Monitoring and Analyzing Switch Operation
Traffic Mirroring
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 Table B-2.
For example, if the MTU on the path to the destination is 1522 bytes, then
untagged mirrored frames leaving the source switch cannot exceed 1518
bytes. Likewise, if the MTU on the path to the destination is 9220 bytes, then
untagged mirrored frames leaving the source switch cannot exceed 9216
bytes.
3500yl
Mirror Source
Router in the
Mirror Path
8212zl
Remote
Mirror
Destination
Traffic
Analyzer
Untagged 1 Gbps
VLAN Links
3500yl
Mirror Source
6200yl
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.
Figure B-57. Effect of Downstream VLAN Tagging on the MTU for Mirrored Traffic
B-94
