A list of functionality that the FIB does not currently provide.
PIC Edge Backup Paths
FIB supports the concept of path 'preference'. Only paths that have the best preference contribute to forwarding. Only once all the paths with the best preference go down do the paths with the next best preference contribute.
In BGP PIC edge, BGP would install the primary paths and the backup paths. With expectation that backups are only used once all primaries fail; this is the same behaviour that FIB's preference sets provide.
However, in order to get prefix independent convergence, one must be able to only modify the path-list's load-balance map (LBM) to choose the paths to use. Hence the paths must already be in the map, and conversely must be in the fib_entry's load-balance (LB). In other words, to use backup paths with PIC, the fib_entry's LB must include the backup paths, and the path-lists LBM must map from the backups to the primaries.
This is change that is reasonably easy w.r.t. to knowing what to change, but hard to get right and hard to test.
Loop Free Alternate Paths
Contrary to the BGP approach for path backups, an IGP could install a loop free alternate (LFA) path to achieve fast re-route (FRR).
Because of the way the LFA paths are calculated by the IGP an LFA backup path is always paired with a primary. VPP FIB does not support this primary-backup pair relationship.
In intent of LFA FRR is/was to get below the magic 50ms mark. To do this the expectation is/was that one would need in the forwarding graph an object that represents a path's state. This object would be checked for each packet being sent. If the path is up, the graph (an adjacency since it's the IGP) for the primary path is taken, if it's down the graph for the backup is taken. When a path goes down only this indirection object needs to be updated to affect all routes. Naturally, the indirection would incur a performance cost, but we know that there are many performance-convergence trade-offs in a FIB design.
Should VPP's FIB support this feature? It all depends on the 50ms. LFA FRR comes from the era when routers ran on lower performance CPUs and interface down was an interrupt. VPP typically has plenty of gas but runs as a user space process. So, can it update all routes in under 50ms on a meaty CPU and can the OS deliver the interface down within the time requirements? I don't have the answers to either question.
Extranets for Multicast
When a unicast prefix is present in two different tables, then it refers to a different set of devices. When the prefix is imported it refers to the same set of devices. If the set of paths to reach the prefix is different in the import and export table, it doesn't matter, since they both refer to the same devices, so either set can be used. Therefore, FIB's usual source preference rules can apply. The 'import' source is lower priority.
When a multicast prefix is present in two different tables, then it's two different flows referring to two different set of receivers. When the prefix is imported, then it refers to the same flow and two different sets of receivers. In other words, the receiver set in the import table needs to be the super set of receivers.
There are two ways one might consider doing this; merging the path-lists or replicating the packet first into each table.
Read Fast Convergence
Collapsing the DPO graph for recursive routes doesn't have to be an all or nothing. Easy cases:
A recursive prefix with only one path and a path-list that is not popular, could stack directly on the LB of the via entry.
A recursive prefix with only multiple paths and a path-list that is not popular, could construct a new load balance using the choices present in each bucket of its via entries. The choices in the new LB though would need to reflect the relative weighting.
The condition of an non-popular path-list means that the LB doesn't have an LB map and hence it needs to be updated for convergence to occur.
The more difficult cases come when the recursive prefix has labels which need to be stack on the via entries' choices.
You might also envision a global configuration that always collapses all chains, which could be used in deployments where convergence is not a priority.