Case Study for a CSPF Failure

Purpose

This case study presents a Multiprotocol Label Switching (MPLS) network topology and CSPF failure scenario designed to demonstrate techniques and commands that are particularly useful when addressing CSPF problems in your network. The focus of the study is the incorrect association of user-provided constraints, specifically administrative groups (also known as link coloring).

When calculating a path, the CSPF algorithm factors in user-provided constraints. The ingress router determines the physical path for each LSP by applying a CSPF algorithm to the information in the traffic engineering database. CSPF is a shortest-path-first (SPF) algorithm that has been modified to take into account constraints when calculating the shortest path across the network. Links that do not comply with the restrictions are removed from the tree and cannot be factored into the resulting SPF calculations.

CSPF integrates topology link-state information that is learned from interior gateway protocol (IGP) traffic engineering extensions and is maintained in the traffic engineering database. The information stored in the traffic engineering database includes attributes associated with the state of network resources.

The network topology shown in Figure 4 illustrates a network in which the LSP is constrained by administrative group coloring (also known as link coloring), and CSPF tracing is configured on the ingress router RI. In this example, the LSP is forced to transit R5 in accordance with the restrictions imposed.

Figure 4: CSPF Topology with Administrative Group Coloring

The network shown in Figure 4 is an MPLS router-only network with SONET interfaces. For more details about the MPLS network topology, see Configuring CSPF Tracing.

The MPLS network shown in Figure 4 is configured with administrative group coloring as follows:

• The LSP R1-to-R6 is established with R1 as the ingress router and R6 as the egress router.
• The required path to R6 transits R5 on the red links. The inclusion of red coloring is not strictly necessary. To force the LSP to transit R5, you could color the links on R3 and R2 blue and then exclude the blue links.
• Both red and blue colors are used with the include and exclude statements to ensure that the LSP always transits R5. For information on configuring administrative group coloring, see the JUNOS MPLS Applications Configuration Guide.

Steps To Take

To check that the network is configured correctly and the LSP is established, follow these steps:

1. Verify That the LSP Is Established
2. Check the Administrative Group Configuration