mpls vpn PE can't ping CE attached to another IGP PE,while interAS works

[wminjuan]

PE A and PE B are in the same AS, PE C is in a different AS. I stand on the PE A to ping the CE attached to PE B , it works. But when I change to PE B to ping the CE attached to A, it return me timeout. when I ping from PE A and B to the CE attached to PE C, both work well. the IOS on PE A is 12.1(5)T7, while PE B is 12.1(2)T.

another thing. all the PE can ping CE attached to itself. no acl used.

[wminjuan]

now I have change the IOS to 12.1(18) on both PE router. the problem is still there. another question comes , CE A which attached to PEA , can't ping the PE A 's loopback 1( I make it a virtual CE). while CE B can ping PE B's loopback1 .