Alcatel Omni Switch/Router, User Manual

Page: 252 / 1100

Duplicate MAC Address Support
Page 9-30
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 FEP
s use the same
MAC
address assigned to the
connecting devices for the purpose of redundancy.
•
DEC
net networks. The
DEC
net protocol assigns the special
MAC
address,
AA000400XXYY
(
XXYY
is an internal protocol ID) to each
DEC
net 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 addres
s moves from one
VLAN
to another
VLAN
within the same Group, the switch will not treat the
MAC
addresses as separate.