Hi,

there is one scenario, I remember, which might require adjustment of those timers:

"Asymmetric Routing and HSRP (Excessive Flooding of Unicast Traffic in Network with Routers That Run HSRP)"

http://www.cisco.com/en/US/tech/tk648/tk362/technologies_tech_note09186a0080094afd.shtml#t8

and also "Unicast Flooding in Switched Campus Networks"

http://www.cisco.com/en/US/products/hw/switches/ps700/products_tech_note09186a00801d0808.shtml

Basically it might happen that the MAC address table entry times out (5 min) whereas an ARP table entry is still present in a host/router (4 h aging ARP time) leading to unwanted unicast flooding. One countermeasure is to configure both timer to be the same.

Regards, Martin