Why computers can communicate on different networks and routers won't?

Question

I have a problem that I can not understand it myself and I need some help. I'm doing a kind of challenge, well network challenge in cisco packet tracer. I have a .pka file that stands for Cisco Packet Tracer Activities. It is a file which has a broken network and we must fix it.

I have done 80% of this troubleshooting network problem and I can't figure why the thing that I'm going to explain now, its not working:

So this is my network and router R1 can successfully communicate with R2 network's computers, and R3 can do the same thing with R2 network. Now, R2 can successfully communicate with each network in picture. Now the thing that I don't understand is why R1 can not communicate with R3 even I manually and statically configured each one with next hop interface(R2>ip route 172.31.1.128 255.255.255.192 172.31.1.193 ; R2> ip route 172.31.1.0 255.255.255.128 172.31.1.198 ; ip addresses are below of each router). Although my computers at network of router1 can communicate with the computers of the network of the router3.....

I'm really trying hard to understand what's is happening here but, since if two router can not communicate with each other, how computers do, and if the computer can communicate why routers won't?

router1

router2

router3

Answer 1

When router1 sends a packet to router3, it uses the “closest” interface as the source. That is on subnet 172.31.1.192/30.

Router3 does not have a route to 172.31.1.192/30

I suspect if you do an extended ping and use router1’s Ethernet interface/IP as the source, it would be successful because router3 does have a static route to 172.31.1.0/25.

Of course, router3 has two IP addresses on it. I’m assuming your ping is destined for router3’s Ethernet IP address. If r1 tried to ping r3’s serial IP that would fail (no route to host error).

This is one reason dynamic routing protocols are more reliable than static routing. Even in your network of 3 routers, you have 5 routes (ignoring type L local routes). Router 1 and router 3 each have 4 non-L routes, so they are each missing one. If you had loopback interfaces you would have 8 routes. This makes static routing, even in the simplest of networks, non-trivial.

Still a critical skill to learn before proceeding to dynamic routing, so rah rah go team go! :-)

Looks like r1 also has an extra static route. Another human error caused by the complexity of static routing :-)