Understanding MPLS: MPLS Terminology September 13
How MPLS works is in fact dependent on a devices topological location within a network. To understand this we must look at a network and define the topological reference names.
CE: Customer Edge - The last customer switch or router before entering the Provider network. This is where the customer provides its internal routes to the provider network. The provider receives the routes on the PE router.
PE: Provider Edge - This is where the customer routes are received. This is also where label imposition/disposition happens through the use of a Shim header. That is, this is where labels are created and/or removed for customer traffic to and from the provider network.
LSR: MPLS Label Switch Router (”P” LSR) -This is any router or switch that performs the function of label switching; the LSR receives a labeled packet and swaps the label with an outgoing label and forwards the new labeled packet from the appropriate interface. The “P” router is often distinguished from the “PE” or “CE” router being that the former does not perform as many functions as the latter. The “P” routers are located in the core of the network. The edge routers are directly connected to customer sites.
Ingress: - The insertion point of the traffic being referred to. It is considered the source of the targeted traffic being discussed. The ingress LSR which performs the action of label imposition (puts the label in to the packet) and forwarding of the packet through the MPLS enabled domain (important distinction).
Egress: - The destination, or the exit point of traffic after it transverses a core of a network. It is the reference point closest to the intended destination of the traffic. The egress LSR performs label disposition (removal of the label from the packet) and the forwarding to its destination.
*Both ingress and egress LSR’s are relative to the source and destination of the traffic being generated or received. Ingress and egress insinuate traffic flows across a network. Because of this, we must look at the concept of “upstream” and “downstream”
Upstream and Downstream: - This concept is absolutely critical in understanding the operation of MPLS networks. This pertains mainly to the operation of label distribution (a control plane function) and data forwarding (a data plane function). Upstream and Downstream are defined in respect to the destination network (prefix or FEC). Data intended for a particular destination network always flows downstream. Keep DAD in your head (destination address downstream), it helps keep perspective. Updates from routing protocols or tag/label distribution protocols always flow upstream. Makes sense eh? Devices need to know how to get to a destination from the source right? If you understand routing protocols, they advertise the networks on their router to the world, thus “pushing them out”, in other words “upstream”. Hope this helps with the concept.
FEC: Forward Equivalence Class - As noted in RFC 3031 (MPLS Architecture) this group of packets is forwarded in the same manner (over the same path with the same forwarding treatment). A FEC in all simplicity is really a network/subnet, in other words a range of addresses that has been subnetted in some way. In MPLS networks they are often called prefixes. An example would be 192.168.1.0/24. This class C address is a FEC. 172.16.1.2/30 is also a FEC, being that 172.16.1.0 - 172.16.1.3 will all be forwarded in the same manner (over the same path with the same forwarding treatment).
CEF: Cisco Express Forwarding - CEF is required to be able to label switch in an MPLS network. CEF has two components, the Forwarding Information Base (FIB) and the adjacency table. The FIB (located in the data plane) is responsible for maintaining next hop IP addresses for all of the routes in the routing table. The adjacency table is responsible for maintaining the layer 2 information for each FIB entry. The adjacency table is responsible for the layer 2 rewrite, and it avoids the need for an ARP request for each IP address lookup. Basically CEF binds the next hop address for a specific network to a physical interface mac address. It relies on recursive updates in and from the routing table to do this. This is essentially what allows layer 3 switching.
FIB: Forwarding Information Base - The FIB is conceptually similar to a routing table or information base. It maintains a mirror image of the forwarding information contained in the IP routing table. When routing or topology changes occur in the network, the IP routing table is updated, and those changes are reflected in the FIB. The FIB maintains next-hop MAC address information based on the information in the IP routing table.
This next section will need a graph to help you understand how these two planes differ in function. Please do not be intimidated by the graph, I am working on cutting out a lot of the ‘extras’ you don’t need to bother with at this point.
Control Plane: - This component identifies reachability to destination networks. It contains ALL Layer 3 routing information to include the processes involved with the operation of routing protocols (OSPF, BGP, RSVP, etc.). It also includes any information responsible for updates between neighboring routers like Tag or Label distribution information exchange.
Data Plane: - Performs the functions related to forwarding data packets, either IP packets or Labeled packets. The information in the data plane like specific Tag or Label numbers for a specific prefix (network - remember what is written above) is completely dependant on the Control Plane. So, the mapping of IP destination networks to labels gets copied to the Data Plane, thus at a Layer 2 level is able to be switched. It eliminates the need of a Layer 3 lookup, saving time, and alleviating or efficating route cache issues.
**Note: These postings are designed to help me retain knowledge as I pursue my CCNP/CCIP/CCIE Service Provider. “MPLS Configuration on Cisco IOS Software” by Lancy Lobo (CCIE# 4690), and Umesh Lakshman, as well as “MPLS and VPN Architectures” CCIP Edition by Ivan Pepelnjak (CCIE# 1354), and Jim Guichard (CCIE# 2069) are the books I reference for this and future postings. A dear thanks for the aforementioned individuals for publishing this content!**