Label Switching Router (LSR)

A label switching router (LSR) makes up the core of a label-switched network. Label-switched networks are made up of predetermined paths, called label-switched paths, (LSPs) which are the result of establishing source-destination pairs by the process called Multi-Protocol Label Switching (MPLS). Label switching routers support MPLS, which ensures that all of the packets carried in a specific route will remain in the same path over a backbone.

LSRs can quickly route data packets without having to check tables or do routing calculations which would add to the send/receive time of data. Because the labels already have directions regarding the path that the data take, the router simply has to direct the data based on the instructions on the labels.

There are four different kinds of LSRs, differentiated by location and position in the LSPs. They are:

1. An ingress router is based at the beginning or entry point of an LSP. It is the only router where normal IP traffic can enter a MPLS path. Ingress routers make use of inbound routers, which receive information from the IP traffic which then goes through the LSP in order to reach its destination. The inbound router makes use of encapsulation for the traffic using an MPLS header.
2. A transit router is found in the middle of an LSP. Unlike the ingress router, which uses inbound routers, transit routers switch the MPLS packets to the next path in the LSP. It uses the interface from which the packet came from and also the MPLS header for its destination information.
3. A penultimate router is located at the second-to-last stop in the LSP. The penultimate router is used for removing the MPLS header before giving it to the last hop in the LSP. MPLS headers are no longer needed since the last hop in an LSP doesn’t have to switch packets forward to another transit router.
4. An egress router is known as the exit point in the label-switched router. It receives the IP traffic that exited from the penultimate router and does a standard IP look-up, then sends the traffic using a normal IP routing.