How DLDP works

Detecting one neighbor

When two devices are connected through an optical fiber or a network cable, enable DLDP to detect unidirectional links to the neighbor. The following illustrates the unidirectional link detection process in two cases:

As shown in Figure 7, before you enable DLDP, the optical fibers between Device A and Device B are cross-connected. After you enable DLDP, the four ports are all up and in unidirectional state, and they send RecoverProbe packets. Take Port 1 as an example to illustrate the unidirectional link detection process.

  1. Port 1 receives the RecoverProbe packet from Port 4, and returns a RecoverEcho packet.

  2. Port 4 cannot receive any RecoverEcho packet from Port 1, so Port 4 cannot become the neighbor of Port 1.

  3. Port 3 can receive the RecoverEcho packet from Port 1, but Port 3 is not the intended destination, so Port 3 cannot become the neighbor of Port 1.

The same process occurs on the other three ports. The four ports are all in unidirectional state.

As shown in Figure 8, Device A and Device B are connected through an optical fiber. After you enable DLDP, Port 1 and Port 2 establish the bidirectional neighborship in the following way:

  1. Port 1 that is physically up enters the unidirectional state and sends a RecoverProbe packet.

  2. After receiving the RecoverProbe packet, Port 2 returns a RecoverEcho packet.

  3. After Port 1 receives the RecoverEcho packet, it examines the neighbor information in the packet. If the neighbor information matches the local information, Port 1 establishes the neighborship with Port 2 and transits to bidirectional state. Port 1 then starts the Entry timer and periodically sends Advertisement packets.

  4. After Port 2 receives the Advertisement packet, it establishes the Unconfirmed neighborship with Port 1. Port 2 then starts the Echo timer and Probe timer, and periodically sends Probe packets.

  5. After receiving the Probe packet, Port 1 returns an Echo packet.

  6. After Port 2 receives the Echo packet, it examines the neighbor information in the packet. If the neighbor information matches the local information, the neighbor state of Port 1 becomes Confirmed. Port 2 then transits to bidirectional state, starts the Entry timer, and periodically sends Advertisement packets.

The bidirectional neighborship between Port 1 and Port 2 is now established.

After that, when Port 2's Rx end fails to receive signals, Port 2 is physically down and enters the Inactive state. Because Port 2's Tx end can still send signals to Port 1, Port 1 stays up. After the Entry timer for Port 2 expires, Port 1 starts the Enhanced timer and Echo timer, and sends a probe packet to Port 2. Because Port 1's Tx line is broken, Port 1 cannot receive the Echo packet from Port 2 after the Echo timer expires. Port 1 then enters the unidirectional state, and sends a Disable packet to Port 2. At the same time, Port 1 deletes the neighborship with Port 2, and starts the RecoverProbe timer. Port 2 stays in Inactive state during this process.

When an interface is physically down, but the Tx end of the interface is still operating, DLDP sends a LinkDown packet to inform the peer to delete the relevant neighbor entry.

Detecting multiple neighbors

When multiple devices are connected through a hub, enable DLDP on all interfaces connected to the hub to detect unidirectional links among the neighbors. When no Confirmed neighbor exists, an interface enters the unidirectional state.

Figure 9: Network diagram

As shown in Figure 9, Device A through Device D are connected through a hub, and enabled with DLDP. When Ports 1, 2, and 3 detect that the link to Port 4 fails, they delete the neighborship with Port 4, but stay in bidirectional state.