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Configuring Backup SPF Options for Protected OSPF Interfaces

By default, if at least one OSPF interface is configured for link-protection or node-link protection, the JUNOS Software calculates backup next hops for all the topologies in an OSPF instance. You can configure the following backup shortest-path-first (SPF) options to override the default behavior:

  • Disable the calculation of backup next hops for an OSPF instance or a specific topology in an instance.
  • Prevent the installation of backup next hops in the routing table or the forwarding table for an OSPF instance or a specific topology in an instance.
  • Limit the calculation of backup next hops to a subset of paths as defined in RFC 5286, Basic Specification for IP Fast Reroute: Loop-Free Alternates

You can disable the backup SPF algorithm for an OSPF instance or specific topology in an instance. Doing so prevents the calculation of backup next hops for that OSPF instance or topology.

To disable the calculation of backup next hops for an OSPF instance or topology:

  • Include the disable statement at the [edit protocols (ospf | ospf3) backup-spf-options] or [edit protocols ospf backup-spf-options topology topology-name] hierarchy level.

In the following example, the calculation of backup next hops is disabled for the OSPF topology voice.

[edit]protocols {ospf {topology voice {backup-spf-options {disable;}}}}

You can configure the router to prevent the installation of backup next hops in the routing table or the forwarding table for an OSPF instance or a specific topology in an OSPF instance. The SPF algorithm continues to calculate backup next hops, but they are not installed.

To prevent the router from installing backup next hops in the routing table or the forwarding table:

  • Include the no-install statement at the [edit protocols (ospf | ospf3) backup-spf-options] or the [edit protocols ospf topology topology-name] hierarchy level.

In the following example, backup next hops for the OSPF topology voice are not installed in the routing table or forwarding table. Any calculated backup next hops for other OSPF instances or topologies continued to be installed.

[edit]protocols {ospf {topology voice {backup-spf-options {no-install;}}}}

You can limit the calculation of backup next hops to downstream paths, as defined in RFC 5286, Basic Specification for IP Fast Reroute: Loop-Free Alternates. You can specify for the JUNOS Software to use only downstream paths as backup next hops for protected interfaces for an OSPF instance or a specific topology in an OSPF instance. In a downstream path, the distance from the backup neighbor to the destination must be smaller than the distance from the calculating router to the destination. Using only downstream paths as loop-free alternate paths for protected interfaces ensures that these paths not result in microloops. However, you might experience less than optimal backup coverage for your network.

To limit the calculation of backup next hops to downstream paths:

  • Include the downstream-paths-only statement at the [edit protocols (ospf | ospf3) backup-spf-options] or [edit protocols ospf backup-spf-options topology topology-name] hierarchy level.

In the following example, only downstream paths are calculated as backup next hops for the topology voice.

[edit]protocols {ospf {topology voice {backup-spf-options {downstream-paths-only;}}}}

Published: 2010-04-14

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