What is an important requirement with MPLS applications like Layer 3 MPLS VPNs?

A.
All the PE routers loopback addresses should be summarized to reduce the number of routing
table entries in the core routers.

B.
Targeted hellos are required between all the PE routers.

C.
An end-to-end LSP is required between the PE routers.

D.
The LSPs that are built between the PE routers must be symmetrical (bidirectional).