Brocade Network OS Administrator’s Guide v4.1.1 - Manual (Page 321)

FIGURE 38 Interconnected Brocade VCS Fabric clusters cause loop
How ELD detects loops
ELD works by multicasting Protocol Data Unit (PDU) packets on edge ports. A device recognizes a loop
when it receives a PDU that it initiated. Once the device recognizes that a Layer 2 loop exists, it can
take action to disable a port and break the Layer 2 loop.
To minimize the number of disabled ports, ELD assigns a priority to each port and a unique receive limit
(pdu-rx-limit) to each Brocade VCS Fabric cluster. The port priority determines whether the sending or
receiving edge port of the cluster is disabled. The pdu-rx-limit determines on which Brocade VCS Fabric
the action takes place. Without these configured values, it is possible that a Layer 2 loop could be
detected in multiple clusters at the same time. As a result, multiple ports would be disabled, stopping
traffic among the Brocade VCS Fabric clusters.
The following figure shows the same interconnections as the previous figure under Edge-loop detection
overview on page 319, but with ELD enabled on each edge port, and with port priorities and receive
limits assigned.
How ELD detects loops
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