The present invention relates to methods and routing and/or switching devices for determining a path maximum transmission unit MTUP(n) size for transmission paths between edge nodes in a data communications network and for updating routing tables associated with the routing and/or switching devices in said edge nodes, which are linked together with link interfaces L(i) constituting transmission paths P(n) for data packet transmission, each link interface L(i) having a link interface maximum transmission unit MTUL(i) size. The methods involve the acquisition of a link interface maximum transmission unit MTUL(i) size for a link interface L(i), and the updating the routing table by calculating the path maximum transmission unit MTUP(n) size for each destination that involves the link interface L(i) for the acquired link interface maximum transmission unit MTUL(i) size. New link-state protocols for distributing the link interface maximum transmission unit MTUL(i) size is also provided.