1

Here's the setup:

R1 -> R2 
 | 
 |--> R3

R1 has 2 serial interfaces - Serial0/0/0 (10.0.0.1) is connected to R3 (Serial0/0/0 - 10.0.0.3), and Serial0/1/0 (10.0.0.4) is connected to R2 (Serial0/0/0 - 10.0.0.2).

Everything connects fine, and I can ping all the IPs from all the routers, except for 10.0.0.1 - it won't successfully ping it from R2 or R3.

I'm new to all this, and just playing around in a home lab I just built. Let me know if you need any config details to help. These are Cisco 1841 routers.

* Update *

Here are some configs:

R1:

interface Serial0/0/0
 ip address 10.0.0.1 255.255.255.0
!
interface Serial0/1/0
 ip address 10.0.0.4 255.255.255.0
!

R2:

interface Serial0/0/0
 ip address 10.0.0.2 255.255.255.0
!

R3:

interface Serial0/0/0
 ip address 10.0.0.3 255.255.255.0
!
3
  • You really need to edit your question to include the router configurations. It sounds like you have a network mask problem.
    – Ron Maupin
    Oct 22 '17 at 23:08
  • @RonMaupin I added the piece of the config that shows the Serial interface configuration. Please let me know if I should add more.
    – xil3
    Oct 22 '17 at 23:27
  • If that actually is Cisco equipment, then that is most certainly NOT your configuration. The same subnet cannot be assigned to more than one active interface -- R1 cannot have 10.0.0.0/24 assigned to both interfaces. (Typically, one would use a /30 per interface, but it can be any unique network of any size -- in the case of cisco, it can even be unnumbered)
    – Ricky
    Oct 23 '17 at 2:33
3

You have a problem with your network addresses and masks. All the interfaces are assigned to the same network, but a router needs to have a different network on each interface (routers route between networks). There are a couple of ways to handle this. Most people use /30 networks for point-to-point links, but RFC 3021, Using 31-Bit Prefixes on IPv4 Point-to-Point Links allows you to use /31 networks (not all devices support this, but Cisco does).


With /30 networks:

R1:

interface Serial0/0/0
 description to R3 S0/0/0
 ip address 10.0.0.1 255.255.255.252
!
interface Serial0/1/0
 description to R2 S0/0/0
 ip address 10.0.0.5 255.255.255.252
!

R2:

interface Serial0/0/0
 description to R1 S0/1/0
 ip address 10.0.0.6 255.255.255.252
!

R3:

interface Serial0/0/0
 description to R1 S0/0/0
 ip address 10.0.0.2 255.255.255.252
!

With /31 networks:

R1:

interface Serial0/0/0
 description to R3 S0/0/0
 ip address 10.0.0.0 255.255.255.254
!
interface Serial0/1/0
 description to R2 S0/0/0
 ip address 10.0.0.2 255.255.255.254
!

R2:

interface Serial0/0/0
 description to R1 S0/1/0
 ip address 10.0.0.3 255.255.255.254
!

R3:

interface Serial0/0/0
 description to R1 S0/0/0
 ip address 10.0.0.1 255.255.255.254
!

You would also need for R2 and R3 to know about the networks on the other side of R1 from them. You can either use static routes (which don't scale), or you can run a routing protocol among your routers.

For example, on R2 you can place a static route to the R1 S0/0/0 network, and on R3, a static route to the R1 S0/1/0 network:

ip route <remote network address> <remote network mask> <next hop (R1) address>

R1 inherently knows about both networks because it is directly connected to both, but R2 has no way to know where to send traffic for the R1-to-R3 network, nor does R3 have any way to know about the R1-to-R2 network, unless you somehow tell those routers about the other networks. In fact, you could cheat, and use default routes:

ip route 0.0.0.0 0.0.0.0 <next hop (R1) address>
3
  • Thank you. I am a bit confused - so for connecting routers together, they must use /30 or /31? Is that just a hard standard? Sorry for the silly questions, this is all new to me. Eventually I would be connecting a switch on R3 fe0/0 and R2 fe0/0, so it wouldn't just be the routers. R1 would act as a hub though.
    – xil3
    Oct 22 '17 at 23:50
  • No, they don't need to use that, but you cannot have the same network on two different interfaces of the same router. Routers route between networks, not from a network back to the same network. If you want to waste addresses using /24 networks, the way you originally did, you need to have two different networks, e.g. 10.0.0.0/24 on one R1 interface and 10.0.1.0/24 on the other R1 interface.
    – Ron Maupin
    Oct 22 '17 at 23:52
  • Ah alright, I get it - sorry for being so dim.
    – xil3
    Oct 22 '17 at 23:56

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