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Catalyst 2940 Switch Software Configuration Guide
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Chapter20 Configuring SPAN
Understanding SPAN
•It cannot be a source port or a reflector port.
•It cannot be an EtherChannel group or a VLAN.
•It can be a physical port that is assigned to an EtherChannel group, even if the EtherChan nel group
has been specified as a SPAN source. The port is removed from the group while it is configured as
a SPAN destination port.
•The port does not transmit any traffic except that required for the SPAN session.
•If ingress traffic forwarding is enabled for a network security device, the destination port for war ds
traffic at Layer 2.
•It does not participate in spanning tree while the SPAN session is active.
•When it is a destination port, it does not participate in any of the Layer 2 protocols (STP, VTP, CDP,
DTP, PagP, or LACP).
•No address learning occurs on the destination port.
•A destination port receives copies of sent and received traffic for all mon ito re d s ource p orts. I f a
destination port is oversubscribed, it could become congested. This could affect traffic forwarding
on one or more of the source ports.

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), Port Aggregation Protocol (PagP), and Link
Aggregation Control Protocol (LACP) packets.
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 Interaction with Other Features
SPAN interacts with these features:
•Spanning Tree Protocol (STP)—A destination port or a reflector port doe s not p artici pat e in ST P
while its SPAN session is active. The destination or reflector port can participate in STP after the
SPAN session is disabled. On a source port, SPAN does not affect the STP status.
•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 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 session. Changes in VLAN
membership or trunk settings for a source port immediately take effect, and the SPAN session
automatically adjusts accordingly.