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Chapter 3 – Configuring the Ports
Using Port Mirroring
Introduction
Port mirroring is a management tool in which all traffic on a designated port is
copied (or mirrored) to a target port. If a protocol analyzer is attached to the
target port, the traffic stream of valid frames on any source port is made available
for analysis.
Note:
Invalid frames received on the source port will not be mirrored. These include CRC
errors, oversize and undersize packets, fragments, jabbers, collisions, late collisions and dropped
events).
Configuring Port Mirroring
Select a target port that has a higher speed than the source port. Mirroring a 100
Mbps port onto a 10 Mbps port may result in an improperly mirrored stream.
Frames will be dropped if the full duplex rate of frames on the source port exceeds
the transmission speed of the target port. Since both transmitted and received
frames on the source port are mirrored to the target port, frames will be discarded
if the sum traffic exceeds the target port’s transmission rate. This problem reaches
its extreme in the case where traffic on a 100 Mbps full duplex port is mirrored
onto a 10 Mbps half duplex port.
A limitation of port mirroring occurs with multicast traffic. Multicast traffic will
be mirrored onto the target port only if the target port is a member of the same
VLANs as the source port.
Limitations of port mirroring occur with VLAN traffic. If the port selected as the
source port receives an untagged frame that will be forwarded to a tagged port, the
target port will incorrectly show the frame as having been received tagged.
If the port selected as the port mirroring target is configured as a tagged edge
VLAN port, all untagged frames received by and copied from the source port will
be sent tagged with the native VLAN for the source port. If the port selected as
the target is configured as an untagged edge VLAN port, all tags in frames copied
from the source port will be removed before transmission on the target port.
A further limitation of port mirroring is that traffic originated by the switch, such
as ping requests, may not be mirrored.
Port Configuration And Status
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