RFC 3137: Reachable Without Carrying Transit

Summary

RFC 3137 kept a router's LSA in the OSPF topology and preserved ordinary costs for its stub links, while setting non-stub link costs to the 16-bit maximum so redundant paths would carry transit traffic instead.
The state was a preference, not disappearance or proof of a completed drain. A router could remain reachable and still be unfit for transit; without an alternate path, later OSPF behavior could continue to use it.

Withdrawal was too blunt

Imagine a router whose control plane has reached its limit. Its CPU is saturated, or its memory can no longer hold every Link State Advertisement and construct a dependable routing table. An operator wants traffic that merely crosses the box to take another path. Yet the router's own addresses still matter: engineers may need to reach it, directly connected prefixes may remain valid, and the node has not necessarily failed.

The obvious action—make the router disappear from OSPF—solves the wrong problem. OSPF does not calculate routes from a list of independent destinations. It calculates a graph. In RFC 2328's shortest-path procedure, routers and transit networks form the first-stage graph; stub networks are added later as leaves. If router X flushes its router-LSA, the graph loses X's advertised links and the backlinks needed to calculate a path to X. The network stops using X for transit, but can also lose the very reachability the operator intended to preserve.

RFC 3137, published as an Informational memo in June 2001, defined a narrower act. Router X remained in the link-state database. Its LSA still described its adjacency surface. What changed was the price attached to different kinds of links.

The maximum metric expressed a middle state

The memo set the costs of all non-stub links in X's router-LSA to the 16-bit value 0xFFFF. It called that value LSInfinity, while carefully distinguishing it from the 24-bit all-ones value used in summary and AS-external LSAs. Stub links retained their interface output costs.

That asymmetry did the work. A non-stub link lets the shortest-path tree pass through a router or transit network. Giving those links the maximum cost made a redundant route around X cheaper. A type-3 stub link represented a directly connected prefix as a leaf. Leaving that cost intact let receivers calculate a route to the router's own surface even while they avoided crossing the router on the way somewhere else.

The resulting state was neither ordinary service nor withdrawal. The router continued to say, in effect, “I am here and these destinations are attached to me, but do not make me your bridge if another route exists.” Reachability and transit eligibility became separate claims in the same LSA.

This was useful for more than impending failure. RFC 3137 listed a critical condition, graceful introduction and removal, and other administrative or traffic-engineering reasons. A newly started router might need time before carrying the domain's transit load. A router approaching maintenance might need traffic shifted away before it was shut down. The memo supplied a protocol signal for the preference; it did not supply the operational evidence that the transition had finished.

Redundancy decided whether the request had force

The word “stub” can sound absolute. Here it was not. RFC 3137 emphasized that the technique produced real benefit in a network with enough redundancy to route around X. The metric did not create a second path. If X remained the only connection to destinations beyond it, behavior depended on how a receiver interpreted the maximum value.

That limit is central to the design. A high metric is an input to a local shortest-path calculation. It is not a command sent to a central controller, not an acknowledgement that traffic has moved, and not a physical block on forwarding. Each other router receives the flooded LSA, constructs its own topology and selects its own best path. Topology supplies the alternatives; the advertisement only changes their relative cost.

The operational evidence therefore has layers. Configuration can show that X was told to enter the special state. An originated LSA can show what X announced. Link-state databases can show which peers received it. SPF and forwarding-table observations can show what each receiver selected. Flow telemetry can show whether transit traffic actually left X. Service probes can show whether destinations remained usable. None of those receipts should silently stand in for the next.

Two generations of OSPF disagreed at the edge case

RFC 3137 also recorded a compatibility boundary. Routers following RFC 1247 discarded link records whose router-LSA cost was the maximum. Routers following RFC 1583 and later did not treat that metric as unreachable. When an alternate path existed, the difference was harmless for the intended preference: both generations chose the cheaper way around X.

When X was the only path, their answers diverged. The older calculation would not use X for transit. The later calculation could use the high-cost route because it was still the only candidate. The memo said that this mixed behavior did not create routing loops in the described case. It did not say that every metric transition, every implementation or every later operational sequence was universally free of transient loops.

The distinction matters because “infinity” was doing two jobs in contemporary text. It could look like an unreachable value, yet later OSPF treated the 16-bit maximum as a finite, least-preferred link cost. The technique relied on that difference to degrade gracefully when no alternative existed.

The 2013 revision named the value more honestly

RFC 6987 obsoleted RFC 3137 twelve years later. It retained the basic technique but introduced the fixed architectural name MaxLinkMetric for 0xFFFF, explicitly to remove confusion about the interpretation of LSInfinity. The new name made the mechanism easier to read: this was the largest link metric, not a universal assertion that the link could never be used.

The revision also applied the method to OSPFv2 and OSPFv3 and discussed OSPFv3's R-bit. Clearing the R-bit indicates that a router should not be used for transit. RFC 6987 left the choice to operators and exposed a decisive policy difference. MaxLinkMetric may still permit the only path through the router; clearing the R-bit consistently excludes it from transit. One option prefers continuity when alternatives vanish. The other enforces isolation more strictly.

Those additions belong to RFC 6987, not retroactively to the 2001 memo. They show how the original middle state survived while its semantics became more explicit.

Graceful restart wanted the opposite forwarding result

It is tempting to group every planned routing transition under “graceful operation.” The protocols ask different questions. RFC 3623's graceful OSPF restart tries to keep a router on the forwarding path while its routing software restarts, provided forwarding state survives, topology remains stable and neighbors can help. RFC 3137 asks other routers to prefer paths that do not cross the router.

One mechanism preserves transit through a temporarily blind control plane. The other suppresses transit through a node that remains visible. Confusing them can reverse the desired outcome.

Later MPLS and GMPLS work in RFC 5817 addressed planned shutdown of traffic-engineered links and nodes with its own signalling and path-selection surface. RFC 6976 addressed transient loops by ordering FIB updates during link-state convergence. Those mechanisms help locate the boundary of RFC 3137: a maximum-metric router-LSA was an OSPF preference signal, not a complete maintenance transaction, a TE-tunnel teardown receipt or a universal loop-free convergence proof.

Reachability was not health

The strongest historical idea in RFC 3137 is not the number 65535. It is the refusal to encode an operational transition as one Boolean fact. A router could be alive enough to advertise, reachable enough to manage and still unsuitable to carry other people's traffic. Conversely, a router could emit the desired LSA while its remote peers had not converged, its forwarding load had not drained, or its attached service had already degraded.

That separation assigns authority precisely. The originating router controls what it advertises. Receivers control how they calculate their paths. The physical topology controls whether alternatives exist. Operators control the maintenance decision. Measurements establish whether traffic moved and services survived. The RFC coordinates these actors without pretending that one artifact owns the whole result.

RFC 3137 did not claim originality, deployment or success. Its authors explicitly declined an originality claim. What the memo preserved was a durable design move: withdraw a role without withdrawing an identity. The router stayed in the graph so the network could still reach it. It raised the cost of being used as someone else's road. In that narrow gap between “present” and “transit,” OSPF gained a language for safer change.

The Router Stayed Reachable After It Stopped Carrying Transit: RFC 3137

Why it matters

Summary

Withdrawal was too blunt

The maximum metric expressed a middle state

Redundancy decided whether the request had force

Two generations of OSPF disagreed at the edge case

The 2013 revision named the value more honestly

Graceful restart wanted the opposite forwarding result

Reachability was not health

Sources

Deeper Profile Context

Strategic Circle

Leadership Alliance

Strategic Circle Briefing

Leadership Alliance Briefing