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I have the following scenario:

  1. I am using two Cisco 1941 routers
  2. Network A connected to Router1 (192.168.10.0/24) (Gigabit connection)
  3. Network C connected to Router 2 (192.168.20.0/24) (Gigabit connection)
  4. Network B connecting Router1 and Router2 (192.168.30.0/30) (Serial connection)
  5. No entries have been added to the routing table (other than what is automatically added for directly connected networks)
  6. There is no gateway of last resort set up.

The difficulty:

  • I can ping from Network A to both interfaces on Router 1 (192.168.10.1, 192.168.30.1)
  • Even though Network B has been automatically added to the routing table on Router 1 as a directly connected network, I cannot ping Router2's interface (192.168.30.2) on Network B from Network A.
  • The only way I can get this to work is by adding the gateway of last resort to Router1.

My question:

Since Network B is a directly connected route on Router 1, why can I only ping (from Network A) Router1's interface on Network B, but I cannot ping Router2's interface on Network B?

5

why can I only ping (from Network A) Router1's interface on Network B, but I cannot ping Router2's interface on Network B?

Because R2 does not know how to get to NetA. This is the purpose of routing protocols: to allow routers to tell each other about networks they can reach.

If you don't want to run a routing protocol, add a static route on R2:

ip route 192.168.10.0 255.255.255.0 192.168.30.1
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4

Routing 101

Routing is unidirectional. A->B and B->A are different paths. For B to receive traffic from A, it doesn't have to know anything about A. (anti-spoofing measures aside)


In this case, machines in Net-A will send traffic to Router1 -- assuming that's their default route. Router1 can forward to Router2 in the connected Net-B. However, Router2 has no knowledge of Net-A, and therefore has nowhere to send any reply.

Router1#show ip route
...
Gateway of last resort is not set

C    192.168.10.0/24 is directly connected, FastEthernet0/0
     192.168.30.0/30 is subnetted, 1 subnets
C       192.168.30.0 is directly connected, Serial1/0

Router2#show ip route
...
Gateway of last resort is not set

C    192.168.20.0/24 is directly connected, FastEthernet0/0
     192.168.30.0/30 is subnetted, 1 subnets
C       192.168.30.0 is directly connected, Serial1/0

A ping from Net-A to Router2 would have src:192.168.10.x/dst:192.168.30.2. The reply has src:192.168.30.2/dst:192.168.10.x, however, Router2 has no route to 192.168.10.x.

Each router needs to know about the networks beyond its peers.
(router1) ip route 192.168.20.0 255.255.255.0 192.168.30.2
(router2) ip route 192.168.10.0 255.255.255.0 192.168.30.1

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