I've been battling with an annoying problem where netflow v9 data from a 3725 router has been going awol when it reaches my F5 load balancers. what i have come down to is finding that the F5 boxes are rejecting the UDP packets as they have a size of 1506 bytes, compared to their MTU of 1500, and i can now see icmp host unreachable messages going back to the router to this effect. but the 3725 router itself is also configured, according to the snmp if table (as that's all i can see... the router is hosted for us) that the fa0.1 interface which is connect to our LAN also has an MTU of 1500, that that device is apparently creating packets of 1506 itself (i've verified this by directly spanning that switch port and capturing data right from it). From what i understand my load balancer is totally correct to reject the packet as it is 6 bytes too big.
what's going on?