NIF - Neighbor Information Frames
The NIF protocol provides three functions:
1.
FDDI stations learn the MAC addresses of their next upstream neighbor. This allows to maintain a ring map for error isolation purposes.
2.
Neighbor stations watch each other. If an upstream neighbor doesn't transmit frames any longer (because it stopped operation), the downstream neighbor will detect the absence of its upstream neighbor.
3.
Duplicate MAC addresses are detected and reported.
Insertion of new FDDI adapters
If an adapter starts operation (insertion into ring) it will announce its presence by transmitting a NIF announcement frame (SMT/NSA frame with MAC broadcast destination address). The next downstream neighbor acknowledges this frame by switching the NIF announcement frame's AC bits to one (AC=11) and sending an additional NIF response frame.
If the new adapter doesn't know its upstream neigbbor's MAC address, yet, it will be replaced by the default value of
00-00-F8-00-00-00
NIF frames provide FDDI station with the following information: MAC address; station type (workstation, concentrator); number of MAC controllers; number of A / B / S / M ports; topology information; treatment of AC bits.
FDDI adapters have to transmit their neighbor information frames in intervals of 228 seconds; otherwise an error message via SRF is sent by the next downstream neighbor.
Duplicate MAC addresses can be detected by means of NIF announcement and NIF response frames. If a new station B uses a MAC address that is already used by an active station A, station A will detect its 'twin' adapter because it receives station B's NIF announcement frame. If this happens, station A will send a corresponding report frame.
NIF announcement and NIF response convey these parameters:
Para-
meter NIF Announcement/Response
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0x0001 UpstreamNeighborAddress
Address of next upstream neighbor.
0x0002 StationDescriptor
Number of MACs including: addresses; number of ports; port type;
station type (workstation or concentrator).
0x0003 StationState
Topology information (order of MACs and ports attached to the ring);
station's wrap status.
0x200B FddiMACFrameStatusFunctions
Treatment of A/C bits (example: FDDI-to-Ethernet bridges).