Basic Static Routing Problem

[kbolderson]

Hello all. I'm scratching my head with a newbie static routing issue. I have created a basic network at home with real hardware to use for CCNA study. I have adapted a simple design that includes 3 routers from a well known network teaching tutorial book. The only difference I have made is that one router in the design has been changed from a Cisco box to my broadband router from ISP (I only have two Cisco routers to use at the moment).

I have followed the tutorial to the letter as far as I can see, and I would say normally that I have made a mistake somewhere but to counter that I also setup the identical network virtually on Cisco Packet Tracer, and I get exactly the same results. Can you see where I am going wrong please! Any help much appreciated.

Cheers.

Problem:

I can ping both RouterDB and PlusnetRouter from RouterBourne (which being in the middle of the two can see both networks as directly connected). But I can't ping PlusnetRouter from RouterDB, although I can ping RouterBourne's Fa0/0 interface IP which is the connected exit interface for 192.168.1.0 / 24 network that PlusnetRouter is on. If I take out RouterDB's static route I can no longer ping that Fa0/0 interface. So I can get to the exit interface, but no further - I don't understand why that should be. Surely if I can get that far with a ping the routing has already taken place. Do I need a route in RouterBourne to get the packet returned to RouterDB? If it knows about 192.168.1.0 and 172.16.10.0 networks as directly connected surely that is not needed?

I assume there is something missing / wrong with RouterBourne as when I ping from a PC on 192.168.1.0 network connected to the PlusnetRouter I get a response from RouterBourne's Fa0/0 interface on same network, but can't ping it's S1 interface that 172.16.10.0 network connects to. I have tried putting in static routes on this router but nothing seems to work. 

I have included loopback interface details only in case I have messed up my subnets. Attached picture shows topology in Packet Tracer.

Details:

RouterDB:
Fa0/0 - 10.10.10.1 /24   / Connects to SwitchDB
S0/1 - 172.16.10.1 /30   / Connects to RouterBourne
L0 - 192.168.0.10 /24

Static Route added: 192.168.1.0 255.255.255.0 S0/1 < I have tried changing this for RouterBourne's S1 interface IP address

RouterBourne:
Fa0/0 - 192.168.1.201 /24   / Connects to PN Router
S1 - 172.16.10.2 /30   / Connects to RouterDB
L0 - 192.168.0.9 /24

PlusnetRouter:
Fa0 - 192.168.1.254 /24   / Connects to RouterBourne

[Rolf Fischer · Accepted solution]

Hi,

to forward a packet with a destination IP address which is not part of any of the directly connected networks, a routing table entry is needed to tell the router which interface to choose for forwarding the packet. As a last resort, this can also be a default-route (0.0.0.0/0).

To make it more interesting, imagine what the packet would look like when you try to ping the PNRouter from RouterDB's interface Fa0/0 (ping 192.168.1.254 source fa0/0):

S: 10.10.10.1
D: 192.168.1.254

RouterDB doesn't have a connected interface in the 192.168.1.0/24 network, so a route is needed which tells RouterDB that the next-hop for this destination can be reached through S0/1.

Now, if the same packet is received by RouterBourne, it can be forwarded to PNRouter because the ip address of interface Fa0/0 is in the same subnet as the destination address.

The reply packet send from PNRouter looks like this:

S: 192.168.1.254
D: 10.10.10.1

PNRouter doesn't have any other routing information than its connected networks, so it will drop the packet unless a route tells it that the other networks can be reached through next-hop 192.168.1.201 (RouterBourne again).

Finally RouterBourne will need to know how to reach network 10.10.10.0/24.

So this little example demonstrates nicely some of the key characteristics of IP routing: It is destination-oriented, unidirectional and forwarding is done hop-by-hop.

Hope that makes sense.

Rolf

[kbolderson]

Cheers Rolf. I will look again at all the routes needed.