Label edge router
A label edge router (LER) is a specific instance of a label switch router that sits at the edge of a Multiprotocol Label Switching (MPLS) network. When the MPLS network is deployed by a common carrier, it is also known as the provider edge (PE) router. The LER plays a vital role in the MPLS network. It is the device with links that extend outside the MPLS network. It receives traffic on these links, applies its local intelligence to determine the appropriate forwarding equivalence class (FEC), and pushes the appropriate MPLS label (or tag) onto the transmission unit. This MPLS label identifies the label switch path (LSP) along which the now labeled transmission unit will flow. The LER then passes the labeled transmission unit onto the network the next label switch router (LSR) that is identified by the LSP over what ever layer 2 network infrastructure that connects them. In many instances the LER will actually push more than one label onto the transmission unit. This applies when a virtual private network (VPN) service is being provided by the MPLS network. For this case the first label pushed on is used to identify which VPN the transmission unit belongs and then the second label identifies the LSP to get to the exit of the MPLS network, the egress LER.
|<mp3>http://podcast.hill-vt.com/podsnacks/2007q4/ler_lsr.mp3%7Cdownload</mp3> | Label edge router vs. label switch router (LSR)|