What Happens To The Mirroring Port; Using Port Mirroring With External Rmon Probes - Alcatel OmniSwitch 6800 Series Network Configuration Manual

Port Mirroring
Note that when port mirroring is enabled, there may be some performance degradation, since all frames
received and transmitted by the mirrored port need to be copied and sent to the mirroring port.
NMS
Workstation
Mirrored
(Active) Port
(w/ Incoming &
Outgoing Frames)

What Happens to the Mirroring Port

When you set up port mirroring and attach cables to the mirrored and mirroring ports, the mirroring port
remains enabled and part of the Bridging Spanning Tree until you protect it from Spanning Tree updates
by specifying an unblocked VLAN as part of the configuration command line. The mirroring port does not
transmit or receive any traffic on its own.

Using Port Mirroring with External RMON Probes

Port mirroring is a helpful monitoring tool when used in conjunction with an external RMON probe. Once
you set up port mirroring, the probe can collect all relevant RMON statistics for traffic on the mirrored
port. You can also move the mirrored port so that the mirroring port receives data from different ports. In
this way, you can roam the switch and monitor traffic at various ports.
Note. If the mirroring port will monitor mirrored traffic on an RMON probe belonging to a different
VLAN than the mirrored port, it should be protected from blocking due to Spanning Tree updates. See
"Unblocking Ports (Protection from Spanning Tree)" on page 24-12
The following diagram illustrates how port mirroring can be used with an external RMON probe, to copy
RMON probe frames and Management frames to and from the mirroring and mirrored ports. Frames
received from an RMON probe attached to the mirroring port can be seen as being received by the
mirrored port. These frames from the mirroring port are marked as if they are received on the mirrored
port before being sent over the switch backplane to an NMS station. Therefore, management frames
destined for the RMON probe are first forwarded out the mirrored port. After being received on the
mirrored port, copies of the frames are mirrored out the mirroring port—the probe attached to the mirror-
ing port receives the management frames.
page 24-10
Relationship Between Mirrored and Mirroring Ports
OmniSwitch 6800 Series Network Configuration Guide
Diagnosing Switch Problems
Mirroring
(Monitoring) Port
(w/ Copied Incoming
& Outgoing Frames)
for details.
November 2004