet	Retrieves the neighbor information.	If the corresponding neighbor entry does not exist, creates the neighbor entry, transits to Probe state, and returns Echo packets.
		If the corresponding neighbor entry already exists, resets the Entry timer and returns Echo packets.
Echo packet	Retrieves the neighbor information.	If the corresponding neighbor entry does not exist, creates the neighbor entry, triggers the Entry timer, and transits to Probe state.
		The corresponding neighbor entry already exists	If the neighbor information it carries conflicts with the corresponding locally maintained neighbor entry, drops the packet.
			Otherwise, sets the flag of the neighbor as two-way connected. In addition, if the flags of all the neighbors are two-way connected, the device transits from Probe state to Advertisement state and disables the Echo timer.
Disable packet	Check to see if the local port is in Disable state.	If yes, no process is performed.
		If not, the local port transits to Disable state.
RecoverProbe packet	Check to see if the local port is in Disable or Advertisement state.	If not, no process is performed.
		If yes, returns RecoverEcho packets.
RecoverEcho packet	Check to see if the local port is in Disable state.	If not, no process is performed.
		If yes, the local port transits to Active state if the neighbor information the packet carries is consistent with the local port information.
LinkDown packet	Check to see if the local port operates in Enhanced mode.	If not, no process is performed.
		If yes and the local port is not in Disable state, the local transits to Disable state.

 

3)        If no echo packet is received from the neighbor, DLDP performs the following processing.

Table 1-6 Processing procedure when no echo packet is received from the neighbor

No echo packet received from the neighbor	Processing procedure
In normal mode, no echo packet is received when the Echo timer expires.	DLDP transits to the Disable state, outputs log and tracking information, and sends Disable packets. In addition, depending on the user-defined DLDP down mode, DLDP shuts down the local port or prompts users to shut down the port, and removes the corresponding neighbor entry.
In enhanced mode, no echo packet is received when the enhanced timer expires.

 

Link auto-recovery mechanism

If the port shutdown mode upon detection of a unidirectional link is set to auto, DLDP sets the state of the port where a unidirectional link is detected to DLDP down automatically. A DLDP down port cannot forward service traffic or send/receive any PDUs except DLDPDUs.

On a DLDP down port, DLDP monitors the unidirectional link. Once DLDP finds out that the state of the link has restored to bidirectional, it brings up the port. The specific process is as follows:

The DLDP down port sends out a RecoverProbe packet, which carries only information about the local port, every two seconds. Upon receiving the RecoverProbe packet, the remote end returns a RecoverEcho packet. Upon receiving the RecoverEcho packet, the local port checks whether neighbor information in the RecoverEcho packet is the same as the local port information. If they are the same, the link between the local port and the neighbor is considered to have been restored to a bidirectional link, and the port will transit from Disable state to Active state and re-establish neighborship with the neighbor.

Only DLDP down ports can send and process Recover packets, including RecoverProbe packets and RecoverEcho packets. The auto-recovery mechanism does not take effect on ports manually shut down.

DLDP neighbor state

A DLDP neighbor can be in one of the three states described in Table 1-7.

Table 1-7 Description on DLDP neighbor states

DLDP neighbor state	Description
Unknown	A neighbor is in this state when it is just detected and is being probed. No information indicating the state of the neighbor is received. A neighbor is in this state only when it is being probed. It transits to Two way state or Unidirectional state after the probe operation finishes.
Two way	A neighbor is in this state after it receives response from its peer. This state indicates the link is a two-way link.
Unidirectional	A neighbor is in this state when the link connecting it is detected to be a unidirectional link. After a device transits to this state, the corresponding neighbor entries maintained on other devices are removed.

 

DLDP Configuration Task List

Complete the following tasks to configure DLDP:

Task	Remarks
Enabling DLDP	Required
Setting DLDP Mode	Optional
Setting the Interval for Sending Advertisement Packets	Optional
Setting the DelayDown Timer	Optional
Setting the Port Shutdown Mode	Optional
Configuring DLDP Authentication	Optional
Resetting DLDP State	Optional

 

Note that:

l          DLDP takes effects only on Ethernet interfaces.

l          DLDP can detect unidirectional links only after all links are connected. Therefore, before enabling DLDP, make sure that optical fibers or copper twisted pairs are connected.

l          To ensure unidirectional links can be detected, make sure these settings are the same on the both sides: DLDP state (enabled/disabled), the interval for sending Advertisement packets, authentication mode, and password.

l          Keep the interval for sending Advertisement packets adequate to enable unidirectional links to be detected in time. If the interval is too long, unidirectional links cannot be terminated in time; if the interval is too short, network traffic may increase in vain.

l          DLDP does not process any link aggregation control protocol (LACP) events. The links in an aggregation group are treated individually in DLDP.

l          When connecting two DLDP-enabled devices, make sure the DLDP software version ID fields of the DLDP packets exchanged between the two devices are the same. Otherwise, DLDP may operate improperly.

Enabling DLDP

Follow these steps to enable DLDP:

To do…		Use the command…	Remarks
Enter system view		system-view	—
Enable DLDP globally		dldp enable	Required Globally disabled by default
Enter Ethernet port view or port group view	Enter Ethernet port view	interface interface-type interface-number	Either of the two is required. The configuration performed in Ethernet port view applies to the current port only. The configuration performed in port group view applies to all the ports in the port group.
	Enter port group view	port-group manual port-group-name
Enable DLDP		dldp enable	Required Disabled on a port by default You can perform this operation on an optical port or an electrical port.

 

 

Setting DLDP Mode

There are two DLDP modes:

l          Normal mode: In this mode, DLDP does not actively detect neighbors when the corresponding neighbor entries age out. The system can identify only one type of unidirectional links: cross-connected fibers.

l          Enhanced mode: In this mode, DLDP actively detects neighbors when the corresponding neighbor entries age out. The system can identify two types of unidirectional links: cross-connected fibers and disconnected fibers.

Follow these steps to set DLDP mode:

To do…	Use the command…	Remarks
Enter system view	system-view	—
Set DLDP mode	dldp work-mode { enhance | normal }	Optional Normal by default

 

Setting the Interval for Sending Advertisement Packets

You can set the interval for sending Advertisement packets to enable unidirectional links to be detected in time.

Follow these steps to set the interval for sending Advertisement packets:

To do…	Use the command…	Remarks
Enter system view	system-view	—
Set the interval for sending Advertisement packets	dldp interval time	Optional 5 seconds by default The interval for sending Advertisement packets applies to all the DLDP-enabled ports.

 

 

Setting the DelayDown Timer

On some ports, when the Tx line fails, the port goes down and then comes up again, causing optical signal jitters on the Rx line. When a port goes down due to a Tx failure, the device transits to the DelayDown state instead of the Inactive state to prevent the corresponding neighbor entries from being removed. In the same time, the device triggers the DelayDown timer. If the port goes up before the timer expires, the device restores the original state; if the port remains down when the timer expires, the devices transits to the Inactive state.

Follow these steps to set the DelayDown timer

To do…	Use the command…	Remarks
Enter system view	system-view	—
Set the DelayDown timer	dldp delaydown-timer time	Optional 1 second by default DelayDown timer setting applies to all the DLDP-enabled ports.

 

Setting the Port Shutdown Mode

On detecting a unidirectional link, the ports can be shut down in one of the following two modes.

l          Manual mode. This mode applies to networks with low performance, where normal links may be treated as unidirectional links. It protects service packet transmission against false unidirectional links. In this mode, DLDP only detects unidirectional links and generates log and traps. The operations to shut down unidirectional link ports are accomplished by the administrator.

l          Auto mode. In this mode, when a unidirectional link is detected, DLDP transits to Disable state, generates log and traps, and set the port as DLDP Down.

Follow these steps to set port shutdown mode:

To do…	Use the command…	Remarks
Enter system view	system-view	—
Set port shutdown mode	dldp unidirectional-shutdown { auto | manual }	Optional auto by default

 

 

Configuring DLDP Authentication

Follow these steps to configure DLDP authentication:

To do…	Use the command…	Remarks
Enter system view	system-view	—
Configure DLDP authentication	dldp authentication-mode { md5 md5-password | none | simple simple-password }	Required none by default

 

 

Resetting DLDP State

After DLDP detects a unidirectional link on a port, the port enters Disable state. In this case, DLDP prompts you to shut down the port manually or shuts down the port automatically depending on the user-defined port shutdown mode. To enable the port to perform DLDP detect again, you can reset the DLDP state of the port in one of the following methods:

l          If the port is shut down with the shutdown command manually, use the undo shutdown command on the port.

l          If the port is shut down by DLDP automatically, use the dldp reset command on the port. Alternatively, you can leave the work to DLDP, which can enable the port automatically upon detecting that the link has been restored to bidirectional. For how to reset DLDP state with the dldp reset command, refer to Resetting DLDP State in System View and Resetting DLDP State in Port view/Port Group View.

The DLDP state that the port transits to upon the DLDP state reset operation depends on its physical state. If the port is physically down, it transits to Inactive state; if the port is physically up, it transits to Active state.

Resetting DLDP State in System View

Resetting DLDP state in system view applies to all the ports shut down by DLDP.

Follow these steps to reset DLDP in system view:

To do…	Use the command…	Remarks
Enter system view	system-view	—
Reset DLDP state	dldp reset	Required

 

Resetting DLDP State in Port view/Port Group View

Resetting DLDP state in port view or port group view applies to the current port or all the ports in the port group shut down by DLDP.

Follow these steps to reset DLDP state in port view/port group view:

To do…		Use the command…	Remarks
Enter system view		system-view	—
Enter Ethernet port view/port group view	Enter Ethernet port view	interface interface-type interface-number	Either is required. The configuration performed in Ethernet port view applies to the current port only; the configuration performed in port group view applies to all the ports in the port group.
	Enter port group view	port-group manual port-group-name
Reset DLDP state		dldp reset	Required

 

Displaying and Maintaining DLDP

To do…	Use the command…	Remarks
Display the DLDP configuration of a port	display dldp [ interface-type interface-number ]	Available in any view
Display the statistics on DLDP packets passing through a port	display dldp statistics [ interface-type interface-number ]	Available in any view
Clear the statistics on DLDP packets passing through a port	reset dldp statistics [ interface-type interface-number ]	Available in user view

 

DLDP Configuration Example

Network requirements

l          Device A and Device B are connected through two fiber pairs, in which two fibers are cross-connected, as shown in Figure 1-4.

l          It is desired that the unidirectional links can be disconnected on being detected; and the ports shut down by DLDP can be restored after the fiber connections are corrected.

Network diagram

Figure 1-4 Network diagram for DLDP configuration

 

Configuration procedure

1)        Configuration on Device A

# Enable DLDP on GigabitEthernet2/0/1 and GigabitEthernet 2/0/2.

<DeviceA> system-view

[DeviceA] interface gigabitethernet 2/0/1

[DeviceA-GigabitEthernet2/0/1] dldp enable

[DeviceA-GigabitEthernet2/0/1] quit

[DeviceA] interface gigabitethernet 2/0/2

[DeviceA-GigabitEthernet2/0/2] dldp enable

[DeviceA-GigabitEthernet2/0/2] quit

# Set the interval for sending Advertisement packets to 6 seconds.

[DeviceA] dldp interval 6

# Set the DelayDown timer to 2 seconds.

[DeviceA] dldp delaydown-timer 2

# Set the DLDP mode as enhanced mode.

[DeviceA] dldp work-mode enhance

# Set the port shutdown mode as auto mode.

[DeviceA] dldp unidirectional-shutdown auto

# Enable DLDP globally.

[DeviceA] dldp enable

2)        Configuration on Device B

Configure Device B as you configure Device A.

3)        Verifying the configurations

You can use the display dldp command to display the DLDP configuration information on ports.

# Display the DLDP configuration information on all the DLDP-enabled ports of Device A.

[DeviceA] display dldp

DLDP global status : enable

DLDP interval : 6s

DLDP work-mode : enhance

DLDP authentication-mode  : none

DLDP unidirectional-shutdown : auto

DLDP delaydown-timer : 2s

The number of enabled ports is 2.

 

Interface GigabitEthernet2/0/1

DLDP port state : disable

DLDP link state : down

The neighbor number of the port is 0.

 

Interface GigabitEthernet2/0/2

DLDP port state : disable

DLDP link state : down

The neighbor number of the port is 0.

The output information indicates that both GigabitEthernet 2/0/1 and GigabitEthernet 2/0/2 are in Disable state and the links are down, which means unidirectional links are detected and the two ports are thus shut down.

Correct the fiber connections after detecting the problem, and perform the following operations:

# Reset DLDP state for the ports shut down by DLDP.

[DeviceA] dldp reset

# Display the DLDP configuration information on all the DLDP-enabled ports of Device A.

[DeviceA] display dldp

 DLDP global status : enable

 DLDP interval : 6s

 DLDP work-mode : enhance

 DLDP authentication-mode  : none

 DLDP unidirectional-shutdown : auto

 DLDP delaydown-timer : 2s

 The number of enabled ports is 2.

 

Interface GigabitEthernet2/0/1

 DLDP port state : advertisement

 DLDP link state : up

 The neighbor number of the port is 1.

    Neighbor mac address : 0000-0000-0101

    Neighbor port index : 59

    Neighbor state : two way

    Neighbor aged time : 11

 

Interface GigabitEthernet2/0/2

 DLDP port state : advertisement

 DLDP link state : up

 The neighbor number of the port is 1.

    Neighbor mac address : 0000-0000-0102

    Neighbor port index : 59

    Neighbor state : two way

    Neighbor aged time : 11

The output information indicates that both GigabitEthernet 2/0/1 and GigabitEthernet 2/0/2 are in Advertisement state and the links are up, which means unidirectional links are not detected and the two ports are restored.

Troubleshooting

Symptom:

Two DLDP-enabled devices, Device A and Device B, are connected through two fiber pairs, in which two fibers are cross-connected. The unidirectional links cannot be detected; all the four ports involved are in Advertisement state.

Analysis:

The problem can be caused by the following.

l          The intervals for sending Advertisement packets on Device A and Device B are not the same.

l          DLDP authentication modes/passwords on Device A and Device B are not the same.

Solution:

Make sure the interval for sending Advertisement packets, the authentication mode, and the password on Device A and Device B are the same.