Monitored Traffic - Cisco Catalyst 3750 Software Configuration Manual

Understanding SPAN and RSPAN
packets that are sent to the RSPAN VLAN. To configure an RSPAN destination session on another
device, you associate the destination port with the RSPAN VLAN. The destination session collects all
RSPAN VLAN traffic and sends it out the RSPAN destination port.
An RSPAN source session is very similar to a local SPAN session, except for where the packet stream
is directed. In an RSPAN source session, SPAN packets are relabeled with the RSPAN VLAN ID and
directed over normal trunk ports to the destination switch.
An RSPAN destination session takes all packets received on the RSPAN VLAN, strips off the VLAN
tagging, and presents them on the destination port. Its purpose is to present a copy of all RSPAN VLAN
packets (except Layer 2 control packets) to the user for analysis.
There can be more than one source session and more than one destination session active in the same
RSPAN VLAN. There can also be intermediate switches separating the RSPAN source and destination
sessions. These switches need not be capable of running RSPAN, but they must handle the requirements
of the RSPAN VLAN (see the
Traffic monitoring in a SPAN session has these restrictions:
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Monitored Traffic

SPAN sessions can monitor these traffic types:
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Catalyst 3750 Metro Switch Software Configuration Guide
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Sources can be ports or VLANs, but you cannot mix source ports and source VLANs in the same
session.
The switch supports up to two source sessions; you can run both a local SPAN and an RSPAN source
session in the same switch. The switch supports a total of 66 source and RSPAN destination
sessions.
ES ports do not support SPAN source sessions.
You can have multiple destination ports in a SPAN session, but no more than 64 destination ports.
You can configure two separate SPAN or RSPAN source sessions with separate or overlapping sets
of SPAN source ports and VLANs. Both switched and routed ports can be configured as SPAN
sources and destinations.
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, can result in dropped
or lost packets.
When RSPAN is enabled, each packet being monitored is transmitted twice, once as normal traffic
and once as a monitored packet. Therefore monitoring a large number of ports or VLANs could
potentially generate large amounts of network traffic.
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.
The switch does not support a combination of local SPAN and RSPAN in a single session. That is,
an RSPAN source session cannot have a local destination port, an RSPAN destination session cannot
have a local source port, and an RSPAN destination session and an RSPAN source session that are
using the same RSPAN VLAN cannot run on the same switch.
Receive (Rx) SPAN—The goal of receive (or ingress) SPAN is to monitor as much as possible all
the 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.
Packets that are modified because of routing or quality of service (QoS)—for example, modified
Differentiated Services Code Point (DSCP)—are copied before modification.
"RSPAN VLAN" section on page
Chapter 21 Configuring SPAN and RSPAN
21-8).
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