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Catalyst 2960 and 2960-S Switch Software Configuration Guide
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Chapter 27 Configuring SPAN and RSPAN
Understanding SPAN and RSPAN
• Transmit (Tx) SPAN—The goal of transmit (or egress) SPAN is to monitor as much as possible all
the packets sent by the source interface after all modification and processing is performed by the
switch. A copy of each packet sent by the source is sent to the destination port for that SPAN session.
The copy is provided after the packet is modified .
Features that can cause a packet to be dropped during transmit processing also affect the duplicated
copy for SPAN. These features include IP standard and extended output ACLs and egress QoS
policing.
• Both—In a SPAN session, you can also monitor a port or VLAN for both received and sent packets.
This is the default.
The default configuration for local SPAN session ports is to send all packets untagged. SPAN also does
not normally monitor bridge protocol data unit (BPDU) packets and Layer 2 protocols, such as Cisco
Discovery Protocol (CDP), VLAN Trunk Protocol (VTP), Dynamic Trunking Protocol (DTP), Spanning
Tree Protocol (STP), and Port Aggregation Protocol (PAgP). However, when you enter the
encapsulation replicate keywords when configuring a destination port, these changes occur:
• Packets are sent on the destination port with the same encapsulation—untagged or IEEE
802.1Q—that they had on the source port.
• Packets of all types, including BPDU and Layer 2 protocol packets, are monitored.
Therefore, a local SPAN session with encapsulation replicate enabled can have a mixture of untagged
and IEEE 802.1Q tagged packets appear on the destination port.
Switch congestion can cause packets to be dropped at ingress source port s, egress source ports, or SPAN
destination ports. In general, these characteristics are independent of one another. For example:
• A packet might be forwarded normally but dropped from monitoring due t o an oversubscribed SPAN
destination port.
• An ingress packet might be dropped from normal forwarding, but still appear on the SPAN
destination port.
• An egress packet dropped because of switch congestion is also dropped from egress SPAN.
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 the Rx
monitor on port A and Tx monitor on port B. If a packet enters the switch through port A and is switched
to port B, both incoming and outgoing packets are sent to the destination port. Both packets are the same .
Source Ports
A source port (also called a monitored port) is a switched port that you monitor for network traffic
analysis. In a local SPAN session or RSPAN source session, you can monitor source ports or VLANs for
traffic in one or both directions. The switch supports any number of source ports (up to the maximum
number of available ports on the switch) and any number of source VLANs (up to the maximum number
of VLANs supported). However, the switch supports a maximum of two sessions (local or RSPAN) with
source ports or VLANs, and you cannot mix ports and VLANs in a single session.
A source port has these characteristics:
• It can be monitored in multiple SPAN sessions.
• Each source port can be configured with a direction (ingr ess, egress, or both) to monitor.
• It can be any port type (for example, EtherChannel, Fast Ether net, Gigabit Ethernet, and so forth).
• For EtherChannel sources, you can monitor traffic for the entire EtherChannel or individually on a
physical port as it participates in the port channel.