AM79C987 AMD [Advanced Micro Devices], AM79C987 Datasheet - Page 29

• Manufacturer Part Number: AM79C987
• Description: Hardware Implemented Management Information Base (HIMIB) Device
• Manufacturer: AMD [Advanced Micro Devices]

IMR+/HIMIB Security Features
The Am79C981 Integrated Multiport Repeater Plus
(IMR+) and the Am79C987 Hardware Implemented
Management Information Base (HIMIB) Ethernet re-
peater chip-set is capable of providing physical network
security features. AMD will only make these features
available to customers who are under an IMR+/HIMIB
security non disclosure agreement (NDA). A description
of the security feature is summarized below. For more
information, contact your local AMD sales office to gen-
erate an IMR+/HIMIB security NDA.
Security Features Summary
The HIMIB incorporates a feature to allow the destina-
tion address (DA) field of a received packet to be com-
pared with the known MAC address connected to each
port. The MAC address for each port is contained in the
HIMIB Last Source Address (LSA) register, which can
be programmed by the user or it will be “learnt” by the
HIMIB device. On receipt of a packet on one port, all
other ports have the contents of the LSA register com-
pared with the DA field of the received frame. If there is a
match on any port, the frame is repeated to that port
normally. For those ports which have the security fea-
ture enabled and do not have a DA/LSA match, the
APPENDIX A
Am79C987
repeated bit stream of the packet will be corrupted (fre-
quently termed “eavesdrop protection”), and the port will
transmit an alternating pattern of 1 and 0 following the
18th (approximate) bit of the Source Address field. This
feature can be enabled/disabled on a port by port basis
using a mask located in the HIMIB Port Control Regis-
ters. Any port with the security feature disabled (using
the field in the Port Controls Registers) will repeat the
packet normally. Note that multicast and broadcast
packets are transmitted to all ports unmodified, regard-
less of the enable/disable state of the security function.
Ports that are connected to single stations can be se-
cured by enabling the eavesdrop protection function
and enabling the Last Source Address Change Inter-
rupt. This prevents unauthorized eavesdropping by sta-
tions on the LAN who are not directly addressed by the
sourcing node, hence the learning of valid source ad-
dresses and “snooping” on data is virtually impossible.
In addition, this allows the management software to de-
tect and possibly disable the port in real time if the HIMIB
indicates via the hardware interrupt line that the Source
Address has changed.
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