Specifications

A.13 Fault Tolerance in Dual Rings
FDDI Configuration Capabilities A–31
A.13 Fault Tolerance in Dual Rings
This section describes the fault tolerant safeguards that are built into DEChub 900
network modules.
A.13.1 Maintaining Network Connectivity
When a concentrator or switch in a dual ring is removed (hot swapped), the FDDI
ring wraps according to FDDI standards. If the removed module has both A and B
ports that are connected to the DEChub 900 backplane, the Hub Manager detects the
removal and reconnects the two neighboring (upstream and downstream) FDDI sta-
tions.
If a network module is detected as being unavailable, the Hub Manager will also
patch-out the module (as long as Auto Healing has been enabled).
This is a distinct advantage that FDDI modules, which are configured into a dual ring
in a DEChub, have over individual nonhub-based FDDI network modules.
NOTE
Because ring wrap occurs much faster than backplane healing, it always
occurs first. Thereafter, the Hub Manager firmware automatically reconnects
the disconnected DAS stations through the DEChub 900 backplane, allow-
ing the dual ring to unwrap. Fault recovery (unwrapping of the dual ring in
the DEChub 900 backplane) is supported in all DEChub 900 FDDI network
modules.
A.13.2 Example of Dual Ring Backplane Healing
As shown in Figure A–27, if Concentrator 1 is removed, Concentrator 2 and Switch
1 continue communications with each other and with all other FDDI stations on the
DAS backbone. This is so, because Concentrator 2 (sensing the change) wraps the
FDDI dual ring.
If Concentrator 2 fails, Concentrator 1 and Switch 1 wrap the dual ring and commu-
nications can continue. Within seconds of this particular ring wrap, where Concen-
trator 2 has become unavailable, the Hub Manager automatically reconnects Switch
1 to Concentrator 1 and the dual ring unwraps.