4–10
DECbridge 500/600 Series Problem Solving
4.3.1
AP2 (or AP) Module Description
The Application Processor (AP or AP2) module contains the main processing unit in
the bridge. It makes most of the packet filtering/forwarding decisions, coordinates
various bridge management tasks, and handles the spanning tree operation.
The module contains several blocks of memory. Volatile memory is used for storing
program code and status information. Nonvolatile (read-only) memory is used for
storing program code; nonvolatile (read/write) memory is used for storing downline
loaded programs, bridge parameters and error logs.
The DECbridge 500 series has one address ROM to store the 48-bit physical address
of the bridge. The DECbridge 600 series has two address ROMs: one stores the ad-
dress of Port 1, and the other stores the address of Port 3. Addresses for Ports 2 and 4
are derived from these two stored addresses.
4.3.2
QM Module Description
The main function of the Queue Manager module is to service packets received from
the FDDI link. The queue manager is supported by a table lookup (TLU) unit, which
compares addresses of received packets with addresses stored in the bridge’s address
table. The results are used to implement address filtering of packets.
4.3.3
FI Module Description
The Fiber Interface (FI) module provides either a single attachment station (SAS) or
dual attachment station (DAS) connection to the FDDI media. (In the DECbridge
600, a null FI module is used that has no FDDI interface.) The major functional
blocks in the FI module are the FDDI chipset and the FI packet memory.
The FDDI chipset provides the FDDI interface and handles the protocol and memory
access control for the FDDI port. In a DAS model, the FDDI chipset contains two
PHY and PMD sections to provide the DAS connection. Each PMD section can han-
dle either a standard ANSI or a single-mode interface.
The FI packet memory stores the frames received from the FDDI ring and the frames
to be transmitted to the FDDI ring.
The DAS multimode version also provides control for an optional optical-bypass
relay. This bypass relay helps maintain FDDI ring integrity if the bridge becomes
broken or is powered down.
