User manual

Chapter 38 ZULD
GS2210 Series User’s Guide
327
CHAPTER 38
ZULD
38.1 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 misconfigured
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 235 ZULD Overview
38.1.1 What You Can Do
• Use the ZULD Status screen (Section 38.2 on page 328) to see details on ZULD.
• Use the ZULD Configuration screen (Section 38.3 on page 329) to enable ZULD on a port, configure a
mode and set the probe time.
38.1.2 What You Need to Know
• ZULD must be enabled on the Switch and the port(s) 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 Advanced
Application > OAM > Configuration and selecting Active. OAM must be enabled on other connected
devices too. If OAM is not enabled initially, ZULD will not work.