818 | Port Monitoring
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Figure 38-5.  Port Monitoring Configurations on the C-Series and S-Series
FTOS Behavior: On the C-Series and S-Series, all monitored frames are tagged if the configured 
monitoring direction is transmit (TX), regardless of whether the monitored port (MD) is a Layer 2 or 
Layer 3 port. If the MD port is a Layer 2 port, the frames are tagged with the VLAN ID of the VLAN to 
which the MD belongs. If the MD port is a Layer 3 port, the frames are tagged with VLAN ID 4095. If 
the MD port is in a Layer 3 VLAN, the frames are tagged with the respective Layer 3 VLAN ID. For 
example, in the configuration source gig 6/0 destination gig 6/1 direction tx, if the MD port 
gigabitethernet 6/0 is an untagged member of any VLAN, all monitored frames that the MG port 
gigabitethernet 6/1 receives are tagged with the VLAN ID of the MD port. Similarly, if BPDUs are 
transmitted, the MG port receives them tagged with the VLAN ID 4095. This behavior might result in a 
difference between the number of egress packets on the MD port and monitored packets on the MG 
port.
FTOS Behavior: The C-Series and S-Series continue to mirror outgoing traffic even after an MD 
participating in Spanning Tree Protocol transitions from the forwarding to blocking.
Port-Pipe 0
Port-Pipe 1
Port-Pipe 0
MD
onitor Session 0 MG
MD
onitor Session 1
onitor Session 2
Port Monitoring 0
Line Card 0 Line Card 1
Port-Pipe 1
MD
MD MG
MD
MG
MD
MG