Editing Multiprotocol Label Switching (section)

===Routing===
When an unlabeled packet enters the ingress router and needs to be passed on to an MPLS [[Tunneling protocol|tunnel]], the router first determines the [[forwarding equivalence class]] (FEC) for the packet and then inserts one or more labels in the packet's newly created MPLS header. The packet is then passed on to the next hop router for this tunnel.

From an [[OSI model]] perspective, the MPLS Header is added between the [[network layer]] header and [[link layer]] header.<ref>Savecall telecommunication consulting company Germany [http://www.savecall.de/mpls/ Savecall - MPLS]</ref>

When a labeled packet is received by an MPLS router, the topmost label is examined. Based on the contents of the label a ''swap'', ''push''{{efn|A.k.a. ''impose''}} or ''pop''{{efn|A.k.a. ''dispose''}} operation is performed on the packet's label stack. Routers can have prebuilt lookup tables that tell them which kind of operation to do based on the topmost label of the incoming packet so they can process the packet very quickly.
* In a ''swap'' operation the label is swapped with a new label, and the packet is forwarded along the path associated with the new label.
* In a ''push'' operation a new label is pushed on top of the existing label, effectively ''encapsulating'' the packet in another layer of MPLS. This allows [[hierarchical routing]] of MPLS packets. Notably, this is used by [[MPLS VPN]]s.
* In a ''pop'' operation the label is removed from the packet, which may reveal an inner label below. This process is called ''decapsulation''. If the popped label was the last on the label stack, the packet ''leaves'' the MPLS tunnel. This can be done by the egress router, or at the penultimate hop.

During these operations, the contents of the packet below the MPLS Label stack are not examined. Indeed, transit routers typically need only to examine the topmost label on the stack. The forwarding of the packet is done based on the contents of the labels, which allows protocol-independent packet forwarding that does not need to look at a protocol-dependent routing table and avoids the expensive IP [[longest prefix match]] at each hop.

At the egress router, when the last label has been popped, only the payload remains. This can be an IP packet or any type of packet. The egress router must, therefore, have routing information for the packet's payload since it must forward it without the help of label lookup tables. An MPLS transit router has no such requirement.

Usually{{efn|This is the default behavior with only one label in the stack, accordingly to the MPLS specification.}}, the last label is popped off at the penultimate hop (the hop before the egress router). This is called [[penultimate hop popping]] (PHP). This is useful in cases where the egress router has many packets leaving MPLS tunnels and thus spends significant CPU resources on these transitions. By using PHP, transit routers connected directly to this egress router effectively offload it, by popping the last label themselves. In the label distribution protocols, this PHP label pop action is advertised as label value 3 (implicit null) and is never found in a label, since it means that the label is to be popped.

Several MPLS services including end-to-end [[Quality of service|QoS]] management,<ref>{{Cite news|url=https://www.networkworld.com/article/912436/cisco-subnet-understanding-mpls-explicit-and-implicit-null-labels.html|title=Understanding MPLS Explicit and Implicit Null Labels|last=Doyle|first=Jeff|work=Network World|access-date=2018-03-13|language=en}}</ref> and [[6PE]],<ref>{{cite web|title=6PE FAQ: Why Does 6PE Use Two MPLS Labels in the Data Plane?|url=https://www.cisco.com/c/en/us/support/docs/multiprotocol-label-switching-mpls/mpls/116061-qa-6pe-00.html|website=Cisco|language=en|access-date=2018-03-13}}</ref> require keeping a label even between the penultimate and the last MPLS router, with a label disposition always done on the last MPLS router, ultimate hop popping (UHP).<ref>{{Cite book|title=Router security strategies : securing IP network traffic planes|last=Gregg.|first=Schudel|date=2008|publisher=Cisco Press|others=Smith, David J. (Computer engineer)|isbn=978-1587053368|location=Indianapolis, Ind.|oclc=297576680}}</ref><ref>{{cite web|title=Configuring Ultimate-Hop Popping for LSPs - Technical Documentation - Support - Juniper Networks|url=https://www.juniper.net/documentation/en_US/junos/topics/task/configuration/mpls-ultimate-hop-popping-enabling.html|website=www.juniper.net|access-date=2018-03-13}}</ref> Some specific label values have been notably reserved<ref>{{Cite journal|url=https://tools.ietf.org/html/rfc3032|title=MPLS Label Stack Encoding|last1=Dino|first1=Farinacci|last2=Guy|first2=Fedorkow|website=tools.ietf.org|language=en|access-date=2018-03-13|last3=Alex|first3=Conta|last4=Yakov|first4=Rekhter|last5=C.|first5=Rosen, Eric|last6=Tony|first6=Li|year=2001 |doi=10.17487/RFC3032 |url-access=subscription}}</ref><ref>{{Cite journal|url=https://tools.ietf.org/html/rfc4182|title=Removing a Restriction on the use of MPLS Explicit NULL|last=<erosen@cisco.com>|first=Eric C. Rosen|website=tools.ietf.org|year=2005 |doi=10.17487/RFC4182 |language=en|access-date=2018-03-13|url-access=subscription}}</ref> for this use. In this scenario the remaining label stack entry conveys information to the last hop (such as its Traffic Class field for QoS information), while also instructing the last hop to pop the label stack using one of the following reserved label values:
* 0: Explicit-null for IPv4 
* 2: Explicit-null for IPv6

An MPLS header does not identify the type of data carried inside the MPLS path. To carry two different types of traffic between the same two routers, with different treatment by the core routers for each type, a separate MPLS path for each type of traffic is required.

====Label-switched path====
A label-switched path (LSP) is a path through an MPLS network set up by the [[Network Management System|NMS]] or by a signaling protocol such as [[Label Distribution Protocol|LDP]], [[RSVP-TE]], [[BGP]] (or the now deprecated [[CR-LDP]]). The path is set up based on criteria in the FEC.

The path begins at an LER, which makes a decision on which label to prefix to a packet based on the appropriate FEC. It then forwards the packet along to the next router in the path, which swaps the packet's outer label for another label, and forwards it to the next router. The last router in the path removes the label from the packet and forwards the packet based on the header of its next layer, for example [[IPv4]]. Due to the forwarding of packets through an LSP being opaque to higher network layers, an LSP is also sometimes referred to as an MPLS tunnel.

The router which first prefixes the MPLS header to a packet is an [[ingress router]]. The last router in an LSP, which pops the label from the packet, is called an [[egress router]]. Routers in between, which need only swap labels, are called transit routers or label switch routers (LSRs).

Note that LSPs are unidirectional; they enable a packet to be label switched through the MPLS network from one endpoint to another. Since bidirectional communication is typically desired, the aforementioned dynamic signaling protocols can automatically set up a separate LSP in the opposite direction.

When [[link protection]] is considered, LSPs can be categorized as primary (working), secondary (backup) and tertiary (LSP of last resort).