Internet Engineering Task Force (IETF) A. Lindem, Ed.
Request for Comments: 5838 Ericsson
Category: Standards Track S. Mirtorabi
ISSN: 2070-1721 A. RoyM. Barnes
April 2010 Support of Address Families in OSPFv3
This document describes a mechanism for supporting multiple address
families (AFs) in OSPFv3 using multiple instances. It maps an AF to
an OSPFv3 instance using the Instance ID field in the OSPFv3 packet
header. This approach is fairly simple and minimizes extensions to
OSPFv3 for supporting multiple AFs.
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 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
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1.2. Requirements Notation
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC-KEYWORDS].
2. Protocol Details
Currently, the entire Instance ID number space is used for IPv6
unicast. This specification assigns different Instance ID ranges to
different AFs in order to support other AFs in OSPFv3. Each Instance
ID implies a separate OSPFv3 instance with its own neighbor
adjacencies, link state database, protocol data structures, and
shortest path first (SPF) computation.
Additionally, the current Link State Advertisements (LSAs) defined to
advertise IPv6 unicast prefixes can be used to advertise prefixes
from other AFs without modification.
It should be noted that OSPFv3 runs on top of IPv6 and uses IPv6 link
local addresses for OSPFv3 control packets. Therefore, it is
required that IPv6 be enabled on an OSPFv3 link, although the link
may not be participating in any IPv6 AFs.
2.1. Instance ID Values for New AFs
Instance ID zero is already defined by default for the IPv6 unicast
AF. When this specification is used to support multiple AFs, we
define the following ranges for different AFs. The first value of
each range is the default value for the corresponding AF.
Instance ID # 0 - # 31 IPv6 unicast AF
Instance ID # 32 - # 63 IPv6 multicast AF
Instance ID # 64 - # 95 IPv4 unicast AF
Instance ID # 96 - # 127 IPv4 multicast AF
Instance ID # 128 - # 255 Unassigned
OSPFv3 Instance IDs
2.2. OSPFv3 Options Changes
A new AF-bit is added to the OSPFv3 Options field. The V6-bit is
only applicable to the IPv6 unicast AF.
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
| | | | | | | | | | | | | | | |AF|*|*|DC|R|N|x|E|V6|
The Options field
The V6-bit is used in OSPFv3 to exclude a node from IPv6 unicast
route calculation but allow it in the SPF calculation for other
address families. Since the Instance ID now denotes the AF
explicitly, this bit is ignored in AFs other than IPv6 unicast.
When an OSPFv3 router is supporting AFs as described in this
specification, it MUST set the AF-bit in the OSPFv3 Options field
of Hello packets, Database Description packets, and LSAs.
2.3. Advertising Prefixes in AFs Other Than IPv6
Each prefix advertised in OSPFv3 has a prefix Length field [OSPFV3].
This facilitates advertising prefixes of different lengths in
different AFs. The existing LSAs defined in OSPFv3 are used for this
and there is no need to define new LSAs.
Prefixes that don't conform to the AF of an OSPFv3 instance MUST NOT
be used in the route computation for that instance.
2.4. Changes to the Hello Packet Processing
When an OSPFv3 router does not support this specification and it is
configured with the corresponding Instance ID, packets could be black
holed. This could happen due to misconfiguration or a router
software downgrade. Black holing is possible because a router that
doesn't support this specification can still be included in the SPF
calculated path as long as it establishes adjacencies using the
Instance ID corresponding to the AF. Note that Router-LSAs and
Network-LSAs are AF independent.
In order to avoid the above situation, Hello packet processing is
changed in order to only establish adjacencies with routers that have
the AF-bit set in their Options field.
Receiving Hello packets is specified in section 188.8.131.52 of [OSPFV3].
The following check is added to Hello packet reception:
o When an OSPFv3 router participates in an AF (sets the AF-bit in
the Options field), it MUST discard Hello packets having the AF-
bit clear in the Options field. The only exception is the Base
IPv6 unicast AF, where this check MUST NOT be done (for backward
2.5. Next-Hop Calculation for IPv4 Unicast and Multicast AFs
OSPFv3 runs on top of IPv6 and uses IPv6 link local addresses for
OSPFv3 control packets and next-hop calculations. Although IPv6 link
local addresses could be used as next hops for IPv4 address families,
it is desirable to have IPv4 next-hop addresses. For example, in the
IPv4 multicast AF, the Protocol Independent Multicast (PIM) [PIM]
neighbor address and the next-hop address should both be IPv4
addresses in order for the Reverse Path Forwarding (RPF) lookup to
work correctly. Troubleshooting is also easier when the prefix
address and next-hop address are in the same AF.
In order to achieve this, the link's IPv4 address will be advertised
in the "link local address" field of the IPv4 instance's Link-LSA.
This address is placed in the first 32 bits of the "link local
address" field and is used for IPv4 next-hop calculations. The
remaining bits MUST be set to zero.
We denote a Direct Interface Address (DIA) as an IPv4 or IPv6 address
that is both directly reachable via an attached link and has an
available layer 3 to layer 2 mapping. Note that there is no explicit
need for the IPv4 link addresses to be on the same subnet. An
implementation SHOULD resolve layer 3 to layer 2 mappings via the
Address Resolution Protocol (ARP) [ARP] or Neighbor Discovery (ND)
[ND] for a DIA even if the IPv4 address is not on the same subnet as
the router's interface IP address.
2.6. AS-External-LSA and NSSA-LSA Forwarding Address for IPv4 Unicast
and IPv4 Multicast AFs
For OSPFv3, this address is an IPv6 host address (128 bits). If
included, data traffic for the advertised destination will be
forwarded to this address. For IPv4 unicast and IPv4 multicast AFs,
the Forwarding Address in AS-external-LSAs and NSSA-LSAs MUST encode
an IPv4 address. To achieve this, the IPv4 Forwarding Address is
advertised by placing it in the first 32 bits of the Forwarding
Address field in AS-external-LSAs and NSSA-LSAs. The remaining bits
MUST be set to zero.
2.7. Database Description Maximum Transmission Unit (MTU) Specification
for Non-IPv6 AFs
For address families other than IPv6, both the MTU for the instance
address family and the IPv6 MTU used for OSPFv3 maximum packet
determination MUST be considered. The MTU in the Database
Description packet MUST always contain the MTU corresponding to the
advertised address family. For example, if the instance corresponds
to an IPv4 address family, the IPv4 MTU for the interface MUST be
specified in the interface MTU field. As specified in Section 10.6
of [OSPFV2], the Database Description packet will be rejected if the
MTU is greater than the receiving interface's MTU for the address
family corresponding to the instance. This behavior will assure that
an adjacency is not formed and address family specific routes are not
installed over a path with conflicting MTUs.
The value used for OSPFv3 maximum packet size determination MUST also
be compatible for an adjacency to be established. Since only a
single MTU field is specified, the M6-bit is defined by this
specification. If the M6-bit is clear, the specified MTU SHOULD also
be checked against the IPv6 MTU, and the Database Description packet
SHOULD be rejected if the MTU is larger than the receiving
interface's IPv6 MTU. An OSPFv3 router SHOULD NOT set the M6-bit if
its IPv6 MTU and address family specific MTU are the same.
If the IPv6 and IPv4 MTUs differ, the M6-bit MUST be set for non-IPv6
address families. If the M6-bit is set, the IPv6 MTU is dictated by
the presence or absence of an IPv6 MTU TLV in the link-local
signaling (LLS) [LLS] block. If this TLV is present, it carries the
IPv6 MTU that SHOULD be compared with the local IPv6 MTU. If this
TLV is absent, the minimum IPv6 MTU of 1280 octets SHOULD be used for
the comparison (refer to [IPV6]).
If the M6-bit is set in a received Database Description packet for a
non-IPv6 address family, the receiving router MUST NOT check the
Interface MTU in the Database Description packet against the
receiving interface's IPv6 MTU.
The figure below graphically depicts the changed fields in octets
20-23 of the OSPFv3 Database Description packet:
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
| Interface MTU | 0 |0|0|0|M6|0|I|M|MS|
OSPFv3 Database Description Packet Changes
The changed fields in the Database Description packet are described
below. The remaining fields are unchanged from [OSPFV3].
The size in octets of the largest address family specific datagram
that can be sent on the associated interface without
fragmentation. The MTUs of common Internet link types can be
found in Table 7-1 of [MTUDISC]. The Interface MTU SHOULD be set
to 0 in Database Description packets sent over virtual links.
The IPv6 MTU bit - this bit indicates that the sender is using a
different IPv6 MTU than the MTU for the AF.
An IPv6 MTU TLV can be optionally carried in an LLS block as
described above. This TLV carries the IPv6 MTU for the interface.
The length field of the TLV is set to 4 bytes.
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
| 17 | 4 |
| IPv6 MTU |
Format of IPv6 MTU TLV
Only one instance of the IPv6 MTU TLV MAY appear in the LLS block.
Instances subsequent to the first are not processed, and the LLS
inconsistency SHOULD be logged.
2.8. Operation over Virtual Links
OSPFv3 control packets sent over a virtual link are IPv6 packets and
may traverse multiple hops. Therefore, there MUST be a global IPv6
address associated with the virtual link so that OSPFv3 control
packets are forwarded correctly by the intermediate hops between
virtual link endpoints. Although this requirement can be satisfied
in IPv6 unicast AFs, it will not function in other AFs as there will
not be a routable global IPv6 address or forwarding path. Therefore,
virtual links are not supported in AFs other than IPv6 unicast.
3. Backward Compatibility
All modifications to OSPFv3 apply exclusively to the support of
address families other than the IPv6 unicast AF using multiple OSPFv3
instances as described in this specification. These modifications
are not applicable to IPv6 unicast topologies and do not preclude
future single instance mechanisms for supporting multiple address
In this section, we will define a non-capable OSPFv3 router as one
not supporting this specification. When multiple AFs are supported
as defined herein, each new AF will have a corresponding Instance ID
and can interoperate with the existing non-capable OSPFv3 routers in
an IPv6 unicast topology. Furthermore, when a non-capable OSPFv3
router uses an Instance ID that is reserved for a given AF, no
adjacency will be formed with this router since the AF-bit in the
Options field will be clear in its OSPFv3 Hello packets. Therefore,
there are no backward compatibility issues. AFs can be gradually
deployed without disturbing OSPFv3 routing domains with non-capable
4. Security Considerations
IPsec [IPsec] can be used for OSPFv3 authentication and
confidentiality as described in [OSPFV3-AUTH]. When multiple OSPFv3
instances use the same interface, they all MUST use the same Security
Association (SA), since the SA selectors do not provide selection
based on data in OSPFv3 Header fields (e.g., the Instance ID). This
restriction is documented in Section 8 of [OSPFV3-AUTH].
Security considerations for OSPFv3 are covered in [OSPFV3].
5. IANA Considerations
The following IANA assignments were made from existing registries.
o The AF-bit was assigned from the OSPFv3 Options registry as
defined in Section 2.2.
o The M6-bit was assigned from the DD Packet Flags registry as
defined in Section 2.7
o The TLV type (17) for the IPv6 MTU TLV was assigned from the OSPF
LLS TLVs registry.
IANA created a new registry, "OSPFv3 Instance ID Address Family
Values", for assignment of the mapping of OSPFv3 Instance IDs to
address families when this specification is used to support multiple
address families. Note that the Instance ID field MAY be used for
applications other than the support of multiple address families.
However, if it is being used for address families as described in
this specification, the assignments herein SHOULD be honored.
6.1. Normative References
[IPV6] Deering, S. and R. Hinden, "Internet Protocol,
Version 6 (IPv6) Specification", RFC 2460,
[IPsec] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, December 2005.
[OSPFV2] Moy, J., "OSPF Version 2", STD 54, RFC 2328,
[OSPFV3] Coltun, R., Ferguson, D., Moy, J., and A. Lindem,
"OSPF for IPv6", RFC 5340, July 2008.
[OSPFV3-AUTH] Gupta, M. and S. Melam, "Authentication/
Confidentiality for OSPFv3", RFC 4552, June 2006.
[RFC-KEYWORDS] Bradner, S., "Key words for use in RFC's to Indicate
Requirement Levels", RFC 2119, March 1997.
6.2. Informative References
[ARP] Plummer, D., "Ethernet Address Resolution Protocol:
Or Converting Network Protocol Addresses to 48.bit
Ethernet Address for Transmission on Ethernet
Hardware", STD 37, RFC 826, November 1982.
[LLS] Zinin, A., Roy, A., Nguyen, L., Friedman, B., and D.
Young, "OSPF Link-Local Signaling", RFC 5613,
[MTUDISC] Mogul, J. and S. Deering, "Path MTU Discovery",
RFC 1191, November 1990.
[ND] Narten, T., Nordmark, E., Simpson, W., and H.
Soliman, "Neighbor Discovery for IP version 6
(IPv6)", RFC 4861, September 2007.
[PIM] Fenner, B., Handley, M., Holbrook, H., and I.
Kouvelas, "Protocol Independent Multicast - Sparse
Mode (PIM-SM): Protocol Specification (Revised)",
RFC 4601, August 2006.
Appendix A. Acknowledgments
The RFC text was produced using Marshall Rose's xml2rfc tool.
Thanks to Tom Henderson and the folks at Boeing for implementing this
document in the Quagga routing suite, http:www.quagga.net.
Thanks to Nischal Sheth for review and comments.
Thanks to Christian Vogt for comments during the Gen-ART review.
Thanks to Adrian Farrel for comments during the IESG review.
Thanks to Alfred Hoenes for comments during the editing process.
Acee Lindem (editor)
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