A strange problem with Cisco 2950 switch

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WAN--->;Frame Relay lease line --->;Router Cisco 2501-->;2950 Switch-->; 2 PCs
when the 2950 connects to the network ,the WAN become down ,PC can access each other ,but Ping any IP in WAN,fail.Removing the switch, all the network recoverred .
有没有高手有兴趣讨论一下？

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没有冲突

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Thanks upstair guy's opinion, Firstly, I don't add the switch to the network , the WAN works well,If I add it, the network doesn't work;secondly,the 2 PCs connect to the network via the switch,that is the architecture.
I'm sure
  1.No IP conflict
  2.No virus cause the network works well once I remove the switch;
  3.Not port problem ,I tried with different port which working.
4.I test again today, when problem occurred, Tracert can get 2 Routers ,not be able to get any other Router.
  Anybody has great idea to share with me ?