Here s a pic from packet tracer

enter image description here

What I'm trying to do is to ping Router3's Se2/0 interface from PC0.(Or Router2's Se2/0 interface from PC1)

However the ping message gets only as far as the Router2's Ethernet interface Shouldn't the ping work since ROUTER3's serial interface is connected to the WAN, which is directly connected to ROUTER2?

Some configuration info


IP :
Default gateway:


Fast Ethernet interface IP:
Serial interface IP:


Serial interface IP:

Subnet Masks are all

  • 1
    Does router 3 have a route back to PC0's subnet in its table? Mar 29, 2015 at 15:08
  • you mean routing table?
    – MattSt
    Mar 29, 2015 at 15:35
  • 1
    Yes. If router 3 doesn't have a route back to PC0 then it can't successfully respond to any echo requests (pings) from PC0. If router 3 has a default gateway that is not router 2, then it needs an explicit route back to the network that PC0 is on. And likewise for Router 2 talking to PC1. Mar 29, 2015 at 20:27

2 Answers 2


First of all that You have route from PC0 to PC1 doesn't mean that You have route from PC1 to PC0.

Thats are Zinin’s 3 routing principles:

  1. "Every router makes its decision alone, based on the information it has in its own routing table.“
  2. "The fact that one router has certain information in its routing table does not mean that other routers have the same information.“
  3. "Routing information about a path from one network to another does not provide routing information about the reverse, or return path."

That means all routers must have full routing table to all networks - R1 to networks beyond R3 and R3 to all networks beyond R2.

Besides R3 probably don't have any information about networks at the other site of R2. You can check it showing R3 routing table - show ip route


You can set a static route in R2 towards the network that PC1 is in and another one in the reverse at R3. Like so: on R2: ip route (PC1's network address) (subnet mask) on R3: ip route

After that you should be able to ping PC-PC and the s2/0 interface as well.

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