Recursive Route Calculator
Trace recursive next-hop resolution depth, its control-plane cost and the platform recursion limit.
Inputs
Control-Plane Resolution Time
8.000µs
Lookups to Reach an Adjacency
4
Data-Plane Lookups per Packet
1
Added Reconvergence Time
60ms
Resolution Status
Resolves — 3 level(s) of recursion reach a directly connected adjacency within the platform limit of 3
Step by step
Values used
Recursion levels to resolve = 3 levels; Platform recursion limit = 3 levels; RIB lookup cost per level = 2 µs; Reconvergence cost per level = 20 ms
Recursive Route
Resolution walks one RIB lookup per recursion level plus a final lookup that lands on a connected adjacency, and the result is flattened into a single FIB entry.
Reconvergence impact
Reconvergence cost ≈ levels × per-level resolution time, because each level must re-resolve in turn.
Control-Plane Resolution Time
= 8.000 µs
Lookups to Reach an Adjacency
= 4
Data-Plane Lookups per Packet
= 1
Added Reconvergence Time
= 60 ms
Resolution Status
= Resolves — 3 level(s) of recursion reach a directly connected adjacency within the platform limit of 3
How it works
Recursion is a control-plane concept only: the RIB walks the chain until it reaches an interface, then writes one flattened FIB entry, so the data plane still does a single lookup. The cost appears during convergence, when every level in the chain has to be re-resolved. This is the mechanism behind BGP next-hop tracking — a BGP route recursing through an IGP route through a connected interface — and an unresolved next hop is the most common reason a route is in the BGP table but never installed.
Formulas
Recursive Route
Resolution walks one RIB lookup per recursion level plus a final lookup that lands on a connected adjacency, and the result is flattened into a single FIB entry.
- recursion
- A route whose next hop is itself learned from another route rather than a connected interface
- FIB flattening
- The resolved outgoing interface and rewrite are stored once, so forwarding is a single lookup
- limit
- Platform cap on recursion depth, commonly 3 levels
Reconvergence impact
Reconvergence cost ≈ levels × per-level resolution time, because each level must re-resolve in turn.
Frequently Asked Questions
How is Recursive Route calculated?
Resolution walks one RIB lookup per recursion level plus a final lookup that lands on a connected adjacency, and the result is flattened into a single FIB entry. Recursion is a control-plane concept only: the RIB walks the chain until it reaches an interface, then writes one flattened FIB entry, so the data plane still does a single lookup. The cost appears during convergence, when every level in the chain has to be re-resolved.
Why does Recursive Route matter?
This is the mechanism behind BGP next-hop tracking — a BGP route recursing through an IGP route through a connected interface — and an unresolved next hop is the most common reason a route is in the BGP table but never installed.
What values do I need to enter?
This calculator takes 4 inputs: Recursion levels to resolve, Platform recursion limit, RIB lookup cost per level, Reconvergence cost per level. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.
Why is my BGP route inactive with a valid next hop?
The next hop is probably not resolvable: it may only be covered by the default route, which most implementations refuse to recurse through for BGP. Advertise the next hop into the IGP or use a static route to it.