Administrator Guide

Layer 2 Switching Commands 653
• Up to 4 sessions with egress (TX) traffic mirroring may be active.
• Up to 2 sessions with both (RX and TX) traffic mirroring may be active.
• Any other combination of up to 4 total ingress or egress mirroring may be
active.
Destination (probe) interfaces do not perform MAC learning and drop
ingress traffic (forwarding is disabled and incoming packets are dropped).
Routing, spanning-tree, and port channel configuration are operationally
disabled on probe ports. Destination interfaces must be dedicated to the
monitoring function (i.e., connected to a PC running WireShark or some
other packet decoder).
VLAN based mirroring is applicable only for ingress (RX) traffic. If neither rx,
tx, nor both are specified in a source session, both ingress and egress traffic
are monitored
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.
On ingress, the port mirroring logic stage is after the VLAN tag processing
stage in the hardware. This means that, on ingress, packets may not appear as
they do on the wire if processing such as VLAN or CoS value rewriting is
programmed or DVLAN tunneling is enabled on the source interface.
Examples of ingress VLAN tag processing are DVLAN tunneling (QinQ) or
VLAN rewriting. Likewise, on egress, the port mirroring logic stage is before
the egress VLAN tag processing stage. This means that, on egress, packets
may not appear as they do on the wire if processing such as VLAN or CoS
value rewriting is programmed.
Reserve a few VLANs across the network for the exclusive use of RSPAN. The
RSPAN VLANs should only be configured on the reflector interfaces
(generally the uplink/transit/downlink interface). RSPAN VLANs are used
exclusively to carry the mirrored traffic across the network and cannot have
source or destination interfaces as members of the RSPAN VLAN. RSPAN
destination interfaces will continue to transmit mirrored traffic even if
blocked by spanning-tree. Each RSPAN session must use a unique reflector
port, destination port and RSPAN VLAN. Reflector ports
(source/transit/destination) should be configured as trunk or general mode