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Cisco Catalyst Blade Switch 3130 for Dell Software Configuration Guide
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Chapter29 Configuring SPAN and RSPAN Understanding SPAN and RSPAN
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 respond to the
requirements of the RSPAN VLAN (see the “RSPAN VLAN” section on page 29-9).
Traffic monitoring in a SPAN session has these restrictions:
•Sources can be ports or VLANs, but you cannot mix source ports and so urce VLAN s in t he sa me
session.
•The switch supports up to two local SPAN or RSPAN source sessions.
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You can run both a local SPAN and an RSPAN source session in the same switch or switch stack.
The switch or switch stack supports a total of 66 source and RSPAN destination sessions.
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On a desktop switch, 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.
•You can have multiple destination ports in a SPAN session, but no more than 64 destination ports
per switch stack.
•SPAN sessions do not interfere with the normal operation of the switch. Ho we ver , an ov ersubscrib ed
SPAN destination, for example, a 10-Mb/s port monitoring a 100-Mb/s port, can result in drop pe d
or lost packets.
•When SPAN or RSPAN is enabled, each packet being monitored is sent 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 ses sion.
•The switch does not support a combination of local SPAN and RSPAN in a single session.
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An RSPAN source session cannot have a local destination port.
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An RSPAN destination session cannot have a local source port.
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An RSPAN destination session and an RSPAN source session that are using the same RSPAN
VLAN cannot run on the same switch or switch stack.
Monitored Traffic
SPAN sessions can monitor these traffic types:
•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 receiv ed by the source is sent to the destinatio n 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 modificatio n.