Label-Switching Routers

Routers that are part of the LSP are label-switching routers (LSRs). Each LSR must be configured with MPLS so that it can interpret MPLS headers and perform the MPLS operations required to pass traffic through the network. An LSP can include four types of LSRs:

• Inbound router—The only entry point for traffic into MPLS. Native IPv4 packets are encapsulated into the MPLS protocol by the inbound router. Each LSP can have only one inbound router.
• Transit router—Any router in the middle of an LSP. An individual LSP can contain between 0 and 253 transit routers. Transit routers forward MPLS traffic along the LSP, using only the MPLS header to determine how the packet is routed.
• Penultimate router—The second-to-last router in the LSP. The penultimate router in an LSP is responsible for stripping the MPLS header from the packet before forwarding it to the outbound router.
• Outbound router—The endpoint for the LSP. The outbound router receives MPLS packets from the penultimate router and performs an IP route lookup. The router then forwards the packet to the next hop of the route. Each LSP can have only one outbound router.

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