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Layer 3 Switching Software Feature and Configuration Guide
78-6235-04, Cisco IOS Release 12.0(10)W5(18)
Chapter4 Configuring Interfaces Configuring Snooping

Restrictions on Port Snooping

The following restrictions apply to port snooping:
•The combined physical bandwidth of the source ports must not exceed the physical bandwidth of
the destination port.
•The snooping source port and destination port cannot be the same port.
•Port snooping is not available on the eight-port Gigabit Ethernet interfaces.

About the Snooping Destination Port

The snooping destination port can be any port in the system, except for the management port on the
route processor (Ethernet0) and ports configuredfor Fast EtherChannel. Typically, the destination port
has a network analyzer or RMON probe attached to it.
Whenin snooping mode, all the existing connections to the snooping destination port are set to the down
state, and the snooping destination port cannot perform any Layer 2 or Layer 3 operations in this state.
The receiveside of the snooping destination port is also disabled when in snooping mode. The snooping
destination port resumes normal operation only when snooping mode is disabled.

About the Snooping Source Port

A source port is a port monitored by the snooping operation. The snooping source port can be on any
interface module.
Thenormal operation of a snooping source port is not altered during snooping operations. Any port with
bandwidth less than or equal to the bandwidth of the snooping destination port can function as a
snooping source port.
Layer3 switching software supports snooping from multiple source ports to a destination port. The total
bandwidthof the snooping source ports must not exceed the bandwidth of the snooping destination port.
Forexample, up to ten Fast Ethernet ports can be configured as snooping source ports to a 1-Gb Ethernet
destination port.
Configuring Snooping
To enable port-based snooping on an interface, perform the following steps, beginning in global
configuration mode:
Note Youmust shut down the destination interface before you enable snooping mode. To bring
the interface up after you have finished configuring snooping, be sure to issue ano
shutdown command.