# Enhanced Multicast Repair Fast Reroute Mechanism for Smart Sensors IoT and Network Infrastructure
## 2 The Fast ReRoute  
One of the main reasons why Fast ReRoute mechanisms have started to be used is that the process of network convergence after a network failure usually takes a longer time than that expected by the network provider.  
In the context of routing, the convergence state of a network is the state in which each network router has up-to-date, complete and consistent routing information [16; 17]. This means that each router is informed of the current network status (up-to-date), of each available network (complete), and each router has chosen the optimal successor/next-hop for each network according to a common criterion (consistent). The convergence process is the operation of each individual routing protocol process to achieve a state of convergence. The time required to complete the process of network convergence depends on various factors, such as the number of devices, especially network routers; topology complexity; the type of routing protocol used (link-state or distance vector); and its individual operational parameter values (like update or hello timers). The time required to finalize the network convergence process usually ranges from hundreds of milliseconds up to a few seconds. The network convergence process may consist of several subprocesses, such as failure detection, response, the distribution of updated routing information, recalculation and, finally, the installation of new routes [7; 18; 19; 20]. The resulting time of network convergence is the sum of durations of each individual subprocesses.  
First, from the router point of view, there is a need for mechanisms that perform failure detection. Failure detection is the time that the router's operating system (OS) needs to detect a failure, i.e., a failed link (network interface) or unavailable neighbor router. Interface failure detection may take up to several milliseconds on the physical layer of a router. The detection of neighbor router unavailability depends on the type of routing protocol used. This failure detection may take from tens of milliseconds when a link state protocols are used (OSPF/IS-IS hello mechanisms), or it may take up to tens of seconds in the case of distance vector routing protocols (RIPv2). During this period, the packets affected by the failure are permanently lost due to outdated routing information and resulting incorrect routing decisions. Next, the local router response to a link failure; it generates and distributes topology/routing updates that reflect actual state. The duration depends on actual load conditions of each individual router. The distribution of information to other routers is required to inform other routers about the situation and starts within 10 ms to 100 ms for each affected next-hop router [18; 19; 20]. All routers that have received actual routing information must recalculate their routing tables. The recalculation of routing tables depends mainly on the size of the network and the amount of topological information. This may take a few milliseconds for link-state routing protocols that use the Dijkstra algorithm. After the recalculation is complete, routers install new routes and update their routing tables. Again, this mainly depends on the type of router and the number of prefixes that were affected by the network failure.