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member is removed from the MLAG interface, the Primary decides if the minimum criteria is satisfied. If it
is not, it will shut down the MLAG interface on both the devices. Shutting down the MLAG interface on the
Secondary is not allowed. The MLAG interface can only be shut down on the Primary.
FDB entries learned on MLAG interfaces are synced between the two devices. In the case where all MLAG
member ports are UP, data traffic does not traverse the peer link.
3.6. Unidirectional Link Detection (UDLD)
The UDLD feature detects unidirectional links on physical ports. UDLD must be enabled on the both sides of
the link in order to detect a unidirectional link. The UDLD protocol operates by exchanging packets
containing information about neighboring devices.
The purpose of UDLD feature is to detect and avoid unidirectional links. A unidirectional link is a forwarding
anomaly in a Layer 2 communication channel in which a bidirectional link stops passing traffic in one
direction.
3.6.1. UDLD Modes
The UDLD supports two modes: normal and aggressive.
In normal mode, a port's state is classified as undetermined if an anomaly exists. An anomaly might be the
absence of its own information in received UDLD messages or the failure to receive UDLD messages. An
undetermined state has no effect on the operation of the port. The port is not disabled and continues
operating. When operating in UDLD normal mode, a port will be put into a disabled state (D-Disable) only in
the following situations:
ï‚·
The UDLD PDU received from a partner does not have its own details (echo).
ï‚·
When there is a loopback, and information sent out on a port is received back exactly as it
was sent.
When operating in UDLD aggressive mode, a port is put into a disabled state for the same reasons that it
occurs in normal mode. Additionally, a port in UDLD aggressive mode can be disabled if the port does not
receive any UDLD echo packets even after
bidirectional connection
was
established.
If a
bidirectional
link is
established,
and packets suddenly stop coming from partner device, the UDLD aggressive-mode port
assumes that link has become unidirectional.
3.6.2. UDLD and Port-channel Interfaces
UDLD is supported on individual physical ports that are members of a Port-channel. If any of the aggregated
links becomes unidirectional, UDLD detects it and disables the individual link, but not the entire Port-
channel. This improves the fault tolerance of the Port-channel.
3.6.3. Configuring UDLD
A network administrator decides to use the UDLD feature while building a loop-free topology with the use
of STP. The administrator configures the ports on both side of the link to use UDLD in aggressive mode to
ensure that ports with unidirectional links will be shut down, and no loops will be introduced into topology.
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