Now it is obvious that process id 1 is already engaged with customer ABCD. so to ensure optimal routing provider assign one domain-id per customer vrf so that this problem can be overridden.
meaning if we do not give common domain-id to vrf then routes from one domain will be installed to other domain as an external route
Now regarding your next question..
ospf has two facility ..downbit and tag
Downbit is used to avoid redistribution of ospf routes back to ospf domain from mp-bgp to avoid loops. Tag is by default is set as bgp as number so if ISPA is receiving routes advertised by same ISPA somewhere is not installed in mp-bgp..this is used specifically for external routes.
is your requirement is ISPA should not allow routes advertised CE which is received from ISPB. I don't think this is possible unless provider ISPA manually match tag (ISPB bgp AS number ) and deny those prefixes from advertising to mp-bgp.
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