MPLS Control Plane and Forwarding Plane Interaction

In this post, we will examine the Control Plane and for Forwarding Plane components in more detail and discuss their interaction. This post will reference the following diagram:

Let us start at the top of this illustration and work our way down, reviewing these components and focusing on their interaction.

First, notice the Routing Protocol is responsible for building the IP Routing Table ( or Routing Information Base RIB). This used to be our only real concern for this lab! (Ahh, the good ole' days.) For your current lab exam, this is where we configure or repair OSPF, RIP version 2, EIGRP, or BGP.

Notice the Label Distribution Protocol (LDP) (covered in the last blog) is our focus in R&S. This important protocol creates, maintains, and distributes the MPLS labels. It takes networks learned via the Routing Protocol and binds prefix information to labels. While our focus is LDP, you recall from previous posts that Tag Distribution Protocol (TDP), BGP, and RSVP are other options for the label creation and exchange.

Two structures are critical in the Forwarding or Data Plane. They are constructed from the information created in the Control Plane. They are the Forwarding Information Base (FIB) and the Label Forwarding Information Base (LFIB). As we configure and verify MPLS, the LFIB will be so critical to examine. Inside this store we will see label values and associations with outgoing interfaces for key network prefixes.

As an example, consider a P router that learns of a prefix via OSPF. The prefix is 10.10.10.0/24 and represents one of the loopback interfaces that will be used to form your iBGP peering in the MPLS network. Thanks to the Label Distribution Protocol, label 16 is received and is to be used for any packets with a destination address of 10.10.10.0/24. The device generates a local label value (let us say 22) for the prefix. This label is sent upstream so the next router can associate this label with the destination prefix.

Thanks to all of these components working together, the data plane is ready to efficiently switch packets based on the LFIB. When the data plane receives a packet with the label 16, it replaces this label with the label of 22 and forwards the packet through the appropriate interface.

It is worth reviewing here something that we documented in an earlier post. Routers in the core (P) of the provider network do not perform routing table lookups. The Forwarding Plane LFIB contains all of the information required to quickly move the packets. As we will examine later on with our L3 MPLS VPNs, the core devices are also free of BGP (a BGP -free core).