Flexible LSP Calculation and Configuration

Traffic engineering involves mapping traffic flow onto a physical topology. You can determine the paths online using constraint-based routing. Regardless of how the physical path is calculated, the forwarding state is installed across the network through RSVP.

The Junos OS supports the following ways to route and configure an LSP:

• You can calculate the full path for the LSP offline and individually configure each router in the LSP with the necessary static forwarding state. This is analogous to the way some Internet service providers (ISPs) configure their IP-over-ATM cores.
• You can calculate the full path for the LSP offline and statically configure the ingress router with the full path. The ingress router then uses RSVP as a dynamic signaling protocol to install a forwarding state in each router along the LSP.
• You can rely on constraint-based routing to perform dynamic online LSP calculation. You configure the constraints for each LSP; then the network itself determines the path that best meets those constraints. Specifically, the ingress router calculates the entire LSP based on the constraints and then initiates signaling across the network.
• You can calculate a partial path for an LSP offline and statically configure the ingress router with a subset of the routers in the path; then you can permit online calculation to determine the complete path.

  For example, consider a topology that includes two east-west paths across the United States: one in the north through Chicago and one in the south through Dallas. If you want to establish an LSP between a router in New York and one in San Francisco, you can configure the partial path for the LSP to include a single loose-routed hop of a router in Dallas. The result is an LSP routed along the southern path. The ingress router uses CSPF to compute the complete path and RSVP to install the forwarding state along the LSP.

• You can configure the ingress router with no constraints whatsoever. In this case, normal IGP shortest-path routing is used to determine the path of the LSP. This configuration does not provide any value in terms of traffic engineering. However, it is easy and might be useful in situations when services such as virtual private networks (VPNs) are needed.

In all these cases, you can specify any number of LSPs as backups for the primary LSP, thus allowing you to combine more than one configuration approach. For example, you might explicitly compute the primary path offline, set the secondary path to be constraint-based, and have the tertiary path be unconstrained. If a circuit on which the primary LSP is routed fails, the ingress router notices the outage from error notifications received from a downstream router or by the expiration of RSVP soft-state information. Then the router dynamically forwards traffic to a hot-standby LSP or calls on RSVP to create a forwarding state for a new backup LSP.