Diagnosing Switch Problems Port Mirroring
OmniSwitch AOS Release 8 Network Configuration Guide December 2017 page 34-10
Relationship Between Mirrored and Mirroring Ports
What Happens to the Mirroring Port
Mirroring Port (MTP), can not be assigned to a port with Tagged VLAN configured on it. Once the
Mirroring Port (MTP) is configured the port does not belong to any VLAN. Inbound traffic into the MTP
is dropped, since it does not belong to any VLAN. When unblocked VLAN is configured , the VLAN ID
specified is assigned to the MTP port as the default VLAN. Hence allowing inbound traffic and handling
traffic for that VLAN ID. Spanning tree remains disabled on MTP port.
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 is not mirrored again. If both mirroring and monitoring are enabled then
the packet is either mirrored or monitored (that is 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.
The diagram on the following page 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
Note. If the mirroring port monitors mirrored traffic on an RMON probe belonging to a different VLAN
than the mirrored port, it must be protected from blocking due to Spanning Tree updates. See “Unblocking
Ports (Protection from Spanning Tree)” on page 34-14 for details.
Workstation
NMS
Mirroring
(Monitoring) Port
(w/ Copied Incoming
& Outgoing Frames)
Mirrored
(Active) Port
(w/ Incoming &
Outgoing Frames)
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