Control Plane Protocols

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The control plane in an MPLS network is responsible for setting up and maintaining the Label Switched Paths (LSPs) between the Provider (P) routers in the MPLS cloud. The P routers do not participate in the data plane forwarding of MPLS packets, but they do play a critical role in establishing and maintaining the LSPs.

The two protocols that work in the control plane of P routers across the MPLS cloud are LDP and RSVP. Here's a brief explanation of each protocol:

1. LDP (Label Distribution Protocol): This is a protocol that runs between the P routers to distribute labels and create LSPs across the MPLS cloud. When an ingress Provider Edge (PE) router sends a packet with a particular destination prefix, it looks up the Label Forwarding Information Base (LFIB) to determine the label that should be attached to the packet. The LFIB is populated by the LDP protocol, which advertises labels from the downstream routers to the upstream routers. Each P router uses LDP to communicate with its neighboring P routers and to distribute labels to them.

2. RSVP (Resource Reservation Protocol): This is a protocol that allows for the reservation of network resources (bandwidth, buffer space, etc.) along the path of an LSP. RSVP operates in two modes: a unicast mode, which is used to reserve resources for a single LSP, and a multicast mode, which is used to reserve resources for a group of LSPs. When a PE router wants to reserve resources for a particular LSP, it sends an RSVP message downstream along the path of the LSP, and each P router along the way reserves the necessary resources.

The other options in the answer are:

• ECMP (Equal-Cost Multipath): This is a forwarding mechanism that allows for the distribution of traffic over multiple paths that have the same cost. It is not a control plane protocol used in MPLS.
• MPLS OAM (Operations, Administration, and Maintenance): This is a set of protocols used for monitoring and troubleshooting MPLS networks. It is not a control plane protocol used in MPLS.
• LSP (Label Switched Path): This is the path that a packet takes through an MPLS network, from the ingress PE router to the egress PE router. LSP is not a control plane protocol used in MPLS.