Device Link Detection Protocol (DLDP)

DLDP Introduction

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 1-2. 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

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.

Configuring DLDP

Network Diagram

Figure 1-3 Network diagram for DLDP configuration

Networking and Configuration Requirements

l          Device A and Device B are connected through two fiber pairs, in which two fibers are cross-connected, as shown in Figure 1-3.

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.

Configuration Example

l          Device A

#

 dldp enable

 dldp interval 6

 dldp work-mode enhance

 dldp delaydown-timer 2

#

interface GigabitEthernet1/0/50

 dldp enable

#

interface GigabitEthernet1/0/51

 dldp enable

l          Device B

The complete configuration on Device B is the same as that on Device A and is thus omitted.

Configuration Guidelines

1)        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.

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

3)        Set the interval for sending Advertisement packets to a value not longer than one-third of the STP convergence time. If the interval is too long, STP loops may occur before unidirectional links are torn down, and it takes a long time for the device to detect unidirectional links, thus causing more traffic forwarding errors; if the interval is too short, unnecessary Advertisement packets can be generated to consume bandwidth. Therefore, you are recommended to use the default value.

4)        To enable DLDP to operate properly, make sure the DLDP authentication modes and the passwords of the two sides of a link are the same.

5)        Destination MAC addresses of DLDP packets fall into two types: class A and class B, which is dependent on the software versions running on the device. To ensure that DLDP operates properly, interconnected devices must use destination MAC addresses of the same class.