Finding a missing (or misconfigured) route is a relatively straightforward process, if the right tools are used in a methodical manner.
For instance, in this example, the user at PC1 reports that he cannot access resources on the R3 LAN. This can be confirmed by pinging the LAN interface of R3 using the LAN interface of R1 as the source (see Figure 1). The results show that there is no connectivity between these LANs.
A traceroute in Figure 2 reveals that R2 is not responding as expected. For some reason, R2 forwards the traceroute back to R1. R1 returns it to R2. This loop would continue until the time to live (TTL) value decrements to zero, in which case, the router would then send an Internet Control Message Protocol (ICMP) destination unreachable message to R1.
The next step is to investigate the routing table of R2, because it is the router displaying a strange forwarding pattern. The routing table in Figure 3 reveals that the 192.168.2.0/24 network is configured incorrectly. A static route to the 192.168.2.0/24 network has been configured using the next-hop address 172.16.2.1. Using the configured next-hop address, packets destined for the 192.168.2.0/24 network are sent back to R1. It is clear from the topology that the 192.168.2.0/24 network is connected to R3, not R1. Therefore, the static route to the 192.168.2.0/24 network on R2 must use next-hop 192.168.1.1, not 172.16.2.1.
Figure 4 shows output from the running configuration that reveals the incorrect ip route statement. The incorrect route is removed and the correct route is then entered.
Figure 5 verifies that R1 can now reach the LAN interface of R3. As a last step in confirmation, the user on PC1 should also test connectivity to the 192.168.2.0/24 LAN.