A label switched path (LSR) is a sequence of Label Switch Routers that switch a labeled packet through an Multiprotocol Label Switching network or part of an Multiprotocol Label Switching network. Basically, the Label Switch Routers is the path through the Multiprotocol Label Switching network or a part of it that packets take. The first Label Switch Router of an Label Switch Routers is the ingress Label Switch Router for that Label Switch Routers, whereas the last Label Switch Router of the Label Switch Routers is the egress Label Switch Router. All the Label Switch Routers in between the ingress and egress Label Switch Routers are the intermediate Label Switch Routers. The ingress Label Switch Router of an Label Switch Routers is not necessarily the first router to label the packet.

The packet might have already been labeled by a preceding Label Switch Router. Such a case would be a nested Label Switch Routers—that is, an Label Switch Routers inside another Label Switch Routers. Another Label Switch Routers starts at the third Label Switch Router and ends on the next-to-last Label Switch Router. Therefore, when the packet enters the second Label Switch Routers on its ingress Label Switch Router (this means the third Label Switch Router), it is already labeled. This ingress Label Switch Router of the nested Label Switch Routers then pushes a second label onto the packet. The label stack of the packet on the second Label Switch Routers has two labels now. The top label belongs to the nested Label Switch Routers, and the bottom label belongs to the Label Switch Routers that spans the entire Multiprotocol Label Switching network. A backup traffic engineering (TE) tunnel is an example of such a nested Label Switch Routers.

Forwarding Equivalence Class

A Forwarding Equivalence Class is a group or flow of packets that are forwarded along the same path and are treated the same with regard to the forwarding treatment. All packets belonging to the same Forwarding Equivalence Class have the same label. However, not all packets that have the same label belong to the same Forwarding Equivalence Class, because their EXP values might differ; the forwarding treatment could be different, and they could belong to a different Forwarding Equivalence Class. The router that decides which packets belong to which Forwarding Equivalence Class is the ingress Label Switch Router. This is logical because the ingress Label Switch Router classifies and labels the packets. Following are some examples of Forwarding Equivalence Classs:

1. Packets with Layer 3 destination IP addresses matching a certain prefix
2. Multicast packets belonging to a certain group
3. Packets with the same forwarding treatment, based on the precedence or IP DiffServ Code Point (DSCP) field
4. Layer 2 frames carried across an Multiprotocol Label Switching network received on one VC or (sub)interface on the ingress Label Switch Router and transmitted on one VC or (sub)interface on the egress Label Switch Router
5. Packets with Layer 3 destination IP addresses that belong to a set of Border Gateway Protocol prefixes, all with the same Border Gateway Protocol next hop

All packets on the ingress Label Switch Router for which the destination IP address points to a set of Border Gateway Protocol routes in the routing table—all with the same Border Gateway Protocol next-hop address—belong to one Forwarding Equivalence Class. It means that all packets that enter the Multiprotocol Label Switching network get a label depending on what the Border Gateway Protocol next hop is. The destination IP address of all IP packets entering the ingress Label Switch Router will be looked up in the IP forwarding table. All these addresses belong to a set of prefixes that are known in the routing table as Border Gateway Protocol prefixes. Many Border Gateway Protocol prefixes in the routing table have the same Border Gateway Protocol next-hop address, namely one egress Label Switch Router.

All packets with a destination IP address for which the IP lookup in the routing table recurses to the same Border Gateway Protocol next-hop address will be mapped to the same Forwarding Equivalence Class. As already mentioned, all packets that belong to the same Forwarding Equivalence Class get the same label imposed by the ingress Label Switch Router.

Label Distribution

The first label is imposed on the ingress Label Switch Router and the label belongs to one Label Switch Routers. The path of the packet through the Multiprotocol Label Switching network is bound to that one Label Switch Routers. All that changes is that the top label in the label stack is swapped at each hop. The ingress Label Switch Router imposes one or more labels on the packet. The intermediate Label Switch Routers swap the top label (the incoming label) of the received labeled packet with another label (the outgoing label) and transmit the packet on the outgoing link. The egress Label Switch Router of the Label Switch Routers strips off the labels of this Label Switch Routers and forwards the packet. Consider the example of plain IPv4-over-Multiprotocol Label Switching, which is the simplest example of an Multiprotocol Label Switching network. Plain IPv4-over-Multiprotocol Label Switching is a network that consists of Label Switch Routers that run an IPv4 Interior Gateway Protocol (IGP) (for example, Open Shortest Path First [OSPF], Intermediate System-to- Intermediate System [IS-IS], and Enhanced Interior Gateway Routing Protocol [EIGRP]). The ingress Label Switch Router looks up the destination IPv4 address of the packet, imposes a label, and forwards the packet.

The next Label Switch Router (and any other intermediate Label Switch Router) receives the labeled packet, swaps the incoming label with an outgoing label, and forwards the packet. The egress Label Switch Router pops the label and forwards the IPv4 packet without labels on the outgoing link. For this to work, adjacent Label Switch Routers must agree on which label to use for each IGP prefix. Therefore, each intermediate Label Switch Router must be able to figure out with which outgoing label the incoming label should be swapped. This means that you need a mechanism to tell the routers which labels to use when forwarding a packet. Labels are local to each pair of adjacent routers. Labels have no global meaning across the network. For adjacent routers to agree which label to use for which prefix, they need some form of communication between them; otherwise, the routers do not know which outgoing label needs to match which incoming label. A label distribution protocol is needed. You can distribute labels in two ways:

1. Piggyback the labels on an existing IP routing protocol
2. Have a separate protocol distribute labels

Piggyback the Labels on an Existing IP Routing Protocol

The first method has the advantage that a new protocol is not needed to run on the Label Switch Routers, but every existing IP routing protocol needs to be extended to carry the labels. This is not always an easy thing to do. The big advantage of having the routing protocol carry the labels is that the routing and label distribution are always in sync, which means that you cannot have a label if the prefix is missing or vice versa. It also eliminates the need of another protocol running on the Label Switch Router to do the label distribution. The implementation for distance vector routing protocols (such as EIGRP) is straightforward, because each router originates a prefix from its routing table. The router then just binds a label to that prefix. Link state routing protocols (such as IS-IS and OSPF) do not function in this way.

Each router originates link state updates that are then forwarded unchanged by all routers inside one area. The problem is that for Multiprotocol Label Switching to work, each router needs to distribute a label for each IGP prefix— even the routers that are not originators of that prefix. Link state routing protocols need to be enhanced in an intrusive way to be able to do this. The fact that a router needs to advertise a label for a prefix it does not originate is counterintuitive to the way link state routing protocols work anyway. Therefore, for link state routing protocols, a separate protocol is preferred to distribute the labels. None of the IGPs has been changed to deploy the first method. However, Border Gateway Protocol is a routing protocol that can carry prefixes and distribute labels at the same time. However, Border Gateway Protocol is not an IGP; it is used to carry external prefixes. Border Gateway Protocol is used primarily for label distribution in Multiprotocol Label Switching Virtual Private Network networks.

Running a Separate Protocol for Label Distribution

The second method—running a separate protocol for label distribution—has the advantage of being routing protocol independent. Whatever the IP routing protocol is, whether it is capable of distributing labels or not, a separate protocol distributes the labels and lets the routing protocol distribute the prefixes. The disadvantage of this method is that a new protocol is needed on the Label Switch Routers. The choice of all router vendors was to have a new label distribution protocol distribute the labels for IGP prefixes. This is the Label Distribution Protocol (Label Distribution Protocol). Label Distribution Protocol, however, is not the only protocol that can distribute Multiprotocol Label Switching labels. Several varieties of protocols distribute labels:

Tag Distribution Protocol, which predates Label Distribution Protocol, was the first protocol for label distribution developed and implemented by Cisco. However, Tag Distribution Protocol is proprietary to Cisco. The IETF later formalized Label Distribution Protocol. Label Distribution Protocol and Tag Distribution Protocol are similar in the way they operate, but Label Distribution Protocol has more functionality than Tag Distribution Protocol. With the widespread availability of Label Distribution Protocol in general-deployment Cisco IOS releases, Tag Distribution Protocol was quickly replaced by Label Distribution Protocol. The result is that Tag Distribution Protocol is becoming obsolete. Therefore, the remainder of this article refers to Label Distribution Protocol only. Label distribution by Resource Reservation Protocol is used for Multiprotocol Label Switching TE only. about Multiprotocol Label Switching TE and to find out how Resource Reservation Protocol does the label distribution.