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FIB Prefix Prioritization

FIB prefix prioritization

FIB prefix prioritization allows user-defined priorities to be assigned to routes when they are exported to the forwarding plane from the routing plane. Route priorities can be assigned in the routing plane, by way of IGP protocol import policies that assign priorities to routes. The user sets the relative priority high and low for prefixes via the existing protocol import policy – see Prefix Prioritization Overview. These route priorities are exported into the forwarding table.

However, there could be situations when there is a need to override the routing plane route prioritization, with user-defined route prioritization at the forwarding plane. FIB (forwarding information base) prefix prioritization allows this. In the forwarding table export policy, on matching routes, a route priority can be assigned.

FIB prefix priorities

  • High – Prefixes assigned this priority have the highest install priority. These routes are always given importance over others.

  • Medium – Prefixes assigned this priority have the second highest install priority.


Prefixes that are not assigned high or medium priorities are unprioritized.

Supported route types for FIB prefix prioritization in order of install preference

  • Interface/local routes – Given highest priority unconditionally and will bypass forwarding table export policy evaluation.

  • Host routes

  • IPv4 and IPv6 routes

  • MPLS – routes protization will be PROTO-based and not prefix-based.

FIB prefix prioritization workflow

The forward table reserves memory buffers for high and medium priority routes, based on user configuration – see fib-prioritization. Once this configuration is set, the number of lower priority routes that can be installed in the forwarding table is limited to the remaining space for these routes.

After setting the percentages for high and medium priority routes, the next step is to configure the forwarding table export policy. See fib-install-priority.

Once routes are installed in the forwarding table and have their route priorities assigned by the FIB prefix prioritization process, the routes can transition to other priorities as well. Because iterations of a forwarding table export policy might mark a previously unprioritized route as high priority for example, such transitions are managed in the forwarding table by the packet forwarding engine.