B-44
Monitoring and Analyzing Switch Operation
Traffic Mirroring
Effect of Downstream VLAN Tagging on
Untagged, 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.
Figure B-26. Effect of Downstream VLAN Tagging on the MTU for Mirrored Traffic
Switch
(Mirror Source)
Router in the
Mirror Path
Switch
(Remote
Mirror
Destination)
Traffic
Analyzer
Untagged 1 Gbps
VLAN Links
Switch
(Mirror Source)
Switch
(Aggregator)
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.
1Gbps
Tagged 10 Gbps VLAN link.
Adds 4 bytes to each frame.
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