As illustrated in Figure 104 on page 167, the dual-ring topology consists of two
counter-rotating rings that provide interconnection for both dual-ring and
single-ring stations. One of the rings is designated as the
primary ring and the
other ring is designated as the
secondary ring.
Figure 104. Dual-Ring Topology
Normally, the primary ring is the operational ring and no data other than the
MAC frames flow on the secondary ring.
Single-ring stations can only be connected to the primary ring through a Trunk
Coupling Unit (TCU) and operate exactly as specified in the IEEE 802.5 standard.
Dual-ring stations on the other hand are connected to both the primary ring and
secondary ring through two TCUs.
Dual-ring reconfiguration employs
wrapback to provide for recovery from all
forms of signal loss failures and from trunk line failures not covered by the IEEE
802.5 standard.
The wrapback method restores normal operation by interconnecting the two
rings on each side of the failure as shown in Figure 105 on page 168. This
results in a single ring being formed from the remaining good parts of the
original two rings.
Chapter 8. 8260 Token-Ring Support 167