Monitored Traffic - Cisco Catalyst 2960 series Configuration Manual

Information About SPAN and RSPAN
Related Topics
Creating a Local SPAN Session, on page 484
Creating a Local SPAN Session and Configuring Incoming Traffic, on page 486
Example: Configuring Local SPAN, on page 501

Monitored Traffic

SPAN sessions can monitor these traffic types:
• Receive (Rx) SPAN—Receive (or ingress) SPAN monitors as much as possible all of 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.
Features that can cause a packet to be dropped during receive processing have no effect on ingress SPAN;
the destination port receives a copy of the packet even if the actual incoming packet is dropped. These
features include IP standard and extended input Access Control Lists (ACLs), ingress QoS policing,
VLAN ACLs, and egress QoS policing.
• Transmit (Tx) SPAN—Transmit (or egress) SPAN monitors as much as possible all of 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.
Packets that are modified because of routing (for example, with modified time-to-live (TTL), MAC
address, or QoS values) are duplicated (with the modifications) at the destination port.
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 ports, egress source ports, or SPAN
destination ports. In general, these characteristics are independent of one another. For example:
Consolidated Platform Configuration Guide, Cisco IOS Release 15.2(4)E (Catalyst 2960-X Switches)
478
◦ 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.