Installing and Administering HP FDDI/9000 Software

28 Chapter 1

FDDI Resources

FDDI Concepts

During normal operation, the two rings in an FDDI network are

independent. The primary ring actively transmits data in accordance

with timed-token protocol, while the secondary ring remains inactive,

providing a redundant LAN capability, until a fault occurs to break the

primary ring.

When stations on an FDDI network are connected to both the primary

and secondary rings simultaneously, these stations can access either or

both rings for data transmission and can reconﬁgure the network, if

necessary. Stations directly connected to the dual ﬁber optic ring are

referred to as dual-attached stations. Dual-attached stations do not

require connection to the FDDI network through a concentrator.

Dual homing improves network availability by allowing the dual ﬁber

connections on the FDDI adapter card to be connected into two separate

concentrators simultaneously. If one connection is broken, the system

will still be able to communicate on the FDDI ring. HP FDDI/9000

supports dual attach and dual homing capabilities for Series 800

systems.

The FDDI Dual Ring in Wrap Mode

In Figure 1-4 a fault has occurred on the primary ring because the

mainframe computer is not operational. The FDDI fault tolerant

capability allows the rings to reconﬁgure automatically and form a new

ring between the two concentrators and the router. An optical bypass

could also be used. In that case, the ring would not wrap. The dotted

lines illustrate connections to the mainframe which are no longer active.

In this new conﬁguration, the concentrator at MAC Address

080009001023 will have an attachment state (CF_State) of Wrap_A,

indicating frames are now being transmitted via the A port, and a new

downstream neighbor, the router at MAC Address 080009002034.

The CF_State of individual stations on the ring varies depending on the

ring conﬁguration at any given time. In Figure 1-3, which shows an

FDDI ring in normal operation, all the dual-attached stations on the ring

have a CF_State of Thru_A, with the data stream entering each MAC

through the A port and exiting through the B port.