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I know the port adapters of which the OP speaks ... they're typical in older 7xxx series routers and they are notorious for their inability to negotiate or correctly negotiate full duplex. Nailing the opposing end up as a full-duplex is really the only solution. Two ports on a managed switch (which is also serving the internal network) dropped into a dedicated/non-routed VLAN is the easiest way to resolve this if you have that gear already installed -- just need to be careful no one picks up the isolated VLAN with the other switch ports when making configuration changes.
So I guess my two choices are to run an extra VLAN on a switch or pick up a PA-2FE-TX.
So the PA-2FE-TX would be cleaner, and the VLAN cheaper. I have some thinking to do. Thanks for the help, Lasagna!
In my case, there's almost always a 24 or 48-port switch installed nearby in the same rack with the 7xxx series router. So nabbing two ports and dropping them into an isolated VLAN and nailing one of the ports to be 100/fd is usually a quick and easy ... albeit less eloquent .. solution. One positive ... inserting the switch in the middle also allows quickly defining a span port in case I need to troubleshoot something on the WAN side of the 7xxx series router. Hope it works out for you.