# Enhanced Bit Repair IP Fast Reroute Mechanism for Rapid Network Recovery
## 5 Evaluation of the Enhanced B-REP FRR Mechanism  
In this chapter, we describe the verification process of the EB-REP FRR extension proposals described above. For this purpose, we used the discrete-event network simulator OMNeT++. OMNeT++ provides a very robust and feature-rich environment suitable for complex network simulations and experiments; however, it only supports static routing. Therefore, OMNeT++ has been extended using the INET extension framework library. INET provided us with the required dynamic routing behavior using the OSPF protocol and allowed us to program and test the EB-REP FRR functionalities. The EB-REP FRR proposal has been tested using several different network topologies consisting of several interconnected OMNeT++ routers (up to tens). Within these topologies, we then tested the EB-REP behavior in the event of one or multiple independent network failures. Performed simulations showed the correct behavior of the Enhanced B-REP algorithm, in which it was able to deliver all packets affected by failures. In this section, as an example of the evaluation process, we describe one of the comprehensive simulation scenarios. The topology consists of a \(4\times 4\) (+1) matrix of interconnected OMNeT++ routers and four end-point devices (Figure 12). For simulation purposes, the OSPF v2 protocol is used as the dynamic IP routing protocol. Routers are interconnected by several links with the link metrics as is shown in Figure 13. Traffic is sent from the source station to the destination, which is the H3. \begin{table}
\begin{tabular}{c c} \hline \hline State: & Router N \\ Event: & Receiving an EB-REP packet with BIER header AND detecting a last bit = 1 in the Bit-String. \\ New state: & D \\ Action: & A BIER header (decapsulation process) is removed from the received packet. \\ The packet is delivered to the next-hop according to the router D unicast routing table. \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Cont. Figure 12: EB-REP simulation topology. Within the simulated topology, we have then simulated two network failures. The first simulated problem is the link failure between routers R1-R6. The second simulated error is a node failure, where a router on the alternate path stopped working. The complete overview of the simulated scenario is in Table 8.  
The exact description of the scenario is as follows: At the beginning of the simulation process, we set a waiting period of 50 simulation seconds (sims) to give the OSPF routing protocol time to complete the convergence process. Subsequently, at the time of 64 sims, the Source host (top left, Figure 14) starts generating a simple User Data protocol (UDP) data flow for host H3. Based on the selected topology and cost evaluation of connected links, the OSPF selects the path through routers R1-R6-R11-R16 as the primary delivery path (Figure 14, red arrows). Then, 70 sims after the start of the simulation, we simulate the first connection failure between routers R1-R6. R1 becomes the EB-REP S router, and it will use the precalculated alternate path through R5-R10-R15-R16 to bypass the gap (Figure 14, orange arrows). At the time of 80 sims, we simulate the second failure as an R10 router fault. The problem, in this case, occurs on the alternative route and router R5 becomes the second router S (exS), where it reroutes packets over a new alternative path, R9-R14-R15-R16 (Figure 14, yellow arrows). \begin{table}
\begin{tabular}{c c} \hline \hline
**Time** & **Description of Action** \\ \hline \(<\)50 & The time necessary for the OSPF convergence and stabilization of network \\  & routing processes. \\
64 & Source host begins generating the flow \\
70 & R1–R6 link failure \\
80 & R10 failure \\
90 & Restoration of failed elements \\ \hline \hline \end{tabular}
\end{table} 
Table 8: Description of the comprehensive simulation scenario. Figure 13: EB-REP simulation topology and overview of link metrics. Let us take a closer look at what happened during the simulation.