Switching nodes are used in communications networks to switch and route data traffic between sources and destinations.  A switching node typically receives data traffic in the form of data packets on input/output (I/O) interface ports and outputs the data packets on appropriate I/O interface ports to direct the data packets to an appropriate destination in the communications network.  A switching node may support multiple interface protocols and maintain multiple communication links on its interface ports with other switching nodes or other devices or networks.  These devices or networks may be, for example, data storage devices, servers, local networks, or other similar communication devices. 
As a switching node functions within a communications network there may be a need to perform an in-switch software upgrade/downgrade (ISSU).  In order to avoid adverse effects on a switching node's communication links, the ISSU may have to be performed while the switching node remains in service in the network and while maintaining the communication links on its interface ports with neighbor switching nodes or devices.  The control plane supervisor software in a switching node maintains the communication links of the switching node by exchanging protocol data units (PDUs) with neighbor nodes.  Any failure of PDU exchange on a link may result in the link being brought down by link timeout at one of the communicating nodes.