I read this in the ICND1 book and was a little surprised to learn that routers make a copy of the original l2pdu before checking the FCS. What does it do with the original packet after it's copied? Send it to null? Why do routers need to copy the frames, why cant it just use the ones sent to their mac-address? They rip it apart and add new data link addressing anyway.
Maybe I'm just reading a little to deep into this, or misunderstanding it..