Alcatel-Lucent 6850-48 网络指南

Diagnosing Switch Problems

Port Mirroring

OmniSwitch AOS Release 6 Network Configuration Guide

page 45-15

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.

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.

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.

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 

 for details.