Editing Multiprotocol Label Switching (section)

===Equipment===
{{MPLS diagram}}
====Label switch router====
An MPLS router that performs routing based only on the label is called a '''label switch router''' ('''LSR''') or '''transit router'''. This is a type of router located in the middle of an MPLS network. It is responsible for switching the labels used to route packets.

When an LSR receives a packet, it uses the label included in the packet header as an index to determine the [[Per-hop behavior|next hop]] on the label-switched path (LSP) and a corresponding label for the packet from a [[Label Information Base]]. The old label is then removed from the header and replaced with the new label before the packet is routed forward.

====Label edge router====
A '''label edge router''' (LER, also '''edge LSR''' (which is "technically more correct")<ref>{{cite web |url=https://ftp.unpad.ac.id/orari/library/library-ref-eng/ref-eng-3/network/mpls/mpls.pdf |title=Cisco MPLS Controller Software Configuration Guide |date=May 2001 |publisher=[[Cisco]]}}</ref> or simply ''edge router''<ref>{{cite web |url=https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/mp_ias_and_csc/configuration/xe-3s/mp-ias-and-csc-xe-3s-book/mp-carrier-ldp-igp.html |title=MPLS VPN Carrier Supporting Carrier Using LDP and an IGP |date=4 April 2014 |publisher=[[Cisco]]}}</ref>) is a router that operates at the edge of an MPLS network and acts as the entry and exit points for the network. LERs ''push'' an MPLS label onto an incoming packet{{efn|In some applications, the packet presented to the LER already may have a label, so that the new LER pushes a second label onto the packet.}} and ''pop'' it off an outgoing packet. Alternatively, under [[penultimate hop popping]] this function may instead be performed by the LSR directly connected to the LER.{{efn|See for example 'Penultimate LSR' in Table 3-1 of {{cite web |url=https://www.ciscopress.com/articles/article.asp?p=426645&seqNum=3 |title=A Network Administrator's View of Multiservice Networks |date=9 December 2005 |publisher=[[Cisco Press]]}}}}

When forwarding an [[IP datagram]] into the MPLS domain, a LER uses routing information to determine the appropriate label to be affixed, labels the packet accordingly, and then forwards the labeled packet into the MPLS domain. Likewise, upon receiving a labeled packet that is destined to exit the MPLS domain, the LER strips off the label and forwards the resulting IP packet using normal IP forwarding rules.

====Provider router====
In the specific context of an MPLS-based [[virtual private network]] (VPN), LERs that function as [[ingress router|ingress]] or [[egress router]]s to the VPN are often called [[provider edge]] (PE) routers. Devices that function only as transit routers are similarly called [[Provider router|provider]] (P) routers.{{Ref RFC|4364}} The job of a P router is significantly easier than that of a PE router.