What type of devices are the 2 interfaces of the IPS-4260 plugged in to?
When power is removed from the sensor, the links are immediately dropped on the interfaces. While power is on, the sensor itself links to the 2 other devices. But when power is removed the links are dropped and the NIC simulates a wire and now the 2 devices link directly to each other.
Any delay in passing of packets is most likely a delay within one of those 2 other devices.
The most common delay is spanning-tree. If one of the 2 devices is a switch, then the default for most ports is to participate in spanning-tree. So before sending the standard packets through the interface it will first send through spanning-tree packets in order to map the local network and prevent packet loops. This generally takes about 40 seconds. Once spanning-tree is run, then the switch will start passing through packets.
THe spanning-tree time cna be removed by setting spanning-tree portfast on the switch interface. This, however, should only be used if you can guarantee no possible spanning-tree loops. It is recommended this only be done for end hosts, and not for networking devices like the sensor. Because if there is a loop it won't be detected and could cause serious network issues.
Ways to see if this is what you are seeing.
Power sensor off
Look at both devices to determine when the link is established.
Watch spanning-tree and determine how long it takes for the ports to be put into a Forwarding state (packets don't get passed until it goes to a Forwarding state).
Determine when packets start making it through.
If the link goes down and back up within a couple of seconds. But takes 40 seconds to get into Forwarding state and start passsing traffic then spanning-tree on the switch is the cause for the delay.
The spanning-tree delay is not unique to hardware bypass. You could remove the sensor and plug the devices directly to each other. If you unplug that cable, and then plug it back in you would still see the same delay before packets start being sent through.
If instead you are seeing 30-75 seconds before it establishes a link, then this could be an issue Cisco is not aware of. Do the same test with just a wire and no sensor to see if the problem is unique to a hardware bypass sensor or if the same delay is also seen with just a wire.
