Disable Constrained Path LSP Computation

If the IGP is a link state protocol and if it supports extensions that allow the current bandwidth reservation on each router's link to be reported, constrained path LSPs are computed by default.

The JUNOS implementations of IS-IS and OSPF include the extensions that support constrained-path LSP computation. In IS-IS, these extensions are enabled by default.
(To disable this support, include the disable statement at the [edit protocols isis traffic-engineering] hierarchy level, as discussed in the JUNOS Internet Software Configuration Guide: Routing and Routing Protocols. In OSPF, these extensions are disabled by default. To enable this support, include the traffic-engineering statement in the configurations of all routers running OSPF, as described in the JUNOS Internet Software Configuration Guide: Routing and Routing Protocols.)

If IS-IS is enabled on a router or if you enable OSPF traffic engineering extensions, MPLS performs the constrained-path LSP computation by default.

Constrained-path LSP computation works as follows: LSPs advertise their link information in the IGP's link-state packets. These packets are flooded throughout the network and hence provide information to all nodes. This link information is placed into the traffic engineering database (TED) and provides each ingress router with LSP topology information and recent LSP bandwidth reservation information. When computing complete paths for LSPs, the ingress router uses the information in the TED, along with the requirements you configure for the LSP, including bandwidth (configured with the bandwidth statement), hop limit (configured with the hop-limit statement), and the address of the egress router (configured with the to statement).

Constrained-path LSPs have a greater chance of being established quickly and successfully for several reasons:

• The LSP computation takes into account the current bandwidth reservation.
• Constrained-path LSPs reroute themselves away from node failures and congestion.

When constrained-path LSP computation is enabled, you can configure the LSP so that it is periodically reoptimized, as described in Optimize Signaled LSPs.

When an LSP is being established or when an existing LSP fails, the constrained-path LSP computation is repeated periodically at the interval specified by the retry timer until the LSP is set up successfully. Once the LSP is set up, no recomputation is done. For more information about the retry timer, see Configure Path Connection Retry Information.

By default, constrained-path LSP computation is enabled. You might want to disable constrained-path LSP computation when all nodes do not support the necessary traffic engineering extensions. To disable constrained-path LSP computation, include the no-cspf statement at the [edit protocols mpls label-switched-path lsp-path-name ] or [edit protocols mpls label-switched-path lsp-path-name (primary | secondary)] hierarchy level:

no-cspf;