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24-5
Catalyst 2950 and Catalyst 2955 Switch Software Configuration Guide
OL-10101-02
Chapter 24 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, and it cannot do protocol filtering.
• It can be a physical port that is assigned to an EtherChannel group, even if the EtherChannel 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, nor can a port be 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 switch. The traffic is then placed on the 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 all network traffic, including multicast and bridge protocol data unit
(BPDU) packets, and CDP, VTP, DTP, STP, PagP, and LACP packets. You cannot use RSPAN to monitor
Layer
2 protocols. See the “RSPAN Configuration Guidelines” section on page 24-11 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 for the
sources a1 Rx monitor and the a2 Rx and Tx monitor to destination port d1. If a packet enters the 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 in STP
while its SPAN or RSPAN session is active. 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.