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LSP Overview

How a Packet Travels Along an LSP

When an IP packet enters an LSP, the ingress router examines the packet and assigns it a label based on its destination, placing the label in the packet’s header. The label transforms the packet from one that is forwarded based on its IP routing information to one that is forwarded based on information associated with the label.

The packet is then forwarded to the next router in the LSP. This router and all subsequent routers in the LSP do not examine any of the IP routing information in the labeled packet. Rather, they use the label to look up information in their label forwarding table. They then replace the old label with a new label and forward the packet to the next router in the path.

When the packet reaches the egress router, the label is removed, and the packet again becomes a native IP packet and is again forwarded based on its IP routing information.

Types of LSPs

There are three types of LSPs:

  • Static LSPs—For static paths, you must manually assign labels on all routers involved (ingress, transit, and egress). No signaling protocol is needed. This procedure is similar to configuring static routes on individual routers. Like static routes, there is no error reporting, liveliness detection, or statistics reporting.

  • LDP-signaled LSPs—See LDP Introduction.

  • RSVP-signaled LSPs—For signaled paths, RSVP is used to set up the path and dynamically assign labels. (RSVP signaling messages are used to set up signaled paths.) You configure only the ingress router. The transit and egress routers accept signaling information from the ingress router, and they set up and maintain the LSP cooperatively. Any errors encountered while establishing an LSP are reported to the ingress router for diagnostics. For signaled LSPs to work, a version of RSVP that supports tunnel extensions must be enabled on all routers.

There are two types of RSVP-signaled LSPs:

  • Explicit-path LSPs—All intermediate hops of the LSP are manually configured. The intermediate hops can be strict, loose, or any combination of the two. Explicit path LSPs provide you with complete control over how the path is set up. They are similar to static LSPs but require much less configuration.

  • Constrained-path LSPs—The intermediate hops of the LSP are automatically computed by the software. The computation takes into account information provided by the topology information from the IS-IS or OSPF link-state routing protocol, the current network resource utilization determined by RSVP, and the resource requirements and constraints of the LSP. For signaled constrained-path LSPs to work, either the IS-IS or OSPF protocol and the IS-IS or OSPF traffic engineering extensions must be enabled on all routers.

Scope of LSPs

For constrained-path LSPs, the LSP computation is confined to one IGP domain, and cannot cross any AS boundary. This prevents an AS from extending its IGP into another AS.

Explicit-path LSPs, however, can cross as many AS boundaries as necessary. Because intermediate hops are manually specified, the LSP does not depend on the IGP topology or a local forwarding table.