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Catalyst2950 Desktop Switch Software Configuration Guide
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Chapter20 Configuring SPAN and RSPAN Understanding SPAN and RSPAN
•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.
If the bandwidth of the reflector port is not sufficient to handle the traffic 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 Cisco Discovery Protocol (CDP), VLAN Trunk Protocol (VTP), Dynamic
Trunking Protocol (DTP), Spanning Tree Protocol (STP), and Port Aggregation Protocol (PagP) packets.
You cannot use RSPAN to monitor Layer2 protocols. See the “RSPAN Configuration Guidelines”
section on page 20-10 for more 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.
•VLAN Trunking Protocol (VTP)—You can use VTP to prune an RSPAN VLAN between switches.
•VLAN and trunking—You can modify VLAN membership or trunk settings for source, destination,
or reflector ports at any time. However, changes in VLAN membership or trunk settings for a
destination or reflector port do not take effect until you disable the SPAN or RSPAN session.
Changes in VLAN membership or trunk settings for a source port immediately take effect, and the
SPAN session automatically adjusts accordingly.
•EtherChannel—You can configure an EtherChannel group as a source port but not as a SPAN
destination port. When a group is configured as a SPAN source, the entire group is mo nit ored.