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Duplicate MAC Address Support
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Duplicate MAC Address Support
When the switch sees the same MAC address sending traffic on a different switch port (a
Duplicate MAC Address), it assumes the original network device moved. The switch sends a
trap notifying network management of this station move event. It sends one trap for a device
move within the same Group and another trap for a device move outside of the home Group.
A station move trap is normally sent after an actual station move. However, certain network
configurations assign the same
MAC address to different network devices (physical and
virtual) as standard practice. In these situations, the duplicate MAC address appears as a
station move when it is really a normal occurrence in these network configurations. These
network configurations that use the same MAC address for different devices include:
•
LAN Emulation under Cisco routers. Cisco routers use the same MAC address for each LAN
Emulation Client (LEC). In LAN Emulation, each ELAN needs to be treated as a separate LAN
and should therefore have a separate MAC address.
•
IBM Front End Processor (FEP). Many IBM FEPs use the same MAC address assigned to the
connecting devices for the purpose of redundancy.
•
DECnet networks. The DECnet protocol assigns the special MAC address, AA000400XXYY
(XXYY is an internal protocol ID) to each DECnet station or routing device regardless of the
number of physical interfaces.
Initially, duplicate
MAC addresses in these special situations may be no more of a problem
than extra traps being sent for an event (station move) that did not really happen. However,
when a large number of these network devices send the same MAC address out the same
port, flooding can occur and the switch will eventually shut the port down.
To prevent a port from being shut down, the switch needs some way of knowing the dupli-
cate
MAC addresses originating from the port are not an error condition.
The switch will treat duplicate
MAC addresses as separate addresses as long as they are
learned from a different Group as the original MAC. Each duplicate MAC address will use one
entry in the CAM. Up to 32 duplications of the same MAC address are supported. Duplicate
MAC addresses learned from virtual ports within the same Group are treated as station moves
and will generate corresponding traps. If the
MAC address moves from one VLAN to another
VLAN within the same Group, the switch will not treat the MAC addresses as separate.