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Catalyst 2950 Desktop Switch Software Configuration Guide
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Chapter 21 Configuring SPAN and RSPAN Understanding SPAN and RSPAN

SPAN and RSPAN Interaction with Other Features

SPAN interacts with these features:
•Routing—Ingress SPAN does not monitor routed traffic. VSPAN only monitors traffic that enters
the switch, not tr affic that is routed between VLANs. For example, if a VLAN is being
Rx-monitored and the multilayer switch routes tr af fic from another VLAN to the monitored VL AN,
that traffic is not monitored and not received on the SPAN destination p ort.
•Spanning Tree Prot ocol (STP)—A dest inatio n port or a re flecto r port does not partici pate in STP
while its SPAN or RSPAN session is active. The destinatio n or refle ctor po rt ca n partic ipate in STP
after the SPAN or RSPAN session is disabled. On a source port, SPAN does not affect the STP status.
STP can be active on trunk ports ca rrying an RSPAN VLA N.
•Cisco Discovery Protocol (CDP) —A SPAN destination port does not participate in CDP while the
SPAN session is active. After the SPAN session is disabled, the port again participates in CDP.
•VLAN Trunking Protoco l (VTP )—You can use VTP to prune an RSPAN VLAN between switches.
•VLAN and trunking—You can modify VLAN membership or trunk settings for source, destination,
or reflec tor ports at any time. However, changes in VLAN memb ership or t runk sett ings for a
destinat ion or r ef lec tor po rt d o n ot ta ke effect u nti l yo u d isab le the SPA N or R SPAN session.
Changes in VLAN m embership or trunk settings for a source port immediately take effect, and the
respective SPAN sessions automatically adjust accordingly.
•EtherChannel—You can co nfigure an EtherCha nnel gro up as a source po rt but not as a SPAN
destinat ion po rt. Wh en a g rou p is co nfigured a s a SPAN sour ce, the ent ire g roup is mo nito red.
If a port is added to a monitored EtherChannel group, the new port is added to the SPAN source port
list. If a por t is removed from a mo nitore d EtherC hannel group, it is aut omatic ally re moved from
the source po rt list. If the port is the only port in the Ether Channel group, the EtherCha nnel gro up
is removed from SPAN.
If a physical port that belongs to an EtherChannel group is configured as a SP AN source, destination,
or refl ector p o rt, it is removed from t he group. After the port is removed from the SPAN session, it
rejoins the EtherChannel group. Ports removed from an EtherChannel group remain members of the
group, but they are in t he down or standalone state.
If a p hys ica l p ort th at be longs to an E ther Cha nn el g ro up is a de sti nat ion or re fle ctor p ort an d th e
EtherChannel group is a source, the port is removed from the EtherChannel group and from the list
of moni tore d port s.
•QoS—For ingress monitoring, the packets sent to the SPAN destination port might be dif ferent from
the packets actually received at the SPAN source port because the packets are forwarded after
ingress QoS classification and policing. The packet DSCP might not be the same as the received
packet.
For egress monitoring, the packets sent out the SPAN destination port might not be the same as the
packets se nt o ut of SPAN source por ts be c ause the egress Q oS po lic ing at the SPAN sour ce p ort
might cha nge the packet classificatio n. QoS polici ng is not ap plied at SPAN destinat ion ports.
•Multica st traffic can be monitor ed. For egress and ingr ess port moni toring, only a single un edite d
packet is sent to the SPAN destination port. It does not reflect the number of times the multicast
packet is sent.
•A secu re p ort ca nn ot be a SPAN destina tio n port.
•You can enab le 802.1X on a port that is a SPAN dest inatio n or refle ctor port; however, 802.1X is
disabled u ntil the po rt is re moved as a SPAN destin ati on or r eflec tor por t. You ca n ena ble 8 02.1X
on a SPAN source port .