What is Label Switching Router?
A label switching router (LSR) forms the core of a label switched network. Label-switched networks consist of predetermined paths, so-called Label-Switched-Paths (LSPs), which are the result of creating source-destination pairs using the process known as Multi-Protocol Label Switching (MPLS). Label switching routers support MPLS, which ensures that all packets transmitted on a specific route stay in the same path via a backbone.
LSRs can route data packets quickly without having to check tables or perform routing calculations that would increase the sending / receiving time of data. Since the labels already contain instructions for the path to the data, the router simply has to control the data using the instructions on the labels.
There are four different types of LSRs, differentiated according to their location and position in the LSPs. You are:
An ingress router is based on the start or entry point of an LSP. It is the only router where normal IP traffic can enter an MPLS path. Ingress routers use inbound routers, which receive information from the IP traffic, which then pass through the LSP to reach its destination. The inbound router uses encapsulation for traffic using an MPLS header.
A transit router is located in the middle of an LSP. In contrast to the ingress router, which uses incoming routers, transit routers switch the MPLS packets to the next path in the LSP. It uses the interface the packet came from and also the MPLS header for its destination information.
A penultimate router is at the penultimate stop in the LSP. The penultimate router is used to remove the MPLS header before it is passed to the last hop in the LSP. MPLS headers are no longer required because the last hop in an LSP does not have to forward the packets to another transit router.
An egress router is known as the starting point in the label switched router. It receives the IP traffic originating from the penultimate router, performs a standard IP lookup, and then sends the traffic via normal IP routing.