Source Ports; Source Vlans - Cisco Catalyst 3120 Software Manual

Chapter 29 Configuring SPAN and RSPAN
Switch congestion can cause packets to be dropped at ingress source ports, egress source ports, or SPAN
destination ports. In general, these characteristics are independent of one another. For example:
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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 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
(unless a Layer-3 rewrite occurs, in which case the packets are different because of the packet
modification).

Source Ports

A source port (also called a monitored port) is a switched or routed 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. You cannot mix ports and VLANs in a single session.
A source port has these characteristics:
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Source VLANs

VLAN-based SPAN (VSPAN) is the monitoring of the network traffic in one or more VLANs. The SPAN
or RSPAN source interface in VSPAN is a VLAN ID, and traffic is monitored on all the ports for that
VLAN.
VSPAN has these characteristics:
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A packet might be forwarded normally but dropped from monitoring due to 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.
It can be monitored in multiple SPAN sessions.
Each source port can be configured with a direction (ingress, egress, or both) to monitor.
It can be any port type (for example, EtherChannel, 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.
It can be an access port, trunk port, routed port, or voice VLAN port.
It cannot be a destination port.
Source ports can be in the same or different VLANs.
You can monitor multiple source ports in a single session.
All active ports in the source VLAN are included as source ports and can be monitored in either or
both directions.
On a given port, only traffic on the monitored VLAN is sent to the destination port.
If a destination port belongs to a source VLAN, it is excluded from the source list and is not
monitored.
Cisco Catalyst Blade Switch 3120 for HP Software Configuration Guide
Understanding SPAN and RSPAN
29-7