Users Guide

570 Monitoring Switch Traffic
• When port mirroring is enabled, all MAC address entries associated with
destination ports are purged. This prevents transmitting packets out of the
port that are not seen on the mirrored port. If spanning tree is enabled, this
is treated as a topology change.
• The spanning tree protocol is disabled on destination ports such that
frames are always received from or transmitted out of the port as soon as
the port is up (spanning tree status is forwarding and role is disabled). This
is analogous to always setting the spanning tree state of the port to
forwarding. When a port is no longer configured to be the destination
port, spanning tree is re-enabled for that port, if configured. Note that the
disabling of spanning tree on a destination port means that administrators
must only connect the destination port to directly attached probes to avoid
the possibility of a network loop.
• GVRP is disabled on destination ports such that GVRP PDUs are never
received from or transmitted to the port. Dynamic registrations are not
allowed on a destination port. The GVRP configuration at the port is
maintained and is reapplied when the port is no longer part of the SPAN.
• All static filters, both ingress and egress, are disabled on destination ports.
• If routing is enabled on a destination port or an RSPAN VLAN, all route
entries associated with that port are purged. From a routing perspective,
the interface is marked as down.
• Generally, the configuration of the source port is undisturbed so that its
behavior remains the same as if it was not mirrored.
• Packets locally generated by the switch and transmitted over a source port
are not mirrored in an RSPAN VLAN mirroring session.
• The internal CPU port is allowed as a source port for local monitoring
sessions only (not allowed for RSPAN). If the internal CPU port is
mirrored, packets received and generated by the CPU for all ports are
mirrored.
• On ingress, the port mirroring logic stage is after the VLAN tag processing
stage in the hardware. This means that mirrored packets may not appear
the same as they do on the wire if VLAN tag processing occurs. Examples
of VLAN tag processing are DVLAN tunneling (QinQ) or VLAN rewriting.
Likewise, on egress, the port mirroring logic stage is before the VLAN tag