Communicate between multiple routers

i think the problem here is that your router might be limited.
technically your setup and ip route is correct and you should not have a problem when pinging from a to c.
Router 2 knows about router 1 and thats why when you ping from c to a it works but router one does not know about router 2 because all traffic is pointing to cable modem.  you could probaly test this by doing trace routes and pings from the routers.
if i had the same situation and it seems like its not really a security issue i would just use on subnet for all computers and use router 2 as a switch only. I would also look into vlans if you need 2 subnets. I think you might have more control using vlans.

You need to disable firewall and network address translation on router 2.

Alternatively, disable dhcp on router 2 (important), and put the connection to router 1 into the one of the lan ports on router 2, not the wan port. You may need to use a crossover cable. This will put all the lan ports into one big bridge group, including the wireless.

The pcs will all be in the 192.168.15.101 range.

you can try to disable firewall NAT From Both routers.

or u can define rule for your abcd computers for outward data.

Yes, just disable DHCP in router 2, and switch to Access Point mode, with this configuration now you have only one network without the need to specify routes and every computer will be able to communicate with each other.

One more thing: When connecting Router 1 to router 2, plug it to ports 1, 2, 3, or 4, this means: don't use the "WAN" port in router 2.

in router 1 you will need to add 192.168.1.0 255.255.255.0 192.168.1.1 (router 2 address)
the problem here is that router 2 knows about router 1 but router 1 is set so that all traffic (0.0.0.0 0.0.0.0)  ISP gateway will be sent to internet.
like i said earlier i do not know if consumer routers will aloow this but you could try adding this line in.

Destination LAN IP          Subnet Mask          Default Gateway          Hop Count       Interface
0.0.0.0                                0.0.0.0                      <ISP gateway>             1                          WAN
<ISP>                                 255.255.254.0         0.0.0.0                            1                          WAN
162.168.15.0                    255.255.255.0         0.0.0.0                            1                           LAN

192.168.1.0                      255.255.255.0   192.168.15.1 (router 2)                                    LAN

I did a little more playing around... I might have success!

I used the third route posted above on router 1.  But, then, on router 2, in the advanced routing page, I changed from using "Gateway" mode to using "Router" mode.

Doing some more testing, but so far, looks promising.  I'll keep you posted.

>>put the connection to router 1 into the one of the lan ports on router 2, not the wan port.
I haven't tested this yet, but if I do this, then my router 2 will not get an IP address, correct?

Yes, that's correct, so you configure a static IP to this router/access point, also, you are able to turn it on and off at any time.

I may be missing something but ¿why are you trying to configure this as two separate routed networks? ¿why can't you just do one network with one router and the other devices acting as switches/access points?

The fact that the WRT (router 2) has routing capabilities doesn't mean you HAVE to use that capability. See it as a switch.

Router 1 does not need to know about Router 2.  Everything in the LAN will communicate on the same subnet and communication will be bridged between all the lan ports on router 2, the wireless and the lan ports on router 1.

The issue, is that unless you disable DHCP on router 2, you will have two dhcp servers on the same network, handing out overlapping or conflicting network addresses.

So that you can continue to manage router 2's wireless, just give it a static IP in the range from router 1's lan address. And Turn off DHCP.

:-)

To do this, log into router 2 as it is currently set up, change the IP to 192.168.15.10. (or another unused address on Router 1s LAN.)  You will lose connection with router 2 for a second, but release and renew the IP address, and you can connect to it on its new IP.

Now Disable DHCP on router 2 in the client configuration menu. Now move the connection between router 1 and router 2 from the wan port of router 2 to one of the lan ports.

You should now do an ipconfig /release and an ipconfig /renew on all workstations connected to router 2. You should now be one happy famuily on the 192.168.15.0/24 network.

You can go to 192.168.15.10 (or whatever IP you gave it .) to change the wireless settings/disable the wireless settings whatever.

I will use your same diagram to help you. This is what we are proposing you as THE solution:

Cable Modem
     |
     |
Router 1 and Only Router (RT31P2)
  |    |    |
  |    |    |
  |    |    \___ Computer A
  |    \_____ Computer B
  |
Access Point and Switch (Linksys WRT)
  |    |    |
  |    |    |
  |    |    \__ Computer C
  |    \____ Computer D
  \______ Computer E

To make the WRT act as Access Point and switch, you should:
1. Assign a static IP address to it: 192.168.15.10
2. Connect it to the router, using one of the WRT's 4 normal ports, not the WAN port
3. Disable the DHCP service
4. Connect wired computers (C, D or E) to ports 2 3 or 4
5. Connect wireless computers as you configured the wireless section of the WRT

Please try hard each of this 5 steps, you shouldn't have any problem, and if there's something you don't know how to do, just look around in the WRT's web configuration page and you will find out.

Ummm. Set up router 2 to permit remote management, give the internal network an ip of 192.168.1.1 , leave the dhcp server off and connect a cable between the lan port and the wan port of router 2. Manage it by whatever ip the wan port gets from dhcp. I'm running out of thoughts. Could you connect to 15.10 from ports connected to lan ports of router2? Do an arp -a on devices that are unable to ping it.

>>Also... When router was setup as a switch, I looked at the clients table on router 1, and router 2 (192.168.15.10) did not show up.

That's perfectly normal because the "client table" is really a DHCP table, and the switch/access point is not getting a dynamic ip, it has a static ip configured.

You were in the right path, you had the solution, please configure everything again and I'll help you to enter the web configuration of your WRT.

>>>>please configure everything again and I'll help you to enter the web configuration of your WRT.
>>Will do.  What should I try next?

1.Make sure every computer can ping each other and browse the internet
2. Open a browser and go to http://192.168.15.10 which will be the WRT static IP address. It should work, and if it doesn't, please report back to us instead of unconfigure everything again,