Label switched path
The Internet is based on the Internet protocol IP, so it is a packet-switched network. This means that data packets are routed individually through the network and it is quite possible that the packets belonging to a connection take different paths. This makes the behavior of the network less predictable than the behavior of a circuit-switched network such as the telephone network. The MPLS protocol allows virtual lines to be set up on a packet-switched network. In MPLS, such lines are called Label Switched Path (LSP).
An LSP is a path switched by means of MPLS through a routed IP network. The routers terminating the path are called label edge routers (LER), of which the start node of an LSP is also called the ingress router, the end point the egress router. These start and end nodes are typically located at the entry and exit points of an autonomous system (AS) (AS Boundary Router). While the label edge routers add or remove the MPLS labels from the data packets, the MPLS routers exchange the MPLS labels inside the LSP and are therefore called Label Switched Routers (LSR).
The LSPs can be switched completely manually, semi-automatically or fully automatically.
- The manual variant requires the configuration of every router that an LSP passes through, but becomes ineffective for autonomous systems on the order of several dozen routers.
- The semi-automatic variant only requires the manual configuration of parts of the LSP, for example the route via the first three routers. The rest of the pathfinding for the LSP is left to the IGP.
- The fully automatic variant relies entirely on a signaling protocol such as the Label Distribution Protocol (LDP), Resource Reservation Protocol (RSVP), RSVP-TE, Border Gateway Protocol (BGP) or CR-LDP when defining the path for an LSP .
Since the forwarding of data packets via LSPs is not transparent for the higher transport layers, an LSP is also referred to as an MPLS tunnel.
LSP knows various functions for error detection:
- An LSP ping checks whether packets of a Forwarding Equivalence Class (FEC) end their MPLS path on an LSR that this FEC uses as an output router. The well-known ICMP Echo Request / Reply serves as a model for this function
- With an LSP traceroute , an LSP traceroute package is sent to the control plane of each LSR of the LSP. It is examined whether the LSRs are actually transit LSRs for the path to be examined. Additional information provided by the LSRs can be used to determine, for example, whether the forwarding corresponds to the path defined by the routing protocols.