If you ping the IP address of your partner's serial interface, it should work; the ping will go to the partner, and the partner will reply.
If you ping the IP address of your own serial interface, it should work: the ping will go out to the partner and back to you. You will generate a reply that will go out to the partner and back to you again.
If you ping another IP address that is in the same subnet as your serial link, but is not the IP address of either end, the ping will bounce backwards and forwards between you and the partner until the "time-to-live" expires. This is normal behaviour; there is nothing to fix.
Kevin Dorrell
Luxembourg