# Enhanced Bit Repair IP Fast Reroute Mechanism for Rapid Network Recovery
## 5 Evaluation of the Enhanced B-REP FRR Mechanism
Upon completion of the OSPF convergence process, as indicated by the timer in the simulation, the initialization of the EB-REP process begins on each router. Their EB-REP processes enter the Init state. In other words, routers from their OSPF LSDBs populate the EB-REP table and EB-REP backup tables for protected interfaces. It should be noted that the EB-REP table is calculated by sorting all area routers in ascending order according to OSPF R-ID. After this process, B-REP IDs and corresponding Bit-String positions are assigned the same way, as we can see for router R1 in Table 9. The EB-REP tables are the same at the end on all routers within the domain. \begin{table}
\begin{tabular}{c c c c} \hline \hline  & **OSPF R-ID** & **B-REP ID** & **Bit-String Position** \\ \hline R7 & 10.0.0.101 & 1 & 0 0000 0000 0000 001 \\ R5 & 10.0.0.109 & 2 & 0 0000 000 000 0010 \\ R9 & 10.0.0.121 & 3 & 0 0000 000 000 0100 \\ R10 & 10.0.0.137 & 4 & 0 0000 000 000 1000 \\ R11 & 10.0.0.149 & 5 & 0 0000 000 000 001 000 \\ R12 & 10.0.0.161 & 6 & 0 0000 000 0010 000 \\ R14 & 10.0.0.169 & 7 & 0 0000 000 0100 000 \\ R0 & 10.0.0.17 & 8 & 0 0000 000 1000 000 \\ R15 & 10.0.0.173 & 9 & 0 0000 001 000 000 000 \\ R6 & 10.0.0.45 & 10 & 0 0000 0010 000 000 \\ R2 & 10.0.0.57 & 11 & 0 0000 000 000 000 \\ R3 & 10.0.0.69 & 12 & 0 0000 1000 000 000 \\ R8 & 10.0.0.89 & 13 & 0 0001 000 000 000 \\ R1 & 192.168.1.2 & 14 & 0 0010 000 000 000 \\ R13 & 192.168.10.2 & 15 & 0 0100 000 000 000 \\ R4 & 192.168.2.1 & 16 & 0 1000 000 000 000 \\ R16 & 192.168.3.1 & 17 & 1 0000 000 000 000 \\ \hline \hline \end{tabular}
\end{table} 
Table 9: EB-REP Table. Figure 14: Visual output from the OMNeT++ simulation. As a next step of the EB-REP initiation, each router calculates its EB-REP Backup Tables. The EB-REP now supports protection for all flows (all destination IPs) and all interfaces of a router. These features were also enabled in the simulation, and each router had all its interfaces enabled as a protected interface. In the real world, this step can take a while in a situation where the EB-REP is enabled on all interfaces and can lead to larger EB-REP tables. Therefore, in real deployment situations, we expect that it will be possible to manually edit and delete the interface list, or if necessary, to define a list of IP addresses of protected flows. A fragment of the EB-REP BT table from router R1 is shown in Table 10.  
According to the present scenario, the first failure is a link failure between R1 and R6. In this case, the Eth2 output interface of the R1 router goes down and the routing to the host H3 is no longer possible. Upon detection of a failure and upon receiving the packet for 192.168.3.2, R1 becomes the S router and its EB-REP mechanism looks through the EB-REP BT table for the Eth2 output interface and the destination IP address. Once it finds the correct table entry, the router reads the corresponding Bit-String value of the alternate route, here 1 0000 0001 0000 1010 (LSB). This BS value defines an alternative path from router S to H3 as the route via R5-R10-R15-R16. R1 then immediately encapsulates the original IP packet with a new EB-REP BIER header, inserts the BS value, and forwards the packet to the R5 next-hop. This behavior is shown by the output from the OMNeT++ simulator console illustrated in Figure 15.  
The EB-REP packet equipped with a BIER header and a corresponding Bit-String receives R5. From the point of view of EB-REP processing, R5 becomes the router R. R5 processes the EB-REP packet as described above and forwards it further. The packet thus passes through a precisely determined path defined in its BS field from router to router, up to router R16. R16, in terms of EB-REP, it is the router D, which detects it is the last router of the alternative path. R16 decapsulates the IP packet (removes the BIER header) and forwards it successfully to H3. At the time 80 sims, R10 fails, which is detected by its neighbor on an alternative path, the R5 router.