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Shared Risk Link Groups for MPLS

SRLG Overview

In MPLS traffic engineering, a Shared Risk Link Group (SRLG) is a set of links sharing a common resource, which affects all links in the set if the common resource fails. These links share the same risk of failure and are therefore considered to belong to the same SRLG. For example, links sharing a common fiber are said to be in the same SRLG because a fault with the fiber might cause all links in the group to fail.

An SRLG is represented by a 32-bit number unique within an IGP (OSPFv2 and IS-IS) domain. A link might belong to multiple SRLGs. The SRLG of a path in a label-switched path (LSP) is the set of SRLGs for all the links in the path. When computing the secondary path for an LSP, it is preferable to find a path such that the secondary and primary paths do not have any links in common in case the SRLGs for the primary and secondary paths are disjoint. This ensures that a single point of failure on a particular link does not bring down both the primary and secondary paths in the LSP.

When the SRLG is configured, the device uses the Constrained Shortest Path First (CSPF) algorithm and tries to keep the links used for the primary and secondary paths mutually exclusive. If the primary path goes down, the CSPF algorithm computes the secondary path by trying to avoid links that share any SRLG with the primary path. In addition, when computing the path for a bypass LSP, CSPF tries to avoid links that share any SRLG with the protected links.

When the SRLG is not configured, CSPF only takes into account the costs of the links when computing the secondary path.

Any change in link SRLG information triggers the IGP to send LSP updates for the new link SRLG information. CSPF recomputes the paths during the next round of reoptimization.

Junos OS Release 11.4 and later supports SRLG based on the following RFCs:

  • RFC 4203, OSPF Extensions in Support of Generalized Multi-Protocol Label Switching (GMPLS).

  • RFC 5307, IS-IS Extensions in Support of Generalized Multi-Protocol Label Switching (GMPLS).


Currently, the “Fate Sharing” feature continues to be supported with the SRLG feature.

Example: Configuring SRLG

This example shows how to configure Shared Risk Link Groups (SRLGs) on a device.


This example uses the following hardware and software components:

  • Seven routers that can be a combination of M Series, MX Series, or T Series routers

  • Junos OS Release 11.4 or later running on all the devices


Junos OS Release 11.4 and later support SRLG configuration in an IGP (OSPFv2 and IS-IS) domain. In this example, you configure SRLG and associate it with the MPLS interface on a device.

The device uses the SRLG cost parameter for the Constrained Shortest Path First (CSPF) algorithm and tries to keep the links used for the primary and secondary paths mutually exclusive by avoiding links that share any SRLG with the primary path.

To configure the SRLG, you first define the SRLG parameters at the [edit routing-options srlg srlg-name] hierarchy level and then associate the SRLG with an MPLS interface at the [edit mpls interface interface-name] hierarchy level.

The srlg srlg-name statement has the following options:

  • srlg-cost—Include a cost for the SRLG ranging from 1 through 65535. The cost of the SRLG determines the level of impact this SRLG has on the CSPF algorithm for path computations. The higher the cost, the less likely it is for a secondary path to share the same SRLG as the primary path. By default, the srlg-cost is 1.

  • srlg-value—Include a group ID for the SRLG ranging from 1 through 4294967295.

In this example:

  • PE1 is the ingress router and PE2 is the egress router.

  • P1, P2, and P3, P4, and P5 are transit routers.

    • P1 has direct primary path connections to both the PE1 ingress and PE2 egress routers.

    • P2 has direct secondary path connections to PE1 and PE2.

    • P3 has a direct secondary path connection to PE1, and an indirect secondary path through P4 and P5 to PE2.

    • P4 has indirect secondary paths to PE1 through P3 and to PE2 through P5.

    • P5 has an indirect path through P4 and P3 to PE1 and a direct secondary path to PE2.

OSPF is configured on all the routers as the interior gateway protocol (IGP). SRLG is configured on all seven routers. The primary path includes SRLG srlg-a. For the standby secondary path, the link P2>PE2 belongs to SRLG srlg-a. The effective link metric, with the added srlg-cost of 10, becomes 11. Therefore, the computed secondary path is PE1>P3>P4>P5>PE2 with a CSPF link metric of 4.



CLI Quick Configuration

To quickly configure this section of the example, copy the following commands, paste them into a text file, remove any line breaks, change any details necessary to match your network configuration, and then copy and paste the commands into the CLI at the [edit] hierarchy level.

Router PE1

Router P1

Router P2

Router P3

Router P4

Router P5

Router PE2


Step-by-Step Procedure

The following example requires you to navigate various levels in the configuration hierarchy. For information about navigating the CLI, see the Junos OS CLI User Guide.

To configure the ingress router PE1:

  1. Configure the device interfaces.

  2. Configure OSPF on the interfaces.

  3. Configure the SRLG definitions.

  4. Configure MPLS and the LSPs.

  5. Enable RSVP on the interfaces.


From configuration mode, confirm your configuration by entering the show interfaces, show protocols ospf, show routing-options, show protocols mpls, and show protocols rsvp commands. If the output does not display the intended configuration, repeat the instructions in this example to correct the configuration.

If you are done configuring the device, enter commit from configuration mode.


Repeat this procedure for every Juniper Networks router in the IGP domain, after modifying the appropriate interface names, addresses, and any other parameters for each router.


Confirm that the configuration is working properly.

Verifying SRLG Definitions


Verify SRLG-to-value mappings and SRLG cost.


Verify TE-Link SRLG


Verify the traffic engineering link SRLG association.


Links P1-PE2 and P2-PE2 are associated with SRLG srlg-a.

Verify Standby Secondary Path


Check the SRLG link cost and its impact on the CSPF computation of the standby secondary path link.


Check the standby secondary path. The effective link cost for P2>PE2 is 11 (with the added srlg-cost of 10). CSPF computes the secondary path as PE1>P3>P4>P5>PE2 with a CSPF link metric of 4.