How Dldp Works - 3Com 4500G Family Configuration Manual

Figure 1-1 Correct and incorrect fiber connections
Correct fiber conecton
Device A
GE1/0/50
GE1/0/50
Device B
Ethernet
optical port
The Device link detection protocol (DLDP) is an technology for dealing with unidirectional links that may
occur in a network. On detecting a unidirectional link, DLDP, as configured, can 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. While
the auto-negotiation mechanism provided by the physical layer detects physical signals and faults,
DLDP 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 can be detected and shut down and prevents failure of other
protocols such as STP. If both ends of a link are operating normally at the physical layer, DLDP detects
whether the link is correctly connected at the link layer and whether the two ends can exchange packets
properly. This is beyond the capability of the auto-negotiation mechanism at the physical layer.

How DLDP Works

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 DLDP link states
State
Initial
Inactive
Active
Unidirectional connection type 1
Cross-connected fibers
Device A
GE1/0/51 GE1/0/50
GE1/0/51 GE1/0/50
Device B
Tx end Rx end
Table 1-1.
DLDP is disabled.
DLDP is enabled but the link is down.
DLDP is enabled and the link is up, or the neighbor entries have
been cleared.
Unidirectional connection type 2
One fiber of a fiber pair Is not
GE1/0/51 GE1/0/50
GE1/0/51 GE1/0/50
Fiber link
Indicates...
1-2
connected or Is broken
Device A
GE1/0/51
GE1/0/51
Device B
Unconnected or broken fiber