User`s guide

Configuring
3-37
Furthermore, mirroring traffic of a higher speed interface out to a
lower speed interface may impose a strain on performance (e.g.
capturing FDDI traffics to a 4 Mbps Token Ring). When the size of
the mirrored packet exceeds the size of the maximum transport
unit (MTU) of the diagnostic port, the packet is labeled as
oversized. As an option for local mirroring in an intermixed mode,
the ATX can be configured to truncate or discard oversized
packets.
3.11.2 Packet Capturing and Mirroring
The mirroring of network traffic is performed by the ATX LAN
Switch software, and the mirror image reflects the ATX LAN
Switch internal representation of the packets. Certain physical
layer information (such as Access Control and Frame Control in
802.5 frames) will not be available. The difference in the physical
layers are minor, and should not impair the normal usage of the
port mirroring as it is mostly used in MAC and network layers.
The ATX LAN Switch mirror software attempts to minimize any
differences between the internal and external formats when the
frame is mirrored out the diagnostic port. Other than the possible
framing translation, MAC layer should have only minor or no
differences between the mirrored image and the raw frame on the
wire. On the network layer, there should be no alteration. For
example, when an inbound routed packet is mirrored, the image
reflects the packet prior to any changes made by the ATX LAN
Switch routing software.
The ATX LAN Switch mirror software maintains the original
packet ordering of bridging frames between the inbound and
outbound interfaces. The bridging packets include the
Transparent, Source Routed and Transparent Source Routed
frames. Network layer routing traffic is not subject to this
requirement, and the sequence of routed packets may occasionally
be out of order (as in the cases without port mirroring).