What Happens To The Mirroring Port; Mirroring On Multiple Ports; Using Port Mirroring With External Rmon Probes - Alcatel-Lucent OmniSwitch 6850-48 Network Configuration Manual

Diagnosing Switch Problems
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 is a 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.

Mirroring on Multiple Ports

If mirroring is enabled on multiple ports and the same traffic is passing through these ports, then only one
copy of each packet is sent to the mirroring destination. When the packet is mirrored for the first time, the
switching ASIC flags the packet as "already mirrored" If the packet goes through one more port where
mirroring is enabled, that packet will not be mirrored again. If both mirroring and monitoring are enabled
then the packet will be either mirrored or monitored (i.e., sent to CPU), whichever comes first.

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 monitors 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
ing Ports (Protection from Spanning Tree)" on page 45-19
OmniSwitch AOS Release 6 Network Configuration Guide
Relationship Between Mirrored and Mirroring Ports
(w/ Copied Incoming
& Outgoing Frames)
for details.
September 2009
Port Mirroring
Mirroring
(Monitoring) Port
"Unblock-
page 45-15