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Shaun GormanFlag for Australia

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Ping between Serial interfaces on two Cisco 2610XM routers stopped working

I have 4 Cisco 2610xm routers and two WS-C2950 switches I am configuring to study my CCNA with.  
Router1 has 10.1.0.31 on FastEthernet0/0, and Router2 has 10.1.0.32 on it's FastEthernet0/0. My computer is 10.1.0.122.
These ports are plugged in to a WS-C2950 switch and I can ping both of them either from my computer which is plugged in to the switch as well, or from either router. Because I am up to the part about natting I would like to plug the two routers together. To this end I plugged a DTE to DCE Crossover cable between the serial interface 0/0 on each router. I successfully setup hdlc encapsulation and a clock rate of 2000000 on router1 which has the DCE end of the crossover cable.
I then gave Router1 serial interface the ip of 192.168.35.5 and Router2 serial interface the ip of 192.168.35.3.
From Router1 I pinged router2 by going "ping 192.168.35.3" It responded with "Success rate is 100 percent (5/5), round-trip min/avg/max = 1/203/1002 ms"
That was the last time it pinged successfully. That happened a few minutes after I configured the serial ports on the two routers and now they can't ping each other. I have done a traceroute 192.168.35.3 from router1 and the first hop goes to 10.1.0.1 like it doesn't know about the serial interface to send 192.168.35.3 traffic. I can still ping the interface 192.168.35.5 on router1 from router1 fine so that interface is up. It seems like I need to configure a route maybe? I have tested routes a little with not much success.
Any ideas?
Thanks,
Shaun
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Miftaul H

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Thanks for both posting Miftaul and Don Johnston. I tried Miftaul's approach first. Removing the Ethernet cable to 10.1.0.31 and pinging again to see if it would go through the serial cable. It did! I put the ethernet cable back in to interface 10.1.0.31 and ping still worked. I tried traceroute and this time it showed: "Tracing the route to 192.168.35.5  1.192.168.35.3 4msec 2 192.168.35.5"  So for whatever reason the route went straight to the serial interface instead of trying the ethernet port. Is that because I removed the Ethernet cable do you think? If it helps any, when I go "sh ip route" it still shows: "Host:192.168.35.3 Gateway 10.1.0.1" .  192.168.35.3 is the serial interface on the router I am checking routes on. Like it thinks the route to the serial interface on the same router is through the ethernet interface.  
Don Johnston: I have gone "show ip int brief" and it shows on both routers Serial10/0 as "up"
So it is fixed, but any ideas why it works now?
Thanks for your help so far!
Shaun