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Industrial Managed
Ethernet Switch – EH9711
User Manual
Page
142
of
223
Figure 2.117 Webpage to Configure DDMI
Table 2.105 Descriptions of DDMI Configuration
Label
Description
Factory
Default
Mode
Indicates the DDMI mode operation. Possible modes are:
Enabled
:
Enable DDMI mode operation.
Disabled
:
Disable DDMI mode operation.
Disabled
Click
Save button
to save changes. Click
Reset button
to undo any changes made locally and revert to previously saved
values.
2.19
UDLD
Unidirectional Link Detection (UDLD) is a layer 2 protocol used to determine the physical status of a link. The purpose of
UDL) is to detect and deter issues that arise from Unidirectional Links. UDLD helps to prevent forwarding loops and
blackholding of traffic by identifying and acting on logical one-way links that would otherwise go undetected. UDLD works
with the Layer 1 mechanisms to determine the physical status of a link. At Layer 1, auto-negotiation takes care of physical
signaling and fault detection. UDLD performs tasks that auto-negotiation cannot perform, such as detecting the identities of
neighbors and shutting down misconnected ports. When users enable both auto-negotiation and UDLD, Layer 1 and Layer 2
detections work together to prevent physical and logical unidirectional connections and the malfunctioning of other protocols.
UDLD works by exchanging
UDLD
protocol packets that include information about the port's
device and port ID
between
the neighboring devices. In order for UDLD to work, both devices on the link must support UDLD and have it enabled on
respective ports. Each switch port configured for UDLD sends UDLD protocol packets that contain the port's own device/port
ID, and the neighbor's device/port IDs seen by UDLD on that port. Neighboring ports should see their own device/port ID
(echo) in the packets received from the other side.
Because of this, a port should receive its own device and port ID information from its neighbor if the link is bi-directional. If
a port does not receive information about its own device and port ID from its neighbor for a specific duration of time, the link
is considered to be unidirectional. This can also occur when the link is up on both sides, but one side is not receiving packets,
or when wiring mistakes occur, causing the transmit and receive wires to not be connected to the same ports on both ends of a
link.
This echo-algorithm allows detection of these issues:
Link is up on both sides; however, packets are only received by one side.
Wiring mistakes when receive and transmit fibers are not connected to the same port on the remote side.
Once the unidirectional link is detected by UDLD, the respective port is disabled. Port shutdown by UDLD remains disabled
until it is manually reenabled, or until errdisable timeout expires (if configured).
UDLD can operate in two modes: normal and aggressive. In normal mode, if the link state of the port was determined to be bi-
directional and the UDLD information times out, no action is taken by UDLD. The port state for UDLD is marked
as undetermined. The port behaves according to its STP state. In aggressive mode, if the link state of the port is determined to