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CX11x, CX31x, CX710 (Earlier Than V6.03), and CX91x Series Switch Modules V100R001C10 Configuration Guide 12

The documents describe the configuration of various services supported by the CX11x&CX31x&CX91x series switch modules The description covers configuration examples and function configurations.
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Principles

Principles

This section describes the implementation of DLDP.

DLDP States

DLDP allows a device to identify the peer device and check connectivity of the unidirectional link by exchanging protocol packets DLDPDUs with the peer device. DLDP defines the following states: Initial, Inactive, Active, Advertisement, Probe, Disable, and DelayDown.

Figure 11-61 State transition during DLDP packet exchange
Table 11-19 DLDP states

State

Description

Initial

DLDP is disabled.

Inactive

DLDP is enabled but the link is Down.

Active

DLDP is enabled and the link is Up, or neighbor entries are cleared.

Advertisement

All neighbors are bidirectionally reachable or have been in Active state for more than 5 seconds. This is a stable state when no unidirectional link has been detected.

Probe

Probe packets are sent to detect whether the link is unidirectional. When an interface enters this state, DLDP starts the probe timer and starts an echo timer for each neighbor to be detected.

Disable

DLDP in enhanced mode detects a unidirectional link and disables the interface that fails to send optical signals.

DelayDown

It is a temporary status. When an interface in Active, Advertisement, or Probe state receives a Port-Down event, it enters this state instead of immediately deleting neighbor entries and transiting to the Inactive state. In DelayDown state, the system reserves DLDP neighbor information and only responds to Port-Up events.

DLDP Timers

DLDP uses the following timers.

Table 11-20 DLDP timers

Timer

Description

Active timer

After DLDP is enabled and the link is Up or neighbor entries are cleared, the interface enters the Active state and starts the Active timer to send Advertisement packets with RSY tags. The default value of the Active timer is 1 second. The interface sends an Advertisement packet with an RSY tag every 1 second, and sends a maximum of five Advertisement packets. If no response packet is received after the interface sends five Advertisement packets with RSY tags, the Active timer times out and the interface enters the Advertisement state.

Advertisement timer

When an interface enters the Advertisement state, the interface starts the Advertisement timer and sends Advertisement packets. This timer determines the interval for sending Advertisement packets, which can be set using a command. The default value of the Advertisement timer is 5 seconds.

Probe timer

When receiving a packet from an unknown neighbor, the interface enters the Probe state and sends a probe packet to check for the unidirectional link. The interface in Probe state starts the Probe timer. This timer determines the interval for sending Probe packets (the default value is 1 second). A DLDP interface in Probe state sends two Probe packets every second.

Echo timer

This timer is triggered when DLDP transits to the Probe state. The default value is 10 seconds. If an interface in Probe state does not receive any echo packet from a neighbor when the Echo timer times out, the interface status is set to unidirectional and the DLDP state machine transits to the Disable state. In this case, the system records logs and alarms information and sends flush packets. In addition, the system shuts down or promotes the user to shut down the interface according to the DLDP Down mode, and deletes the neighbor entry.

Entry aging timer

When a new neighbor joins, a neighbor entry is created and the aging timer is triggered for the entry. When a DLDP packet is received from a neighbor, DLDP updates the corresponding neighbor entry and resets the entry aging timer.
  • In normal mode, if no DLDP packet is received from a neighbor when the corresponding entry aging timer times out, the local device sends Advertisement packets with RSY tags and deletes the neighbor entry.
  • In enhanced mode, the enhanced timer is triggered if no DLDP packet is received from a neighbor when the entry aging timer times out.

The value of the entry aging timer is three times the value of the Advertisement timer.

Enhanced timer

In enhanced mode, the enhanced timer is triggered if no DLDP packet is received from a neighbor when the entry aging timer times out. The local device consecutively sends eight probe packets to the neighbor, at the rate of 1 pps. If no echo packet is received from the neighbor when the enhanced timer times out, DLDP enters the Disable state. The value of the enhanced timer is 10 seconds.

DelayDown timer

In enhanced mode, when DLDP in Active, Advertisement, or Probe state detects a Port-Down event, it transits to the DelayDown state instead of deleting the neighbor entry and transiting to the Inactive state. At this time, the system reserves DLDP neighbor information and only responds to Port-Up events.
  • If DLDP does not receive any Port-Up event when the DelayDown timer times out, DLDP deletes the neighbor entry and enters the Inactive state.
  • If DLDP receives a Port-Up event before the DelayDown timer times out, DLDP returns to the previous state.

RecoverProbe timer

An interface in Disable state sends one RecoverProbe packet every 2 seconds to detect whether the unidirectional link fault is rectified. Upon receiving a RecoverEcho packet from the peer end, the local interface checks whether the neighbor information in the RecoverEcho packet is the same as that on the local interface. If they are the same, the link between the local interface and the neighbor has recovered. The local interface transits from Disable state to Active state and re-establishes the neighbor relationship.

DLDP Working Modes

DLDP can work in normal mode or enhanced mode.

DLDP Working Mode

When a Neighbor Entry Expires

Unidirectional Link Detection

Normal

DLDP does not automatically probe a neighbor before aging out a neighbor entry. When the entry aging timer times out, DLDP ages out the neighbor entry.

DLDP can identify only unidirectional links caused by intersected fibers.

Enhanced

DLDP automatically probes a neighbor before aging out a neighbor entry. When the entry aging timer times out, DLDP starts the enhanced timer and echo timer. When the echo timer times out, the local link status is set to unidirectional and the neighbor entry is deleted.

DLDP can identify unidirectional links caused by intersected fibers or fiber disconnection.

NOTE:

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.

DLDP Authentication Modes

If a link of the DLDP interface is in Up state, the DLDP interface sends DLDP packets to the peer device and processes the DLDP packets received from the peer device. To ensure packet validity on an insecure network, users can configure one of the following authentication modes for DLDP packets.

Table 11-21 Authentication modes of DLDP packets

Authentication Mode

Description

Simple authentication mode

The receiver compares the authentication key and authentication type of the packet with those configured on the local end. If they are different, the receiver discards the packet.

MD5 authentication mode

The receiver compares the authentication key and authentication type of the packet with the MD5-encrypted password and authentication type that are configured on the local end. If they are different, the receiver discards the packet.

SHA authentication mode

The receiver compares the authentication key and authentication type of the packet with the SHA-encrypted password and authentication type that are configured on the local end. If they are different, the receiver discards the packet.

DLDP Working Process

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.

For details about how DLDP detects unidirectional links, see Unidirectional Link Detection When a Single Neighbor Exists and Unidirectional Link Detection When Multiple Neighbors Exist. For details about how DLDP processes unidirectional links, see Unidirectional Link Processing. For details about how DLDP recovers a link, see Link Auto-Recovery Mechanism.

Unidirectional Link Detection When a Single Neighbor Exists

This section describes how DLDP detects unidirectional links when a single neighbor exists.

A Link Is Unidirectional Before DLDP Is Enabled

As shown in Figure 11-62, optical fibers between switches are cross connected.

Figure 11-62 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 11-63, optical fibers connect switches.

Figure 11-63 Correct optical fiber connections when a single neighbor exists

When the Tx and Rx optical fibers are working properly, Switch A and Switch B establishes 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 neighbor information carried by the echo packet is the same as that saved on the local device. Therefore, Interface 2 marks this neighbor as a bidirectionally connected neighbor. Interface 2 transits from the Probe state to 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.

  5. After DLDP is enabled, the procedure for sending packets from Interface 2 and setting up a neighbor on Interface 1 is similar to steps 1 to 4.

  6. At last, Interface 1 and Interface 2 regard each other as bidirectionally connected neighbors and enter the Advertisement state.

If the Rx optical fiber of Interface 2 fails (as shown in Figure 11-64) and cannot receive any optical signal. When this occurs, Interface 2 enters the Inactive state and stops sending and receiving packets. The Tx optical fiber of Interface 2 is normal, so Interface 1 can receive signals and keep in Up state. Interface 1 cannot receive DLDP packets from Interface 2 before the entry aging timer times out. The procedure for detecting unidirectional links varies depending on the configured DLDP working mode.

Figure 11-64 Disconnection of one optical fiber when a single neighbor exists
  • In 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.

  • In 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, so Interface 1 cannot 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. Meanwhile, 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.

    NOTE:
    • 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.

Unidirectional Link Detection When Multiple Neighbors Exist

DLDP can be configured for devices connected by hubs to detect unidirectional links. Each interface detects at least one neighbor.

As shown in Figure 11-65, a hub connects SwitchA to SwitchB, SwitchC, and SwitchD through copper twisted pairs or optical fibers. All switches support DLDP. To detect unidirectional links on this network, enable DLDP on all switch interfaces connected to the hub.

Figure 11-65 Networking diagram of multiple neighbors

On a network with multiple neighbors, an interface immediately enters the Disable state if it receives Disable packets from a neighbor or detects that a neighbor is unidirectional. When SwitchA, SwitchB, and SwitchC detect a unidirectional link fault on SwitchD, they transit to Disable state. This can prevent traffic forwarding errors when the topology changes. If the Rx optical fiber between SwitchB and the hub is disconnected when SwitchA is forwarding traffic to SwitchB, SwitchA shuts down its interface connected to the hub and stops sending packets to SwitchB, SwitchC, and SwitchD. If a backup link exists between SwitchA and SwitchB, STP immediately starts the backup link when SwitchA shuts down the interface.

If the switches in the preceding networking diagram are connected by a device that has DLDP disabled but supports DLDPDU forwarding, DLDP still detects unidirectional links as if a network has multiple neighbors.

Unidirectional Link Processing

When a unidirectional link is detected, DLDP shuts down the corresponding interface in either of the following ways:

  • Manual mode: DLDP detects the unidirectional link, and the network administrator shuts down the interface manually. Upon detecting a unidirectional link, the local device only records logs and traps and prompts the network administrator to shut down the interface.

  • Automatic mode: DLDP detects the unidirectional link, records logs and traps, and sets the interface state to DLDP Down. A DLDP Down interface receives and forwards only BPDUs, not user traffic.

NOTE:

The automatic mode is recommended on networks with high performance. When the network performance is poor, there may be a delay in receiving DLDP packets, causing DLDP to mistakenly identify a unidirectional link. In this scenario, the manual mode is recommended. The network administrator manually shuts down the interface, preventing packet forwarding from being affected.

Link Auto-Recovery Mechanism

  • An interface that is manually shut down cannot automatically go Up using the link auto-recovery mechanism.

  • An interface that is automatically shut down can automatically go Up using the link auto-recovery mechanism.

    In automatic shutdown mode, DLDP sets the interface state to DLDP Down when detecting a unidirectional link. A DLDP Down interface receives and forwards only BPDUs, not user traffic. A DLDP Down interface can be recovered upon link recovery. The DLDP Down interface periodically sends RecoverProbe packets. If the interface receives correct RecoverEcho packets, the unidirectional link becomes a bidirectional link and the interface becomes Up.

    The link auto-recovery process is as follows:

    1. The DLDP Down interface sends one RecoverProbe packet every 2 seconds. A RecoverProbe packet carries only information about the local interface.

    2. Upon receiving a RecoverProbe packet, the peer end returns a RecoverEcho packet.

    3. When receiving the RecoverEcho packet, the local interface checks whether the neighbor information in the RecoverEcho packet is the same as that on the local interface. If they are the same, the link between the local interface and the neighbor has recovered. The local interface transits from Disable state to Active state and re-establishes the neighbor relationship.

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Updated: 2019-08-09

Document ID: EDOC1000041694

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