When we plug the new cisco router to the our company network, there is several time out session from user workstation to the server. This symptom only happen for a while and after that, it back to normal. May I know what could be the reason?
I would suggest to edit the title of your thread in something like " problems in introduction of a new router in our network"
There is no sense in having a so long title.
if the router has to take the role of an older device and uses the same IP addresses of that device, when you introduce it all devices including user PCs have in their ARP table the MAC address of the older router.
So until all devices don't refresh this information they try to send ethernet frames to a device that is not there anymore: the new router has different MAC addresses associated to its interfaces.
This is probably the most likely reason and ARP timeouts are long and the effect can be lnoticeable to end users.
Unless HSRP or other first hop redundancy protocol is in use.
If it is only a question of some tens of seconds it can be routing convergence as explained by Leo
Hope to help
This is a very vague statement and without knowing anything else I can only say routing convergence.