Hello Joerg,
Oh, I see. I apologize for the confusion.
I am not sure about this requirement as I do not have access to IOS-XR. Purely technically, it should not be necessary to require that BGP-to-OSPF redistribution must be configured in order to make a sham-link work.
The opposite endpoint of a sham-link is an IP address learned within the same VRF where the sham-link also resides. Because this IP address is actually an IP address of the opposite PE router in the corresponding VRF (a loopback interface assigned to that VRF), the only correct way to learn it is via BGP. This loopback shall not be advertised into the OSPF running over the sham-link (see this document).
However, the Routing Configuration Guide for IOS-XR 4.1 does indicate that the BGP-to-OSPF redistribution is configured as a part of the sham-link configuration procedure. However, that may be a pure sanity check whether it makes sense to establish the sham-link at all: without that redistribution, the routes learned from the other PE router are not forwarded in OSPF to the customer's CE, hence establishing the sham-link itself won't make any difference.
But this is just a guess, and certainly, I see no technical reason why the sham-links should be inoperable without BGP-to-OSPF redistribution.
Anyone else to share his/her knowledge and expertise here?
Best regards,
Peter
