Internet Engineering Task Force (IETF) K. Patel
Request for Comments: 8770
Category: Standards Track PPE Consulting
ISSN: 2070-1721 M. Bhardwaj
Host Router Support for OSPFv2
The Open Shortest Path First Version 2 (OSPFv2) protocol does not
have a mechanism for a node to repel transit traffic if it is on the
shortest path. This document defines a bit called the Host-bit
(H-bit). This bit enables a router to advertise that it is a non-
transit router. This document also describes the changes needed to
support the H-bit in the domain. In addition, this document updates RFC 6987
to advertise Type 2 External and Not-So-Stubby Area (NSSA)
Link State Advertisements (LSAs) (RFC 3101
) with a high cost in order
to repel traffic effectively.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at https://www.rfc-editor.org/info/rfc8770
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Table of Contents 1.
Requirements Language 3.
Host-Bit Support 4.
SPF Modifications 5.
Autodiscovery and Backward Compatibility 6.
OSPF AS-External-LSAs / NSSA-LSAs with Type 2 Metrics 7.
IANA Considerations 8.
Security Considerations 9.
Normative References 9.2.
The OSPFv2 protocol specifies a Shortest Path First (SPF) algorithm
that identifies transit vertices based on their adjacencies.
Therefore, OSPFv2 does not have a mechanism to prevent traffic
transiting a participating node if it is a transit vertex in the only
existing or shortest path to the destination. The use of metrics to
make the node undesirable can help to repel traffic only if an
alternative better route exists.
A mechanism to move traffic away from the shortest path is
particularly useful for a number of use cases: 1.
Graceful isolation of a router, to avoid blackhole scenarios when
there is a reload and possible long reconvergence times.
2. Closet switches that are not usually used for transit traffic but
need to participate in the topology.
3. Overloaded routers that could use such a capability to 4.
temporarily repel traffic until they stabilize.
BGP route reflectors, known as virtual Route Reflectors, that are
not in the forwarding path but are in central locations such as
data centers. Such route reflectors are typically used for route
distribution and are not capable of forwarding transit traffic.
However, they need to learn the OSPF topology to perform SPF
computation for optimal routes and reachability resolution for
their clients [BGP-ORR].
This document describes the functionality provided by the Host-bit
(H-bit); this functionality prevents other OSPFv2 routers from using
the host router by excluding it in path calculations for transit
traffic in OSPFv2 routing domains. If the H-bit is set, then the
calculation of the shortest-path tree for an area, as described in
Section 16.1 of [RFC2328
], is modified by including a check to verify
that transit vertices DO NOT have the H-bit set (see Section 4
Furthermore, in order to repel traffic effectively, this document
] so that Type 2 External and Not-So-Stubby Area
(NSSA) Link State Advertisements (LSAs) [RFC3101
] are advertised with
a high cost (see Section 6
). OSPFv3 [RFC5340
] defines an option bit,
known as the R-bit, for router-LSAs; the H-bit supports similar
The key words "MUST
", "MUST NOT
", "SHALL NOT
", "SHOULD NOT
", "NOT RECOMMENDED
" in this document are to be interpreted as described in
BCP 14 [RFC2119
] when, and only when, they appear in all
capitals, as shown here.3.
This document defines a new router-LSA bit, known as the Host-bit or
the H-bit. An OSPFv2 router advertising a router-LSA with the H-bit
set indicates that it MUST NOT
be used as a transit router (see Section 4
) by other OSPFv2 routers in the area that support the H-bit
If the H-bit is not set, then backward compatibility is achieved, as
the behavior will be the same as in [RFC2328
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
| LS age | Options | 1 |
| Link State ID |
| Advertising Router |
| LS sequence number |
| LS checksum | length |
|H|0|0|N|W|V|E|B| 0 | # links |
| Link ID |
| Link Data |
| Type | # TOS | metric |
| ... |
| TOS | 0 | TOS metric |
| Link ID |
| Link Data |
| ... |
Figure 1: OSPF Router-LSA
Bit H is the high-order bit of the OSPF flags, as shown below.
0 1 2 3 4 5 6 7
Figure 2: OSPF Router-LSA Option Bits
When the H-bit is set, the OSPFv2 router is a host (non-transit)
router and is incapable of forwarding transit traffic. In this mode,
the other OSPFv2 routers in the area MUST NOT
use the host router for
transit traffic but may send traffic to its local destinations.
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
When the H-bit is set, an Area Border Router (ABR) MUST
same H-bit setting in its self-originated router-LSAs for all
attached areas. The consistency of the setting will prevent
inter-area traffic transiting through the router by suppressing
advertisements of prefixes from other routers in the area in its
summary-LSAs. Only IPv4 prefixes associated with its local
be advertised in summary-LSAs to provide reachability
to end hosts attached to a router with the H-bit set.
When the H-bit is set, the host router cannot act as an Autonomous
System Border Router (ASBR). Indeed, ASBRs are transit routers to
prefixes that are typically imported through redistribution of
prefixes from other routing protocols. Therefore, non-local IPv4
prefixes, e.g., those imported from other routing protocols, SHOULD
be advertised in AS-external-LSAs if the H-bit is set. Some use
cases, such as an overloaded router or a router being gracefully
isolated, may benefit from continued advertisements of non-local
prefixes. In these cases, the Type 2 metric in AS-external-LSAs MUST
be set to LSInfinity [RFC2328
] to repel traffic (see Section 6
4. SPF Modifications
The SPF calculation described in Section 16.1 of [RFC2328
modified to ensure that the routers originating router-LSAs with the
H-bit set will not be used for transit traffic. Step (2) is modified
to include a check on the H-bit, as shown below. (Please note that
all of the sub-procedures of Step (2) remain unchanged and are not
included in the excerpt below.)
(2) Call the vertex just added to the tree "vertex V". Examine
the LSA associated with vertex V. This is a lookup in
Area A's link state database based on the Vertex ID. If this
is a router-LSA, and the H-bit of the router-LSA is set, and
vertex V is not the root, then the router should not be used
for transit and Step (3) should be executed immediately. If
this is a router-LSA and bit V of the router-LSA (see
Appendix A.4.2) is set, set Area A's TransitCapability to
TRUE. In any case, each link described by the LSA gives the
cost to an adjacent vertex. For each described link (say it
joins vertex V to vertex W):
5. Autodiscovery and Backward Compatibility
To reduce the possibility of any routing loops due to partial
deployment, this document defines an OSPF Router Information (RI) LSA
capability bit [RFC7770
]. See Section 7
The RI LSA MUST
Autodiscovery via announcement of the OSPF Host Router capability
) ensures that the H-bit functionality and its associated
SPF changes MUST
only take effect if all the routers in a given OSPF
area support this functionality.
In normal operation, it is possible that the RI LSA will fail to
reach all routers in an area in a timely manner. For example, if a
new router without H-bit support joins an area that previously had
only H-bit-capable routers with the H-bit set, then it may take some
time for the RI LSA to propagate to all routers. While it is
propagating, the routers in the area will gradually detect the
presence of a router that does not support the capability and will
revert back to the normal SPF calculation. During the propagation
time, the area as a whole is unsure of the status of the new router;
this type of situation can cause temporary transient loops.
The following recommendations will mitigate transient routing loops:
* Implementations are RECOMMENDED
to provide a configuration
parameter to manually override enforcement of the H-bit
functionality in partial deployments where the topology guarantees
that OSPFv2 routers not supporting the H-bit do not compute routes
resulting in routing loops.
* All routers with the H-bit set MUST
advertise all of the router's
non-stub links with a metric equal to MaxLinkMetric [RFC6987
its LSAs in order to prevent OSPFv2 routers (unless a last-resort
path) that do not support the H-bit from attempting to use the
non-stub links for transit traffic.
* All routers supporting the H-bit MUST
check the RI LSAs of all
nodes in the area to verify that all nodes support the H-bit
before actively using the H-bit feature. If any router does not
advertise the OSPF Host Router capability (Section 7
), then the
SPF modifications described in Section 4 MUST NOT
be used in the
6. OSPF AS-External-LSAs / NSSA-LSAs with Type 2 Metrics
When calculating the path to a prefix in an OSPF AS-external-LSA or
] with a Type 2 metric, the advertised Type 2 metric
is taken as more significant than the OSPF intra-area or inter-area
path. Hence, advertising the links with MaxLinkMetric as specified
] does not discourage transit traffic when calculating AS-
external or NSSA routes with Type 2 metrics.
Consequently, this document updates [RFC6987
] so that the Type 2
metric in any self-originated AS-external-LSAs or NSSA-LSAs is
advertised as LSInfinity-1 [RFC2328
]. If the H-bit is set, then the
Type 2 metric MUST
be set to LSInfinity.
7. IANA Considerations
IANA has registered the following value in the "OSPFv2 Router
| Value | Description | Reference |
| 0x80 | Host (H-bit) | RFC 8770
Table 1: H-Bit
IANA has registered the following in the "OSPF Router Informational
Capability Bits" registry.
| Bit Number | Capability Name | Reference |
| 7 | OSPF Host Router | RFC 8770
Table 2: OSPF Host Router Capability Bit
8. Security Considerations
This document introduces the H-bit, which is a capability feature
that restricts the use of a router for transit, while only its local
destinations are reachable. This is a subset of the operations of a
normal router and therefore should not introduce new security
considerations beyond those already known in OSPFv2 [RFC2328
feature introduces the advertisement of host router capability
information to all OSPFv2 routers in an area. This information can
be leveraged for discovery and verification that all routers in the
area support the capability before the feature is turned on. In the
event that a rogue or buggy router incorrectly advertises its
capability, possible scenarios are as follows:
* The router does not have the capability but sends the H-bit set in
its LSAs. In this case, a routing loop is possible. However,
this is mitigated by the fact that this router should be avoided
anyway. Moreover, the link metrics cost (MaxLinkMetric) of this
router will mitigate this situation. In any case, a router
advertising the H-bit capability without its link metrics cost
equal to MaxLinkMetric could be a rogue router and should be
* The router has the capability but sends the H-bit clear in its
LSAs. In this case, the router merely prevents the support of
other H-bit routers in the area and prevents all the routers from
running the modified SPF. Any impacts are also mitigated in this
scenario, as other H-bit routers in the area also advertise the
MaxLinkMetric cost, so they will still be avoided unless they are
the last-resort path.
* The rogue router is on the only transit path for some destinations
and sends the H-bit set (for no good/valid reason) in its LSAs,
and effectively partitions the network. This case is
indistinguishable from the normal case where an operator may
consciously decide to set the H-bit to perform maintenance on a
router that is on the only transit path. The OSPF protocol will
continue to function within the partitioned domains.
9.1. Normative References
] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119
, March 1997,
] Moy, J., "OSPF Version 2", STD 54, RFC 2328
, April 1998,
] Retana, A., Nguyen, L., Zinin, A., White, R., and D.
McPherson, "OSPF Stub Router Advertisement", RFC 6987
, September 2013,
] Lindem, A., Ed., Shen, N., Vasseur, JP., Aggarwal, R., and
S. Shaffer, "Extensions to OSPF for Advertising Optional
Router Capabilities", RFC 7770
, DOI 10.17487/RFC7770
February 2016, <https://www.rfc-editor.org/info/rfc7770
] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119
Key Words", BCP 14, RFC 8174
, DOI 10.17487/RFC8174
May 2017, <https://www.rfc-editor.org/info/rfc8174
9.2. Informative References
[BGP-ORR] Raszuk, R., Ed., Cassar, C., Aman, E., Decraene, B., and
K. Wang, "BGP Optimal Route Reflection (BGP-ORR)", Work in
Progress, Internet-Draft, draft-ietf-idr-bgp-optimal-
route-reflection-20, 8 January 2020,
] Murphy, P., "The OSPF Not-So-Stubby Area (NSSA) Option", RFC 3101
, DOI 10.17487/RFC3101
, January 2003,
] Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF
for IPv6", RFC 5340
, DOI 10.17487/RFC5340
, July 2008,
The authors would like to acknowledge Hasmit Grover for discovering
the limitation in [RFC6987
], and Acee Lindem, Abhay Roy, David Ward,
Burjiz Pithawala, and Michael Barnes for their comments.
170 W. Tasman Drive
San Jose, CA 95134
United States of America
170 W. Tasman Drive
San Jose, CA 95134
United States of America