H3C S5120-EI Series - Configuring DLDP; DLDP Overview; Background

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Configuring DLDP
DLDP overview
Background
Unidirectional links occur when one end of a link can receive packets from the other end, but the other
end cannot receive packets sent by the first end. Unidirectional links result in problems such as loops in
an STP-enabled network.
For example, the link between two switches, Switch A and Switch B, is a bidirectional link when they are
connected via a fiber pair, with one fiber used for sending packets from A to B and the other for sending
packets from B to A. This link is a two-way link. If one of the fibers gets broken, the link becomes a
unidirectional link (one-way link).
There are two types of unidirectional fiber links. One occurs when fibers are cross-connected. The other
occurs when a fiber is not connected at one end, or when one fiber of a fiber pair gets broken. Figure
7 sho
ws a correct fiber connection and the two types of unidirectional fiber connection.
Figure 7 Correct and incorrect fiber connections
The Device link detection protocol (DLDP) detects unidirectional links (fiber links or twisted-pair links) and
can be configured to shut down the related port automatically or prompt users to take actions to avoid
network problems.
As a data link layer protocol, DLDP cooperates with physical layer protocols to monitor link status. When
the auto-negotiation mechanism provided by the physical layer detects physical signals and faults, DLDP
Correct fiber conecton
Unidirectional connection type 1
Cross-connected fibers
Unidirectional connection type 2
One fiber of a fiber pair Is not
connected or Is broken
Port 1 Port 2
Device A
Device B
Port 1
Device A
Device B
Port 1
Device A
Device B
Ethernet
optical port
Unconnected or broken fiberFiber link
Port 2 Port 2
Port 1 Port 2
Port 1 Port 2
Port 1 Port 2
Tx end Rx end