Prerequisites for MPLS over GRE
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Label Distribution Protocol (LDP)—for MPLS label distribution.
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Routing protocol (ISIS or OSFP) between the core devices P1-P-P2
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MPLS between PE1-P1 and PE2-P2
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Since the ingress traffic enters the IP core from MPLS network and egress traffic leaves the IP core to enter the MPLS network, it is recommended to use QoS group value for defining QoS policies as we traverse the protocol boundary.