Host Router Support for OSPFv2
updates
- rfc-6987 — OSPF Stub Router Advertisement
Extracted elements (22)
Implementations are RECOMMENDED to provide a configuration parameter to manually override H-bit enforcement in partial deployments where topology guarantees that non-H-bit routers do not create routing loops, mitigating transient loop risk during incremental rollout.
OSPFv3 already defines the R-bit in router-LSAs for similar non-transit functionality; the H-bit provides analogous capability for OSPFv2, filling a gap in the OSPFv2 protocol that OSPFv3 had already addressed.
RFC 6987's approach of advertising MaxLinkMetric is insufficient to repel AS-external or NSSA routes with Type 2 metrics because Type 2 metrics are taken as more significant than intra-area or inter-area path costs, necessitating the use of LSInfinity instead.
Use cases motivating the H-bit include graceful router isolation, closet switches participating in topology without carrying transit, overloaded routers temporarily repelling traffic, and BGP route reflectors (e.g., virtual RRs) that participate in OSPF for SPF computation but cannot forward transit traffic.
During partial deployment, if a new router without H-bit support joins an area of H-bit-capable routers, RI LSA propagation delay can cause temporary transient routing loops while routers detect and react to the non-capable node.
If the H-bit is not set in a router-LSA, backward compatibility is achieved: behavior is identical to RFC 2328, allowing routers without H-bit support to coexist without disruption.
A router advertising a router-LSA with the H-bit set MUST NOT be used as a transit router by other OSPFv2 routers in the area that support the H-bit functionality.
All routers supporting the H-bit MUST check the RI LSAs of all nodes in the area. If any router does not advertise the OSPF Host Router capability, the SPF modifications described in Section 4 MUST NOT be used in the area.
All routers with the H-bit set MUST advertise all non-stub links with a metric equal to MaxLinkMetric in their LSAs so that routers not supporting H-bit will still avoid using those links for transit (unless last-resort).
An OSPFv2 router originating a router-LSA with the H-bit set MUST advertise all its non-stub links with a link cost of MaxLinkMetric (per RFC 6987).
H-bit functionality and its associated SPF modifications MUST only take effect if all routers in a given OSPF area support this functionality, verified via RI LSA capability advertisements.
The Router Information (RI) LSA advertising OSPF Host Router capability MUST be area-scoped, ensuring autodiscovery is limited to the local area.
This document updates RFC 6987 so that self-originated AS-external-LSAs or NSSA-LSAs advertise the Type 2 metric as LSInfinity-1 in overload scenarios; if the H-bit is set, the Type 2 metric MUST be set to LSInfinity.
When the H-bit is set, an ABR MUST advertise the same H-bit setting in its self-originated router-LSAs for all attached areas, and MUST only advertise IPv4 prefixes associated with its local interfaces in summary-LSAs.
When the H-bit is set, non-local IPv4 prefixes (e.g., redistributed from other protocols) SHOULD NOT be advertised in AS-external-LSAs. If they are advertised, the Type 2 metric MUST be set to LSInfinity to repel traffic.
The H-bit (Host-bit) is the high-order bit (bit H) of the OSPF router-LSA flags byte. When set, it indicates the router is a non-transit host router incapable of forwarding transit traffic.
IANA registered value 0x80 with description 'Host (H-bit)' in the 'OSPFv2 Router Properties Registry', and bit number 7 with name 'OSPF Host Router' in the 'OSPF Router Informational Capability Bits' registry.
A rogue router on the only transit path that sets the H-bit can effectively partition the network; this scenario is indistinguishable from an operator intentionally isolating a router for maintenance, and OSPF will continue to function within the partitioned domains.
A rogue router sending the H-bit set without matching MaxLinkMetric cost could cause routing loops; however, the MaxLinkMetric advertisement by legitimate H-bit routers mitigates the risk by making the rogue router unattractive for transit regardless.
The H-bit introduces advertisement of host router capability to all OSPFv2 routers in an area. Since H-bit restricts router behavior to a subset of normal router operations, it does not introduce new security considerations beyond those already known in OSPFv2 (RFC 2328).
SPF Step (2) is modified: when examining a router-LSA for vertex V, if the H-bit is set and V is not the SPF root, the router is excluded from transit and Step (3) executes immediately, skipping further link examination for transit purposes.
The H-bit occupies the most significant bit (bit 0) of the 8-bit OSPF Router-LSA flags field, arranged as H|0|0|N|W|V|E|B. It is carried in the router-LSA header alongside existing flag bits.