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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 1 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 1: CSPF Topology with Administrative Group Coloring
CSPF Topology with Administrative
Group Coloring

The network shown in Figure 1 is an MPLS router-only network with SONET interfaces.

The MPLS network shown in Figure 1 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 redlinks. 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 MPLS Applications User Guide.

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



Verify That the LSP Is Established

Purpose

Check that the LSP shown in CSPF Topology with Administrative Group Coloring is established and traversing the path from R1 to R6 through the red links.

Action

To verify that the LSP is established, enter the following Junos OS command-line interface (CLI) operational mode command:

Sample Output

Sample Output 2

Meaning

Sample Output 1 from ingress router R1 shows that LSP R1-to-R6 is successfully established as indicated by the Explicit Route Object (ERO) 10.1.15.2 S 10.1.56.2 S, the log message CSPF: computation result accepted, and State: Up. Also, the LSP is routing packets correctly over the red links, avoiding the blue links or the links without any coloring.

Sample Output 2 from transit router R5 shows that LSP R1-to-R6 is transiting R5 as expected.



Check the Administrative Group Configuration

Purpose

Check that the administrative group coloring is correct and the relevant interfaces are associated with each administrative group correctly

Action

To check the administrative group configuration, enter the following Junos OS CLI operational mode commands, or issue the show command at the [edit protocols mpls] hierarchy level, as shown in the example below:

Sample Output 1

Sample Output 2

Sample Output 3

Meaning

Sample Output 1 shows that administrative group coloring is correctly configured on all relevant routers. Administrative groups redand blue are configured at the [edit protocols mpls] hierarchy level, and relevant interfaces are associated with each administrative group correctly.

R3 is configured with blue coloring and the include and exclude statements are included in the configuration of R1 to ensure that LSP R1-to-R6 always transits R5. The inclusion of red coloring is not strictly necessary. To force the LSP to transit R5,you could color the links on R2 and R3 blue and then exclude the blue links. Red coloring is included in this example to demonstrate the fact that the CSPF algorithm excludes links that do not have a color configured, when the include statement is configured at the [edit protocols mpls] hierarchy level.

In addition, ingress router R1 has CSPF tracing configured in preparation for gathering information when the CSPF algorithm fails later in this example.

Sample Output 2 shows that the correct interfaces are associated with the red and blue administration groups on R1, R3, R5, and R6.

Sample Output 3 confirms that link coloring is correctly reported in the traffic engineering database for R1. Not shown is the traffic engineering database output for the remaining routers, which is similar to the R1 output, and correct.