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Multiprotocol Label Switching
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====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.
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