We're having a somewhat strange issue with MTU. We have a 50Mbps ethernet handoff from our provider (which, apparently, comes off of a fiber ring) which terminates directly on our ASA 5510. Speed and duplex are set to 100/full on both sides. The MTU for the outside interface is set to 1500 and our provider reports that the MTU on their router is set to 2450. From a consumer-class DSL connection at my home, I show the path MTU to the provider's router as 1500, which is expected. However, the path MTU to my ASA drops to 1020, which, theoretically, is only one hop back from the ISP's router. From inside the LAN behind the ASA, the path MTU to any site on the Internet is 1020, but the path MTU to anywhere on my LAN is 1500. We have noticed some latency to Internet sites which makes me wonder about packet fragmentation. Also, we have been testing a video conference unit that streams HD video using UDP packets and we are seeing a lot of loss. I'm wondering if the MTU change is part of the equation with the streaming as well. Any thoughts or places to check would be very much appreciated. Thanks!
I have this problem too.