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In order to have a second "backup" gateway for my company network, I tried to implement this topology:enter image description here

It is pretty straightforward, with the network between the modem and the router being the 192.168.1.0/24 and the network between the router and the stations being the 192.168.10.0/24. The actual IP addresses are the ones on the sketch.

The status of the interfaces on the router is like this:

cisco1#show ip interface brief
Interface                  IP-Address      OK? Method Status                Protocol
FastEthernet0/0            192.168.1.69    YES NVRAM  up                    up
FastEthernet0/1            192.168.10.69   YES NVRAM  up                    up
Serial0/0/0 

The routing table is like this:

cisco1#show ip route
Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2
       i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
       ia - IS-IS inter area, * - candidate default, U - per-user static route
       o - ODR, P - periodic downloaded static route, H - NHRP, l - LISP
       + - replicated route, % - next hop override

Gateway of last resort is 192.168.1.1 to network 0.0.0.0

S*    0.0.0.0/0 [1/0] via 192.168.1.1
      192.168.1.0/24 is variably subnetted, 2 subnets, 2 masks
C        192.168.1.0/24 is directly connected, FastEthernet0/0
L        192.168.1.69/32 is directly connected, FastEthernet0/0
      192.168.10.0/24 is variably subnetted, 2 subnets, 2 masks
C        192.168.10.0/24 is directly connected, FastEthernet0/1
L        192.168.10.69/32 is directly connected, FastEthernet0/1

As you can see I have configured the modem's IP address as the gateway of last resort, so that the packets take that way towards the Internet.

I have also configured my test PC with an IP of 192.168.10.13, mask of 255.255.255.0 and default gateway of 192.168.10.69.

Now this is where it gets weird: From the router's CLI I can successfully ping 192.168.1.1, 192.168.1.69, 192.168.10.69 and 192.168.10.13. From the test PC I can successfully ping only 192.168.1.69 and 192.168.10.69. When trying to ping 192.168.1.1 from the test PC it fails.

Of course I have no Internet connectivity from the test PC.

Now, is there something I am overlooking? Why is it that I cannot connect to the internet?

UPDATE:

After some more inspection, my routing table looks really fishy. Why have the directly connected networks been broken up like that? I have never before seen The interface IP addresses mentioned in the routing table as separate "/32" directly connected subnets.

Does anyone have any idea why this is happening? I have a strong feeling this may be related to my connectivity issue.

4

If you want to ping your modem (192.168.1.1) from your test PC (192.168.10.13), you must to add a route in your modem for the network 192.168.10.0/24 via 192.168.1.69 (your router). By default, your modem doesn't know where is the address of your test PC.

Your ICMP Echo arrives on your modem but it can't send the reply because it has no route for this network: 192.168.10.0/24

You ping from your router all IP's addresses beacuse they are all connected to him. The router knows, by default, all the networks it have been directly connected to him.

3

Your router have 2 networks connected directly:

192.168.1.0/24
192.168.10.0/24

It can reach the two networks, but, your modem router, only knows that it wil connect to internet across the serial link or something like this, i´m talking about the link connected with the cloud.

Then, the same happens when the modem receive a packet from an equipment located in the network 192.168.10.0/24. The modem knows this network cause it has connected DIRECLTY.

YOU MUST ADD some route on your modem router with destination 192.168.1.0/24 across gateway 192.168.1.69/24. This is the way that packets from 192.168.1.0 will be reply to that destination. The problem is this, the REQUEST arrive without problem, but the modem UNKNOWN the location of the network 192.168.1.0/24.

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