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Chapter 21 Configuring SPAN and RSPAN
Understan din g SPAN and RSPAN
•Spanning tree is au tomati cally disabl ed on a refl ecto r port.
•A reflector port receives copies of sent and received traffic for all monitored source ports. If a
reflector port is oversubscrib ed, it could bec o me congested. This could affect traffic forwarding on
one or more of the source p orts.
If the bandwidth of the reflector port is not sufficient to handle the traffic from the corresponding source
ports a nd V LAN s, the excess packe ts a re dr opp ed. A 10 /100 port ref lec ts a t 10 0 Mbps. A Gi gabit por t
reflect s at 1 Gbps.

VLAN-Based SPAN

VLAN-ba sed SPAN (VSPAN) is the mon itori ng of the net work traffic in one or more VL ANs. You can
configure VSPAN to monitor only received (Rx) traffic, which applies to all the ports for that VLAN.
Use these guidelines for VSPAN sessions:
•Only traffic on the monitored VLAN is sent to the destination port.
•If a destina tion port bel ongs to a sourc e VLAN, it is excluded from th e source l ist and is not
monitored.
•If ports a re adde d to o r re moved fro m the sour ce V LA Ns, t h e tra ffic on the sour ce V LAN rec eived
by those ports is added t o or removed from the source s being moni tored.
•VLAN p run ing and the VL AN allowed l ist have no effect o n SPAN mo ni toring .
•VSPAN only monitors traffic that enters the switch, not traffic that is routed between VLANs. For
example, if a VLAN is being Rx-monitored and the multilayer switch routes traffic from another
VLAN to the monitored VLAN, that traffic is not monitored and is not received on the SPAN
destinati on port .
•You cannot use filter VLANs in the same session with VLAN sources.
•You can mon itor onl y Ether net VLAN s.

SPAN Traffic

You can use local SPAN to monitor al l netw ork traffic, includin g multi cast and bri dge protoc ol data un it
(BPDU) packets, and Cisco Discovery Protocol (CDP), VLAN Trunk Protocol (VTP), Dynamic
T runking Protocol (DTP), Spanning T ree Protocol (STP), and Port Aggregation Protocol (PagP) packets.
Yo u cannot use RSPAN to monitor La yer 2 proto cols. See the “RSPAN Configuration Guidelines”
section on page 21-14 for m ore i nfor ma tio n.
In some S PAN configura tio ns, mul tipl e copies of the sam e sour ce packet are se nt to the SPA N
destinat ion po rt. For exam ple , a bidire c tiona l (b ot h Rx and T x) SPAN se ssion is con figured fo r th e
source s a1 Rx monito r and th e a2 Rx and T x moni tor to des tination por t d1. If a pack et e nter s the switch
through a1 and is switc hed to a2, both incomi ng and outgo ing packet s are sent to destinati on port d1.
Both packets are the same (unless a Layer-3 rewrite occurs, in which case the packets are different
because of the added La yer 3 infor mation) .