can't link testplan with testcase external ID

I got 3 testcase on RQM server, and tried to link them to a newly created test plan.xls
as bellow:
testcase1 is the RQM name of the case.
then I tried this cfg-use external ID to link, it didnt work.
plan
Column_A |
case
Column_B |
testplan1 |
testcase1
testcase2 testcase3 |
testplan2 |
urn:com.ibm.rqm:testcase:95418
urn:com.ibm.rqm:testcase:95416 urn:com.ibm.rqm:testcase:95417 |
testplan.dc:title=A1
testplan.testcase=Link(B1)
then I tried this cfg-use internal ID to link, it worked.
testplan.dc:title=A1
testplan.testcase=Link(B2)
whats wrong,thx
Accepted answer

Hi Zhao Li
As testcase1-testcase3 are existing, how do you determine their external id in your scenario?
According to https://jazz.net/wiki/bin/view/Main/RqmApi#External_ID
===
A unique ID provided by the user when creating a new resource with a PUT. Resources created in the Rational Quality Manager UI will not have external IDs.
==
so if your existing 3 testcases were created over GUI, they don't have external id and you would need to use
testplan.testcase=Link(B2)
to make the link work because here the internal id should be used.