In case of IS-IS as a PE-CE routing protocol for MPLS VPNs , when route is redistributed from BGP to IS-IS , does 'Down Bit' is also set to avoid routing loops?.

Background Information

We know that when OSPF is used as PE-CE routing protocol and when route is redistributed from BGP at PE to OSPF towards CE , then PE also sets the 'Down Bit'. This is done to avoid routing loops. When other PE connected to the same CE (or set of CE's) receives the LSA with 'Down Bit' set in database , it doesnot install the route in the routing protocol to avoid routing loop (which will occut when PE-2 will again redistribute the OSPF route into BGP VPNv4).

We also know that in case of IS-IS , where L1/L2 router 'leake' the routes from L2 to L1 it also sets the 'Down Bit' so that other L1/L2 router connected to same set of L1 routers upon receiving the L1 routes will not inadvertently advertise them as L2 routes.This is done to avoid routing loops.

Because in case of IS-IS L1 routes are leaked to L2 automatically without requiring explicit configuration. So 'Down Bit' helps to prevent routing loops.

Please help to clarify that 'Down Bit' is also set when IS-IS is used as a PE-CE routing protocol with regards to MPLS VPN?

If 'Down Bit' is not set , then how to prevent routing loop?

Many Thanks

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