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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).

Note

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.

Requirements

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

Overview

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. 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.

Configuration

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 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.

Results

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.

Note

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.

Verification

Confirm that the configuration is working properly.

Verifying SRLG Definitions

Purpose

Verify SRLG-to-value mappings and SRLG cost.

Action

user@PE1> show mpls srlg

Purpose

Verify the traffic engineering link SRLG association.

Action

user@PE1> show ted link detail

Meaning

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

Verify Standby Secondary Path

Purpose

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

Action

user@PE1> show mpls lsp ingress extensive

Meaning

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.

This example shows how to configure the exclude-srlg option to exclude Shared Risk Link Group (SRLG) links for the secondary label-switched path (LSP).

Requirements

This example uses the following hardware and software components:

  • M Series, MX Series, or T Series devices

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

Overview

For critical links where it is imperative to keep the secondary and primary paths completely disjoint from any common SRLG, you can optionally configure the exclude-srlg statement at the [edit protocols mpls] or [edit protocols mpls label-switched-path path-name] hierarchy levels. For logical systems, you configure the exclude-srlg statement at the edit logical-systems protocols mpls[edit logical-systems logical-system-name protocols mpls label-switched-path path-name] hierarchy level.

If exclude-srlg is configured, the Constrained Shortest Path First (CSPF) algorithm excludes any link belonging to the set of SRLGs in the primary path. If exclude-srlg is not configured, and if a link belongs to the set of SRLGs in the primary path, CSPF adds the SRLG cost to the metric, but still accepts the link for computing the path.

In this example, PE1 is the ingress router and PE2 is the egress router. P1, P2, and P3, P4, and P5 are transit routers. 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. Because exclude-srlg is configured, CSPF rejects link P2>PE2 as the link belongs to the SRLG srlg-a. Therefore, the computed standby secondary path is PE1>P3>P4>P5>PE2.

Configuration

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

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 CLI User Guide.

  1. Configure the device interfaces.
  2. Configure OSPF on the interfaces.
  3. Configure the SRLG definitions.
  4. Configure MPLS and the LSPs.
  5. Configure the exclude-srlg statement to forcibly keep the links for the secondary path completely disjoint from the primary LSP path.
  6. Enable RSVP on the interfaces.

Results

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.

Note

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.

Verification

Confirm that the configuration is working properly.

Purpose

Verify that the link for the secondary path is completely disjoint from the primary path.

Action

user@PE1> show mpls lsp detail

Meaning

Primary path includes SRLG srlg-a. For the standby secondary path, the link P2>PE2 belongs to SRLG srlg-a. CSPF rejects link P2>PE2 because the link belongs to the SRLG srlg-a.

This example shows how to configure SRLG with link protection without the exclude-srlg option.

Requirements

This example uses the following hardware and software components:

  • M Series, MX Series, or T Series devices

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

Overview

In this example, PE1 is the ingress router and PE2 is the egress router. P1, P2, and P3, P4, and P5 are transit routers. OSPF is configured on all the routers as the interior gateway protocol (IGP). SRLG is configured on all seven routers. The link P1>PE2 (primary path) and the link P2>PE2 belong to SRLG srlg-a.

You configure link protection for the interface P1>PE2 by including the link-protection statement.

When SRLG srlg-a is configured on the link P1>PE2 and P2>PE2, the bypass takes the longer path P1>P4>P5>PE2, not selecting the link P2>PE2 because of the added SRLG cost for srlg-a.

Configuration

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

Configuring Device PE1

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 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 and configure link protection for the pe1-pe2 LSP.
  5. Enable RSVP on the interfaces.

Results

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.

Configuring Device P1

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 CLI User Guide.

To configure device P1:

  1. Configure the device interfaces.
  2. Configure OSPF on the interfaces.
  3. Configure the SRLG definitions.
  4. Configure MPLS on the interfaces and associate the SRLG srlg-a with interface ge-0/0/2.0 for the P1>PE2 link.
  5. Enable RSVP on the interfaces and configure link-protection for interface ge-0/0/2.0.

Results

From configuration mode, confirm your configuration by entering the show interfaces, show protocols ospf, show protocols mpls, show protocols rsvp, and show routing-options 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.

Configuring Device P2

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 CLI User Guide.

To configure P2:

  1. Configure the device interfaces.
  2. Configure OSPF on the interfaces.
  3. Configure the SRLG definitions.
  4. Configure MPLS on the interfaces and associate the SRLG srlg-a with interface ge-0/0/2.0 for the P2>PE2 link.
  5. Enable RSVP on the interfaces.

Results

From configuration mode, confirm your configuration by entering the show interfaces, show protocols ospf, show protocols mpls, show protocols rsvp, and show routing-options 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.

Configuring Device P3

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 CLI User Guide.

To configure P3:

  1. Configure the device interfaces.
  2. Configure OSPF on the interfaces.
  3. Configure the SRLG definitions.
  4. Configure MPLS on the interfaces.
  5. Enable RSVP on the interfaces.

Results

From configuration mode, confirm your configuration by entering the show interfaces, show protocols ospf, show protocols mpls, show protocols rsvp, and show routing-options 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.

Configuring Device P4

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 CLI User Guide.

To configure P4:

  1. Configure the device interfaces.
  2. Configure OSPF on the interfaces.
  3. Configure the SRLG definitions.
  4. Configure MPLS on the interfaces.
  5. Enable RSVP on the interfaces.

Results

From configuration mode, confirm your configuration by entering the show interfaces, show protocols ospf, show protocols mpls, show protocols rsvp, and show routing-options 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.

Configuring Device P5

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 CLI User Guide.

To configure P5:

  1. Configure the device interfaces.
  2. Configure OSPF on the interfaces.
  3. Configure the SRLG definitions.
  4. Configure MPLS on the interfaces.
  5. Enable RSVP on the interfaces.

Results

From configuration mode, confirm your configuration by entering the show interfaces, show protocols ospf, show protocols mpls, show protocols rsvp, and show routing-options 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.

Configuring Device 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 CLI User Guide.

To configure PE2:

  1. Configure the device interfaces.
  2. Configure OSPF on the interfaces.
  3. Configure the SRLG definitions.
  4. Configure MPLS on the interfaces.
  5. Enable RSVP on the interfaces.

Results

From configuration mode, confirm your configuration by entering the show interfaces, show protocols ospf, show protocols mpls, show protocols rsvp, and show routing-options 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.

Verification

Confirm that the configuration is working properly.

Purpose

Verify that the SRLG cost is added to the TE link if it belongs to the SRLG of the protected link. Issue the show ted link detail and show rsvp session extensive bypass commands on device P1.

Action

user@P1> show ted link detail
user@P1> show rsvp session extensive bypass

Meaning

The shortest path for the bypass protecting the link P1->PE2 would have been P1->P2->PE2. Because the links P1>PE2 and P2>PE2 both belong to SRLG srlg-a, the SRLG cost of 10 for srlg-a is added to the metric for the link P2>PE2. This makes the metric for the link P2>PE2 too high to be selected for the shortest path. Therefore, the CSPF result for the computed path for the bypass becomes P1>P4>P5>PE2.

This example shows how to configure SRLG with link protection with the exclude-srlg option.

Requirements

This example uses the following hardware and software components:

  • M Series, MX Series, or T Series devices

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

Overview

In this example, PE1 is the ingress router and PE2 is the egress router. P1, P2, and P3, P4, and P5 are transit routers. OSPF is configured on all the routers as the interior gateway protocol (IGP). SRLG is configured on all seven routers. The link P1>PE2 (primary path) and the link P2>PE2 belong to SRLG srlg-a.

You configure link protection for the interface P1>PE2 by including the link-protection statement along with the exclude-srlg option. This makes the bypass LSP and the protected link completely disjoint in any SRLG.

When SRLG srlg-a is configured on the link P1>PE2 and P2>PE2, the link P2>PE2 is rejected for CSPF consideration due to the exclude-srlg configuration. Therefore, the computed path for the bypass becomes P1>P4>P5>PE2.

Configuration

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

Configuring Device PE1

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 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 and configure link protection for the pe1-pe2 LSP.
  5. Enable RSVP on the interfaces.

Results

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.

Configuring Device P1

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 CLI User Guide.

To configure device P1:

  1. Configure the device interfaces.
  2. Configure OSPF on the interfaces.
  3. Configure the SRLG definitions.
  4. Configure MPLS on the interfaces and associate the SRLG with interface ge-0/0/2.0 for the P1>PE2 link.
  5. Enable RSVP on the interfaces and include the link-protection statement with the exclude-srlg option for interface ge-0/0/2.0.

Results

From configuration mode, confirm your configuration by entering the show interfaces, show protocols ospf, show protocols mpls, show protocols rsvp, and show routing-options 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.

Configuring Device P2

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 CLI User Guide.

To configure P2:

  1. Configure the device interfaces.
  2. Configure OSPF on the interfaces.
  3. Configure the SRLG definitions.
  4. Configure MPLS on the interfaces and associate the SRLG with interface ge-0/0/2.0 for the P2>PE2 link.
  5. Enable RSVP on the interfaces.

Results

From configuration mode, confirm your configuration by entering the show interfaces, show protocols ospf, show protocols mpls, show protocols rsvp, and show routing-options 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.

Configuring Device P3

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 CLI User Guide.

To configure P3:

  1. Configure the device interfaces.
  2. Configure OSPF on the interfaces.
  3. Configure the SRLG definitions.
  4. Configure MPLS on the interfaces.
  5. Enable RSVP on the interfaces.

Results

From configuration mode, confirm your configuration by entering the show interfaces, show protocols ospf, show protocols mpls, show protocols rsvp, and show routing-options 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.

Configuring Device P4

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 CLI User Guide.

To configure P4:

  1. Configure the device interfaces.
  2. Configure OSPF on the interfaces.
  3. Configure the SRLG definitions.
  4. Configure MPLS on the interfaces.
  5. Enable RSVP on the interfaces.

Results

From configuration mode, confirm your configuration by entering the show interfaces, show protocols ospf, show protocols mpls, show protocols rsvp, and show routing-options 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.

Configuring Device P5

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 CLI User Guide.

To configure P5:

  1. Configure the device interfaces.
  2. Configure OSPF on the interfaces.
  3. Configure the SRLG definitions.
  4. Configure MPLS on the interfaces.
  5. Enable RSVP on the interfaces.

Results

From configuration mode, confirm your configuration by entering the show interfaces, show protocols ospf, show protocols mpls, show protocols rsvp, and show routing-options 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.

Configuring Device 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 CLI User Guide.

To configure PE2:

  1. Configure the device interfaces.
  2. Configure OSPF on the interfaces.
  3. Configure the SRLG definitions.
  4. Configure MPLS on the interfaces.
  5. Enable RSVP on the interfaces.

Results

From configuration mode, confirm your configuration by entering the show interfaces, show protocols ospf, show protocols mpls, show protocols rsvp, and show routing-options 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.

Verification

Confirm that the configuration is working properly.

Purpose

Verify that the TE link is excluded if it belongs to the SRLG of the protected link when link-protection is configured with exclude-srlg. Issue the show ted link detail and show rsvp session extensive bypass commands on device P1.

Action

user@P1> show ted link detail
user@P1> show rsvp session extensive bypass

Meaning

The shortest path for the bypass protecting the link P1>PE2 would have been P1>P2>PE2. Because the links P1>PE2 and P2>PE2 both belong to SRLG srlg-a, the link P2>PE2 is rejected for CSPF consideration due to the exclude-srlg constraint. Therefore, the computed path for the bypass becomes P1>P4>P5>PE2.

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