Router loopback and ping questions

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Hello all, got a question for you Cisco/router types. I encountered this at work today and kind of wondered. Suppose I have a 7500 serie routers on either end of a dedicated link and they are not getting connectivity. The only thing that separates these routers on the line side is a DSX (digital cross connect) patch panel. Suppose I looped one side of the DSX panel. Now, most routers when looking at the interface will say line up, protocol up, looped, when there is a loop up somewhere down the line. My question is, if you telnetted into router A, pinged the IP address of router B, would the echo packets come back clean/no loss, or would the echo packets come back as "destination unreachable" as there really isn't another router on the other end, but merely a loopback imitating one. I figure that ICMP echo packets are the same coming to, and from destination (afterall, that's why they call them echo packets). I also figured if I did an extended ping with a loopback, I could further my argument that the problem is not on our side, but after the demarcation point. Anything can notice a loop on line side, but true connectivity is another thing. That is what I'm wanting to accomplish with this scenario. I am also wondering if there is anything related to this that may have to do with frequency of routing table updates. Thanks for any thoughts.

Nocturnal

You can ping it if the encapsulation is HDLC, but it won't work with ppp. When I'm troubleshooting connectivity issues I will often change the encap to HDLC to do the very thing you have described.