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Ok, I was trying to connect routers in a parallel topology , as shown in the figure

enter image description here

I've configured the IP addresses for the fast Ethernet and serial interfaces.

Here are the static IP routes for each router

Router-1

20.0.0.0/8 via 192.168.1.2
30.0.0.0/8 via 192.168.1.2

Router-2

10.0.0.0/8 via 192.168.2.2
30.0.0.0/8 via 192.168.2.2

Router-3

10.0.0.0/8 via 192.168.3.2
20.0.0.0/8 via 192.168.3.2

for the Router 0 my static IP routes are

10.0.0.0/8 via 192.168.1.1
20.0.0.0/8 via 192.168.2.1
30.0.0.0/8 via 192.168.3.1
192.168.1.0/24 via 192.168.1.1
192.168.2.0/24 via 192.168.2.1
192.168.3.0/24 via 192.168.3.1

After this I had configured the IP , mask and default Gateway for the PC's .

Then I try to send the packet from PC0->PC1 and hence all the other combinations of 2 PC's. And I get packet Failed in the PDU list window.

Where am I going wrong ? How can I successfully send packets from PC's to PC's ?

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  • Ok, I tinkered around with the addresses a bit . Got this . If the default gateway IP for the PC is same as network IP then the packet does not send. Also if lets say the fa0/0 IP of router 1 is 10.0.0.1 and I set the default gateway in PC0 to be the same 10.0.0.1, and do the same for other router-switch-PC branches (with their own IP's), then the packet is sent successfuly to wherever its destination is . However , if the fa0/0 IP in the router , network IP , PC IP , PC default Gateway IP are all unique - The packets Fail to reach destination , again ?? whats wrong here? I'm confused. Jun 11, 2015 at 18:55
  • there is no need of this steps for router 0 192.168.1.0/24 via 192.168.1.1 192.168.2.0/24 via 192.168.2.1 192.168.3.0/24 via 192.168.3.1 as they are directly connected
    – user63725
    Dec 6, 2019 at 11:00

1 Answer 1

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The default gateway of each PC needs to be the same as the subnet router port IP in its network. So the subnet router port in the first network would have the address 10.0.0.1 and the PC would have 10.0.0.2 and the default gateway would point to 10.0.0.1. This tells the PC "when I have a packet that must go to a destination that is not on my own network, send it to 10.0.0.1, and that device will know what to do with it".

The "default gateway" is really just a route to 0.0.0.0/0 which means "the rest of the IP address space that I don't have a more specific route for".

In fact, you can eliminate the routes on the subnet routers, too, and just give them a default gateway to the central router. So instead of having a route to 20/8 and 30/8 on the subnet routers, you could just put a route to 0.0.0.0/0 pointing to 192.168.1.2 on the first router, 192.168.2.2 on the second one, and 192.168.3.2 on the third. The only router that needs to know explicitly how to reach any of the 10.x.x.x routes is the central router at the top of your diagram. All of the rest can just have default routes.

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  • Clarification example: PC1: IP address 10.0.0.2 netmask 255.0.0.0 default gateway 10.0.0.1 Router 1 fa0/0 ip address 10.0.0.1 netmask 255.0.0.0 fa0/1 ip address 192.168.1.1 netmask 255.255.255.0 ip route 0.0.0.0 0.0.0.0 192.168.1.2 This tells PC1 that in order to reach anything outside of 10.0.0.0/8 it must send the packet to Router 1. Router 1's configuration says "to send traffic to 10.0.0.0/8 I use fa0/0, to send traffic to 192.168.1.0/24 I use fa0/1 and to send traffic anywhere else, I forward it to 192.168.1.2 and that router needs a route to all of the various subnets.
    – GeorgeB
    Jun 11, 2015 at 20:03
  • Thank you so much. Now I understand. Pretty strange why I couldn't find this explanation anywhere else on the net. Jun 11, 2015 at 20:19
  • uhm , just curious but -how did you know this ? could you substantiate your answer with some credible resource or reference to a document? Jun 11, 2015 at 20:23
  • I've been a network engineer for nearly 20 years. I do this for a living every day here in Silicon Valley. But maybe this will help you: kb.iu.edu/d/ajfx
    – GeorgeB
    Jun 11, 2015 at 20:29
  • oh great ! and sir I meant no offence , just that I took almost 4 hours redoing the network again and again ,and couldn't find why it wouldn.t work. I even went through the cisco documentation , but couldn't point to this. ha, the little details we tend to miss,but those that change the game! So anyways, I was just being curious. Jun 11, 2015 at 23:16

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