[j-nsp] BGP L3 VPN and backdoor between customers

[Roman]

Dear All 

How would you handle the following situation -  suppose you provide L3 BGP VPN service and your PE-CE protocol is BGP . Customer uses private AS 65000 in all its sites so as-override in place in PE config .  To complicate things customers also have backdoor links between them .  The question is how to ensure that there is no loop forming in provider AS for intercustomer traffic ? My first thought was to watch out for extended bgp community in customer updates ( for example target: ) and   drop this particular update in case these communities present . Any other ideas are very welcome 

Thanks in advance 
Roman