The error message that you are seeing is generated by CDP. In the newer version of CDP when it is sent out Ethernet interfaces the CDP packet indicates the duplex setting. So I am guessing that you probably see the error message every 60 seconds when the 1601 receives a CDP packet from the 2612. Probably the CDP code on the 1601 is assuming half duplex operation since half duplex is pretty standard on 10M Ethernet.
If the show interface indicates that the 1601 is operating in full duplex, and if there is no collisions detected (and especially if there are no late collisions - since late collisions are a typical sign of duplex mismatch) then the routers are working ok and the error message is cosmetic. You can leave the settings at full duplex, get somewhat better performance, and endure the cosmetic error message, or you can put the interfaces back to half duplex, get no error message, and get 10M half duplex performance.
I do not have much experience with the 1601 so can not address this from direct experience. But I believe that the 1601 has a simple Ethernet interface which is only capable of 10M speed. It is normal for 10M interfaces to operate in half duplex mode (and many of the 10M interfaces do not allow the duplex to be changed). It seems that your interface on the 1601 does allow the duplex to be set to full.
It seems to me that there are two things at work here: 1) what is the interface really doing; and 2) what does CDP think that the interface is doing. If the interfaces are operating in full duplex on both sides there will not be collisions on either interface. If one interface is operating in full duplex and one interface is operating in half duplex there should be collisions reported on the half duplex interface (and also late collisions should be reported on the half duplex interface).
So let the routers run for a while and pass a good amount of traffic over the interfaces. Then check both interfaces for any collisions. If there are no collisions then full duplex is operating successfully and the message is cosmetic.
I suspect that the show interface on the 1601 showing speed unknown reflects the fact that it is not operating with its default settings. So long as the routers do communicate successfully over the Ethernet then the speed is correct.
This document gives several answers on frequently asked questions for PFRv3 channel state behavior.
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