Chapter 39 ZULD
XGS2220 Series User’s Guide
266
CHAPTER 39
ZULD
39.1 ZULD Overview
A unidirectional link is a connection where the link is up on both ends, but only one end can receive
packets. This may happen if OAM was initially enabled but then disabled, there are mis-configured
transmitting or receiving lines or the hardware is malfunctioning. Zyxel Unidirectional Link Detection
(ZULD) is a layer-2 protocol that can detect and disable these physical one-way links before they cause
loops or communication malfunction.
In the figure below, S1–A is a bidirectional link as both ends can send packets to each other. S1–B is
unidirectional as B cannot send packets to S1 (although the S1–B link is up). Similarly, S2–S1 is
unidirectional as S1 cannot send packets to S2 (although the S1–S2 link is up).
Figure 188 ZULD Overview
39.1.1 What You Can Do
• Use the ZULD Status screen (Section 39.2 on page 267) to see details on ZULD.
• Use the ZULD Setup screen (Section 39.3 on page 268) to enable ZULD on a port, configure a mode
and set the probe time.
39.1.2 What You Need to Know
• ZULD must be enabled on the Switch and the ports in order to detect unidirectional links by monitoring
OAMPDUs.
• Ports advertise their unidirectional link detection capability using OAMPDUs, so all connected devices
must support OAM as well as ZULD. You need to enable OAM on the Switch by going to PORT > OAM
> OAM Setup and enable the switch in the Active field. OAM must be enabled on other connected
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