What could have caused this?
Today we added a monitor port on one of our routers.
This is a standard procedure that we have performed in the past on several other devices.
A physical interface (uplink port) was set as the source and the remote vlan was set as the destination.
The remote vlan was only trunked across a single link to another router.
The other router was setup with a monitor source of the remote vlan and destination of the physical uplink to the sniffer.
Both of the routers in question are part of an HSRP pair.
Both of the routers in question are running FWSM.
When the monitor port was enabled, the cpu usage spiked to 100% causing HSRP flapping.
The remote router was no longer responding to pings due to cpu usage.