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Software Configuration Guide—Release 12.2(25)SG
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Chapter37 Configuring SPAN and RSPAN
Overview of SPAN and RSPAN
SPAN and RSPAN Concepts and Terminology
This section describes concepts and terminology associated with SPAN and RSPAN configuration and
includes the following subsections:
•SPAN Session, page 37-3
•Traffic Types, page 37-3
•Source Port, page 37-4
•Destination Port, page 37-5
•VLAN-Based SPAN, page 37-5
•SPAN Traffic, page 37-6

SPAN Session

A local SPAN session associates a destination port with source ports. You can monitor incoming or
outgoing traffic on a series or range of ports and source VLANs. An RSPAN session associates source
ports and source VLANs across your network with an RSPAN VLAN. The destination source is the
RSPAN VLAN.
You configure SPAN sessions by using parameters that specify the source of network traffic to monitor.
You can configure multiple SPAN or RSPAN sessions with separate or overlapping sets of SPAN
sources. Both switched and routed ports can be configured as SPAN sources or destination ports .
An RSPAN source session associates SPAN source ports or VLANs with a destination RSPAN VLAN.
An RSPAN destination session associates an RSPAN VLAN with a destination port.
SPAN sessions do not interfere with the normal operation of the switch; however, an oversubscribed
SPAN destination (for example, a 10-Mbps port monitoring a 100-Mbps port) results in dropped or lost
packets.
You can configure SPAN sessions on disabled ports; however, a SPAN session does not become active
unless you enable the destination port and at least one source port or VLAN for that session.
A SPAN session remains inactive after system startup until the destination port is operational.

Traffic Types

SPAN sessions include these traffic types:
•Receive (Rx) SPAN—The goal of receive (or ingress) SPAN is to monitor as much as possible all
packets received by the source interface or VLAN before any modification or processing is
performed by the switch. A copy of each packet received by the source is sent to the destination port
for that SPAN session. You can monitor a series or range of ingress ports or VLANs in a SPAN
session.
On tagged packets (Inter-Switch Link [ISL] or IEEE 802.1Q), the tagging is removed at the ing ress
port. At the destination port, if tagging is enabled, the packets appear with the ISL or 802.1Q
headers. If no tagging is specified, packets appear in the native format.
Packets that are modified because of routing are copied without modification for Rx SPAN; that is,
the original packet is copied. Packets that are modified because of quality of service (QoS)—for
example, modifi ed Differentiated Services Code Point (DSCP)—are copied wi th modificati on for Rx
SPAN.