Effect of downstream VLAN tagging on untagged, mirrored traffic
In a remote mirroring application, if mirrored traffic leaves the switch without 802.1Q VLAN tagging, but is forwarded through a downstream device that adds 802.1Q VLAN tags, the MTU for untagged mirrored frames leaving the source switch is reduced below the values shown in Maximum frame sizes for mirroring.
For example, if the MTU on the path to the destination is 1522 bytes, untagged mirrored frames leaving the source switch cannot exceed 1518 bytes. Likewise, if the MTU on the path to the destination is 9220 bytes, untagged mirrored frames leaving the source switch cannot exceed 9216 bytes.