Monitoring and Analyzing Switch Operation

Traffic Mirroring

Effect of Downstream VLAN Tagging onUntagged, Mirrored Traffic

In a 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, then the MTU for untagged, mirrored frames leaving the source switch is reduced below the values shown in table B-2. That is, if the MTU on the path to the destination is 1522 bytes, then untagged, mirrored frames leaving the source switch cannot exceed 1518 bytes. If the MTU on the path to the destination is 9220 bytes, then untagged, mirrored frames leaving the source switch cannot exceed 9216 bytes.

Tagged 10 Gbps VLAN link.

Adds 4 bytes to each frame.

Switch

(Aggregator)

Untagged 1 Gbps

VLAN Links

Switch

 

Switch

(Mirror Source)

 

(Mirror Source)

 

 

 

 

 

 

 

 

 

 

Router in the

 

 

Switch

Mirror Path

1Gbps

 

(Remote

 

 

 

 

Mirror

 

 

 

 

Destination)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Traffic

 

 

 

 

 

Analyzer

 

Due to VLAN tagging on the 10 Gbps link, untagged traffic from the mirror sources must be at least 4 bytes smaller than the MTU for the path to the mirror destination.

Figure B-26. Effect of Downstream VLAN Tagging on the MTU for Mirrored Traffic

B-44