Mirroring Rules and Restrictions
This section summarizes the rules and restrictions for configuring mirroring:
- Each configured mirror instance that you configure is saved,
regardless of its state.
- To change monitor ports you must first remove all the filters.
- You cannot mirror the monitor port.
- The mirroring configuration is removed only if the configuration
matches the removed VLAN or slot. If you have a match you can
do the following:
- Delete a VLAN (for all VLAN-based filters).
- Delete a port from a VLAN (for all VLAN-, port-based
- Unconfigure a slot (for all port-based filters on that slot).
- Any mirrored port can also be enabled for load sharing (or link
aggregation); however, each individual port of the load-sharing group must be explicitly
configured for mirroring.
- When traffic is modified by hardware on egress, egress mirrored
packets may not be transmitted out of the monitor port as they egressed the port containing
the egress mirroring filter. For example, an egress mirrored packet that undergoes VLAN
translation is mirrored with the untranslated VLAN ID. In addition, IP multicast packets
which are egress mirrored contain the source MAC address and VLAN ID of the unmodified
- The monitor port is automatically removed from all VLANs; you cannot
add it to a VLAN.
- You cannot use the management port in mirroring configurations.
- You cannot run ELSM and mirroring on the same port. If you attempt to
enable mirroring on a port that is already enabled for ELSM, the switch returns a message
similar to the following: Error: Port mirroring cannot be enabled on an ELSM enabled
- With one-to-many mirroring, you need to enable jumbo frame support in
the mirror-to port and loopback port, if you need to mirror tagged packets of length 1519 to
- The loopback port is dedicated for mirroring, and cannot be used for
other configurations. This is indicated through the glowing LED.
- Egress mirrored packets are always tagged when egressing the monitor
port. If an egress mirrored packet is untagged on the egress mirrored port, the mirrored
copy contains a tag with an internal VLAN ID.
- As traffic approaches line rate, mirroring rate may decrease. Since
mirroring makes copies of traffic, the bandwidth available will be devoted mostly to regular
traffic instead of mirrored traffic when the load is high.
- A mirror port cannot be a LAG.