A special kind of links, namely, unidirectional links, may occur in a network. When a unidirectional link appears, the local device can receive packets from the peer device through the link layer, but the peer device cannot receive packets from the local device. Unidirectional link can cause problems such as loops in a Spanning Tree Protocol (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 fiber

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

1.1.1 DLDP Introduction

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. The auto-negotiation mechanism provided by physical layer protocols detects physical signals and faults. DLDP, however, 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.

1.1.2 DLDP Fundamentals

I. 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	Description
Initial	This state indicates that DLDP is not enabled.
Inactive	This state indicates that DLDP is enabled but the link is down.
Active	This state indicates that: l DLDP is enabled and the link is up. l The neighbor entries are cleared.
Advertisement	This state indicates that a device can communicate normally with all its neighbors in both directions or DLDP remains in active state for more than five seconds. It is the normal state where no unidirectional link is detected.
Probe	A device enters this state if it receives a packet from an unknown neighbor. In this state, DLDP sends packets to check whether the link is a unidirectional link. After a device enters this state, the probe sending timer is triggered, and an echo waiting timer is triggered for each neighbor to be detected.
Disable	A device enters this state when: l A unidirectional link is detected. l The contact with a neighbor in enhanced mode gets lost. In this state, no DLDP packet is sent or accepted.
DelayDown	A device in the Active, Advertisement, or Probe DLDP link state transits to this state rather than remove 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.

II. DLDP timers

Table 1-2 DLDP timers

DLDP timer	Description
Active timer	Determines the Interval to send Advertisement packets with RSY tag, which defaults to 1 second. When a device transits to the active DLDP link state, it sends Advertisement packets with RSY tag according to this timer. The maximum number of this type of packets 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. The maximum number of this type of 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 waiting timer expires and no Echo packet is received from a neighbor device, the link is set as a unidirectional link 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. And 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 10 seconds. 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. If no Echo packet is received from the neighbor when the Echo timer expires, the link is set as a unidirectional link 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.
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. The setting of the timer ranges from 1 to 5 (in seconds). 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	Determines the interval to RecoverProbe packets, which are used to detect whether a unidirectional link is restored. This timer is set to 2 seconds.

III. 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

& Note:

l In normal DLDP mode, only fiber cross-connected unidirectional links (as shown in Figure 1-1 ) can be detected.

l In enhanced DLDP mode, two types of unidirectional links can be detected. One is fiber cross-connected links (as shown in Figure 1-1). The other refers to fiber pairs with one fiber not connected or disconnected (as shown in Figure 1-2). To detect unidirectional links that are of the latter type, you need to configure the ports to operate at specific speed and in full duplex mode. Otherwise, DLDP cannot take effect. When a fiber of a fiber pair is not connected or gets disconnected, the port that can receive optical signals is in Disable state; the other port is in Inactive state.

IV. 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.

V. 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

& Note:

When a device transits from a DLDP state other than Inactive state or Disable state to Initial state, it sends Flush packets.

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.
Advertisement packet with RSY tag	Retrieving the neighbor information.	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.
Normal Advertisement packet	Retrieves the neighbor information.	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.
Flush packet		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.
Probe packet	Retrieves the neighbor information.	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.
		The corresponding neighbor entry already exists	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.
Disable packet	Check to see if the local port is in Disable state.	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.
RecoverProbe packet		If yes, returns RecoverEcho packets.
RecoverEcho packet	Check to see if the local port is in Disable state.	If not, no process is performed.
RecoverEcho packet	Check to see if the local port is in Disable state.	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.
LinkDown packet	Check to see if the local port operates in Enhanced mode.	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.

VI. DLDP neighbor state

A DLDP neighbor can be in one of the three states described in Table 1-7. You can check the state of a DLDP neighbor by using the display dldp command.

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.

1.2 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 works only when the link is up.

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 LACP (Link Aggregation Control Protocol) events have no effect on DLDP. Links in an aggregation group are treated individually in DLDP.

l 802.1X has no effect on DLDP.

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

1.2.1 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 Ethernet port view or port group view	Enter port group view	port-group { aggregation agg-id \| 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.

& Note:

DLDP takes effect only when it is enabled both globally and on a port.

Table of Contents

09-DLDP Configuration

Chapter 1 DLDP Configuration

1.1 Overview

1.1.1 DLDP Introduction

1.1.2 DLDP Fundamentals

I. DLDP link states

II. DLDP timers

III. DLDP mode

IV. DLDP authentication mode

V. DLDP implementation

VI. DLDP neighbor state

1.2 DLDP Configuration Task List

1.2.1 Enabling DLDP