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Catalyst2950 and Catalyst2955 Switch Software Configuration Guide
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Chapter25 Configuring SPAN and RSPAN Understanding SPAN and RSPAN

Reflector Port

The reflector port is the mechanism that copies packets onto an RSPAN VLAN. The reflector port
forwards only the traffic from the RSPAN source session with which it is affiliated. Any device
connected to a port set as a reflector port loses connectivity until the RSPAN source session is disabled.
The reflector port has these characteristics:
•It is a port set to loopback.
•It cannot be an EtherChannel group, it does not trunk, a nd it can not do pro toco l filteri ng .
•It can be a physical port that is assigned to an EtherChannel group, even if the EtherChanne l group
is specified as a SPAN source. The port is removed from the group while it is configured as a
reflector port.
•A port used as a reflector port cannot be a SPAN source or destination port , n or ca n a po rt b e a
reflector port for more than one session at a time.
•It is invisible to all VLANs.
•The native VLAN for looped-back traffic on a reflector port is the RSPAN VLAN.
•The reflector port loops back untagged traffic to the sw itch. The tr af fic i s then pl aced on th e RSPAN
VLAN and flooded to any trunk ports that carry the RSPAN VLAN.
•Spanning tree is automatically disabled on a reflector port.
•A reflector port receives copies of sent and received traffic for all monitored source ports. If a
reflector port is oversubscribed, it could become congested. This could affect traffic forwarding on
one or more of the source ports.
If the bandwidth of the reflector port is not sufficient for the traffic volume from the corresponding
source ports, the excess packets are dropped. A 10/100 port reflects at 100 Mbps. A Gigabit port reflects
at 1 Gbps.

SPAN Traffic

You can use local SPAN to monitor al l netw ork traffic, including multicast and bridge protocol data unit
(BPDU) packets, and CDP, VTP, DTP, ST P, PagP, and LA C P packe ts. You cannot use RSP AN to monito r
Layer2 protocols. See the “RSPAN Configuration Guidelines” section on page25-12 fo r mo re
information.
In some SPAN configurations, multiple copies of the same source packet are sent to the SPAN
destination port. For example, a bidirectional (both Rx and Tx) SPAN session is configured fo r th e
sources a1 Rx monitor and the a2 Rx and Tx monitor to destination port d 1. If a p ack et ent ers t he switch
through a1 and is switched to a2, both incoming and outgoing packets are sent to destination port d1.
SPAN and RSPAN Interaction with Other Features
SPAN interacts with these features:
•Spanning Tree Protocol (STP)—A destination port or a reflector port does not participate i n STP
while its SPAN or RSPAN session is acti ve. The destination or reflector port can participate in STP
after the SPAN or RSPAN session is disabled. On a source port, SPAN does not affect the STP status.
STP can be active on trunk ports carrying an RSPAN VLAN.
•Cisco Discovery Protocol (CDP)—A SPAN destination port does not participate in CDP while the
SPAN session is active. After the SPAN session is disabled, the port again participates in CDP.