Users Guide

Monitoring Switch Traffic 601
The packet that is mirrored to the destination port is normally in the same
format as the original packet on the wire, except as noted in the following
section: Port Mirroring Behaviors. This means that the mirrored packet is
VLAN tagged or untagged as it was received/transmitted on the source port.
Destinations include physical interfaces and RSPAN VLANs.
Mirrored traffic is subject to the same QoS constraints as normal traffic.
Oversubscribed traffic (both mirrored and un-mirrored) will be dropped in
accordance with the configured or default policy. The operator may assign
CoS or DiffServ policies to the mirrored traffic in the same manner as for
normal traffic. RSPAN traffic is transmitted with a PCP of 0.
After configuring the port mirroring session, enable or disable the
administrative mode of the session to start or stop the probe port from
receiving mirrored traffic.
Port Mirroring Behaviors
The following behaviors are applicable to port mirroring:
• The following source port types may be configured in more than one
session in support of M:N mirroring:
– Physical ports
– LAGs
– CPU
VLANs and RSPAN may not be configured as mirroring sources in more
than a single session. VLAN mirroring is not recommended for RSPAN if
sources on multiple switches are members of the VLAN. This is because as
stations communicate with each other over the mirrored VLAN, duplicate
packets will be sent to the probe: once for the source port, and once for
each switch over which the packet is received in the source VLAN.
• Only one destination (probe) is supported for a session. Logical and CPU
ports cannot be configured as a probe port. The destination (probe) port
loses its VLAN configuration when port mirroring is enabled. The VLAN
configuration is restored when the port is no longer configured for a
monitor session. Traffic transmitted by the connected station into a probe
port is dropped. The mirrored source and the transit ports retain their
VLAN configuration. VLAN-based mirroring is applicable only for ingress
traffic.