Network OS Administration Guide

Supporting Network OS 6.0.1a

Part Number: 53-1003768-04

Enabling a fabric ISL

The fabric isl enable command controls whether an ISL should be formed between two cluster members. With the default setting of ISL discovery to auto and the ISL formation mode to enable , an ISL automatically forms between two cluster switches.

Performing a fabric isl enable command on an operational ISL has no effect. However, performing a no fabric isl enable command on an interface toggles its link status and subsequently disables ISL formation. In addition, the no fabric isl enable command triggers the switch to inform its neighbor that the local interface is ISL disabled. Upon receiving such information, a neighbor switch stops its ISL formation activity regardless of its current interface state.

After you repair any segmented or disabled ISL ports, toggle the fabric ISL in order to propagate the changes.
A shutdown command on an operating ISL interface not only brings down the physical link but also its FSPF adjacency. The main difference between a shutdown command and a no fabric isl enable command is that the link stays up after a no fabric isl enable, while the link stays down after a shutdown.
Upon a fabric reconvergence that due to a topology change involving the ECMP fabric-isl path, there may be sub-second flooding of known unicast traffic.