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Configuring Backup Selection Policy for the IS-IS Protocol

Support for IS-IS loop-free alternate (LFA) routes essentially adds IP fast-reroute capability for IS-IS. Junos OS precomputes multiple loop-free backup routes for all IS-IS routes. These backup routes are pre-installed in the Packet Forwarding Engine, which performs a local repair and implements the backup path when the link for a primary next hop for a particular route is no longer available. The selection of LFA is done randomly by selecting any matching LFA to progress to the given destination. This does not ensure best backup coverage available for the network. In order to choose the best LFA, Junos OS allows you to configure network-wide backup selection policies for each destination (IPv4 and IPv6) and a primary next-hop interface. These policies are evaluated based on admin-group, srlg, bandwidth, protection-type, metric, and neighbor information.

Before you begin to configure the backup selection policy for the IS-IS protocol:

  • Configure the router interfaces. See the Junos OS Network Management Administration Guide for Routing Devices

  • Configure an interior gateway protocol or static routing. See the Junos OS Routing Protocols Library for Routing Devices

To configure the backup selection policy for the IS-IS protocol:

  1. Configure per-packet load balancing.
  2. Enable RSVP on all the interfaces.
  3. Configure administrative groups.
  4. Configure srlg values.
  5. Enable MPLS on all the interfaces.
  6. Configure srlg on the interfaces.
  7. Configure the administrative groups on the interfaces.
  8. Enable link protection and configure the metric value on all the interfaces.
  9. Apply the routing policy to all equal cost multipaths exported from the routing table to the forwarding table.
  10. Configure the administrative group of the backup selection policy for an IP address. You can choose to exclude, include all, include any, or prefer the administrative groups from the backup path.

    • Specify the administrative group to be excluded.

      The backup path is not selected as the loop-free alternate (LFA) or backup nexthop if any of the links in the path have any one of the listed administrative groups.

      For example, to exclude the group c1 from the administrative group:

    • Configure all the administrative groups if each link in the backup path requires all the listed administrative groups in order to accept the path.

      For example, to set all the administrative groups if each link requires all the listed administrative groups in order to accept the path:

    • Configure any administrative group if each link in the backup path requires at least one of the listed administrative groups in order to select the path.

      For example, to set any administrative group if each link in the backup path requires at least one of the listed administrative groups in order to select the path:

    • Define an ordered set of administrative group that specifies the preference of the backup path.

      The leftmost element in the set is given the highest preference.

      For example, to set an ordered set of administrative group that specifies the preference of the backup path:

  11. Configure the backup path to allow the selection of the backup next hop only if the bandwidth is greater than or equal to the bandwidth of the primary next hop.
  12. Configure the backup path to specify the metric from the one-hop neighbor or from the remote router such as an RSVP backup label-switched-path (LSP) tail-end router to the final destination. The destination metric can be either highest or lowest.
    • Configure the backup path that has the highest destination metric.

    • Configure the backup path that has the lowest destination metric.

  13. Configure the backup path that is a downstream path to the destination.
  14. Set the order of preference of the root and the destination metric during backup path selection. The preference order can be :
    • [root dest] — Backup path selection or preference is first based on the root-metric criteria. If the criteria of all the root-metric is the same, then the selection or preference is based on the dest-metric.

    • [dest root] — Backup path selection or preference is first based on the dest-metric criteria. If the criteria of all the dest-metric is the same, then the selection is based on the root-metric.

  15. Configure the backup path to define a list of loop-back IP addresses of the adjacent neighbors to either exclude or prefer in the backup path selection. The neighbor can be a local (adjacent router) neighbor, remote neighbor, or any other router in the backup path.
    • Configure the list of neighbors to be excluded.

      The backup path that has a router from the list is not selected as the loop-free alternative or backup next hop.

    • Configure an ordered set of neighbors to be preferred.

      The backup path having the leftmost neighbor is selected.

  16. Define the backup path per-neighbor policy, to either exclude or prefer a backup path.
    • Configure to not select the backup path as the loop-free alternative or backup-next hop if any node or router with route-tag is present in the path.

      For example, to not select the backup path as the loop-free alternative or backup-next hop if any node or router with 1004 route-tag is present in the path:

    • Configure the set of route tags in descending order of preference.

      For example, to configure the set of route tags in descending order of preference:

  17. Configure the backup path to specify the required protection type of the backup path to be link, node, or node-link.
    • Select the backup path that provides link protection.

    • Select the backup path that provides node protection.

    • Select the backup path that allows either node or link protection LFA where node-protection LFA is preferred over link-protection LFA.

  18. Specify the metric to the one-hop neighbor or to the remote router such as an RSVP backup label-switched-path (LSP) tail-end router.
    • Select the path with highest root metric.

    • Select the path with lowest root metric.

  19. Configure the backup selection path to either allow or reject the common shared risk link groups (SRLGs) between the primary link and each link in the backup path.
    • Configure the backup path to allow common srlgs between the primary link and each link in the backup path.

      A backup path with a fewer number of srlg collisions is preferred.

    • Configure the backup path to reject the backup path that has common srlgs between the primary link and each link in the backup path.

  20. Configure the backup path to control the order and the criteria of evaluating the backup path based on the administrative group, srlg, bandwidth, protection type, neighbor, neighbor-tag, and metric. The default order of evaluation is admin-group, srlg, bandwidth, protection-type, neighbor, neighbor-tag, and metric.