I recently replaced a 3640 router with a brand new 3845 and one of my WAN links is a 100Mbps fiber optic link, lit with Transition Networks media converters. It worked fine when a dedicated interface port on the the 3640 was connected to a dedicated port on the 3620 across the fiber link. Both the 3640 and 3620 have the old "version 1" NM-2FE2W dual 10/100 network modules in them. My new 3845 has NM-2FE2W-V2 modules and it seems that the old style and the new V2 interfaces refuse to talk to one another. I tried every combination of speed and duplex setting at each router's respective interface (this is a router-to-router dedicated link) and also ever possible combination of speeds and duplex setting on the fiber optic media converters in conjunction wih all possible speed and duplex settings on the router interfaces themselves.
What I would get would be that the link would come up for about 120 seconds, carry traffic for that short while, and suddenly the interface on the NM-2FE2W-V2 would shut itself down and the link LED would go dark. There are no messages in either router's logs complaining of duplex mismatch or any other messages giveing any clue other than simple the "Interface FastEthernet1/0, changed state to down" message. This does not happen when a "V2" interface talks to another "V2" interface in the same router, but it seems like there may have been some kind of limitation coded into either the firmware or IOS to make a "V2" interface not compatible with an original "version 1" 10/100 interface... or is something else happening here?
The 10/100/1000 interface built into the 3845 has no problem at all talking to the "v1" 10/100 interface in the 3620 router across the fiber. That works just fine, but I hate using up a precious 10/100/1000 interface port for this router-to-router link when a plain 100Mbps port would do just fine.
Anybody else encounter this strange problem?