Internet Engineering Task Force (IETF) P. Pillay-Esnault

Internet Engineering Task Force (IETF) P. Pillay-Esnault
Request for Comments: 6565 Cisco Systems
Category: Standards Track P. Moyer
ISSN: 2070-1721 Pollere, Inc.
J. Doyle
Jeff Doyle and Associates
E. Ertekin
M. Lundberg
Booz Allen Hamilton
June 2012

OSPFv3 as a Provider Edge to Customer Edge (PE-CE) Routing Protocol

Abstract

Many Service Providers (SPs) offer Virtual Private Network (VPN)
services to their customers using a technique in which Customer Edge
(CE) routers are routing peers of Provider Edge (PE) routers. The
Border Gateway Protocol (BGP) is used to distribute the customer's
routes across the provider's IP backbone network, and Multiprotocol
Label Switching (MPLS) is used to tunnel customer packets across the
provider's backbone. Support currently exists for both IPv4 and IPv6
VPNs; however, only Open Shortest Path First version 2 (OSPFv2) as
PE-CE protocol is specified. This document extends those
specifications to support OSPF version 3 (OSPFv3) as a PE-CE routing
protocol. The OSPFv3 PE-CE functionality is identical to that of
OSPFv2 except for the differences described in this document.

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
http://www.rfc-editor.org/info/rfc6565.

Pillay-Esnault, et al. Standards Track [Page 1]

RFC 6565 OSPFv3 as a PE-CE Routing Protocol June 2012

Copyright Notice

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not be created outside the IETF Standards Process, except to format
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than English.

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RFC 6565 OSPFv3 as a PE-CE Routing Protocol June 2012

Table of Contents

1. Introduction ....................................................4
2. Specification of Requirements ...................................4
3. Requirements ....................................................4
3.1. OSPFv3 Specificities .......................................5
4. BGP/OSPFv3 Interaction Procedures for PE Routers ................5
4.1. VRFs and OSPFv3 Instances ..................................5
4.1.1. Independent OSPFv3 Instances in PEs .................6
4.1.2. OSPFv3 Domain Identifier ............................6
4.2. OSPFv3 Areas ...............................................7
4.3. VRFs and Routes ............................................7
4.3.1. OSPFv3 Routes on PEs ................................8
4.3.2. VPN-IPv6 Routes Received from MP-BGP ................9
4.4. BGP Extended Communities Attribute ........................12
4.5. Loop Prevention Techniques ................................14
4.5.1. OSPFv3 Down Bit ....................................15
4.5.2. Other Possible Loops ...............................15
5. OSPFv3 Sham Links ..............................................15
5.1. Creating a Sham Link ......................................16
5.2. OSPF Protocol on Sham Link ................................16
5.3. OSPF Packet Forwarding on Sham Link .......................17
6. Multiple Address Family Support ................................17
7. Security Considerations ........................................18
8. IANA Considerations ............................................18
9. Acknowledgments ................................................18
10. References ....................................................18
10.1. Normative References .....................................18
10.2. Informative References ...................................19

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RFC 6565 OSPFv3 as a PE-CE Routing Protocol June 2012

1. Introduction

[RFC4364] offers Service Providers (SPs) a method for providing Layer
3 Virtual Private Network (VPN) services to subtending customer
networks. Using the procedures defined in [RFC4364], Provider Edge
(PE) routers separate customer VPN routing information into Virtual
Routing and Forwarding (VRF) tables. The Border Gateway Protocol
(BGP) is used to disseminate customer network VPN routes between PE
VRFs configured in the same VPN.

The initial BGP/MPLS IP VPN specification enabled PE routers to learn
routes within customer sites through static routing, or through a
dynamic routing protocol instantiated on the PE-CE link.
Specifically, [RFC4364] (and its predecessor, [RFC2547]) included
support for dynamic routing protocols such as BGP, RIP, and OSPFv2.
The OSPFv2 as the Provider/Customer Edge Protocol specification
[RFC4577] further updates the operation of OSPFv2 as the PE-CE
routing protocol by detailing additional extensions to enable intra-
domain routing connectivity between OSPFv2-based customer sites.

While [RFC4364] was defined for IPv4-based networks, [RFC4659]
extends support to IPv6 VPNs. It is expected that OSPFv3 will be
used as the IGP for some IPv6 VPNs just as the OSPFv2 was used for
IPv4 VPNs. The advantages of using OSPFv3 as a PE-CE protocol are
the same as for the IPv4 VPN deployment.

This document defines the mechanisms required to enable the operation
of OSPFv3 as the PE-CE routing protocol. In doing so, it reuses, and
extends where necessary, methods defined in [RFC4659] and [RFC4577].
This document also includes the specifications for maintaining intra-
domain routing connectivity between OSPFv3-based customer sites
across an SP backbone.

We presuppose familiarity with the contents of [RFC4364], [RFC4659],
[RFC4577], [RFC4576], [RFC5340], and [RFC2328].

2. Specification of Requirements

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 [RFC2119].

3. Requirements

The benefits and considerations associated with deploying OSPFv3 as
the PE-CE routing protocol are similar to those described in
[RFC4577]. The requirements described in Section 3 of [RFC4577]
remain semantically identical for the deployment of OSPFv3.

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[RFC5340] describes the modifications required to OSPF to support
IPv6. In that specification, many of the fundamental mechanisms
associated with OSPFv2 remain unchanged for OSPFv3. Consequently,
the operation of OSPFv3 as the PE-CE routing protocol is very similar
to OSPFv2 as the PE-CE protocol.

3.1. OSPFv3 Specificities

Section 2 of [RFC5340] describes the differences between OSPFv3 and
OSPFv2. Several of these changes will require modifications to the
architecture described in [RFC4577]. These differences and their
corresponding impact to [RFC4577] are described below:

New LSA types:

For an IPv6 VPN architecture where customers interface with
providers through OSPFv3, traditional BGP/OSPF interactions
specify that VPN-IPv6 reachability information redistributed into
OSPFv3 will be expressed as AS-External OSPFv3 LSAs. Instead, it
may be desirable to view these LSAs as inter-area-prefix LSAs.
The OSPF Route Type Extended Communities attribute defined in
[RFC4577] is extended to include OSPFv3 route types. These new
encodings are defined in Section 4.4.

Multiple instances over a link:

OSPFv3 operates on a per-link basis as opposed to OSPFv2, which
operates on a per-IP-subnet basis. The support of multiple OSPFv3
protocol instances on a link changes the architecture described in
[RFC4577]. [RFC4577] specifies that each interface belongs to no
more than one OSPF instance. For OSPFv3, multiple instances can
be established over a single interface and associated with the
same VRF.

In addition to establishing multiple OSPFv3 instances over a
single PE-CE link, multiple OSPFv3 instances can also be
established across a sham link. This enables multiple OSPFv3
instances associated with a VRF to independently establish intra-
area connectivity to other OSPFv3 instances attached to a remote
PE VRF. Support for multiple OSPFv3 instances across the sham
link is described in Section 5.

4. BGP/OSPFv3 Interaction Procedures for PE Routers

4.1. VRFs and OSPFv3 Instances

The relationship between VRFs, interfaces, and OSPFv3 instances on a
PE router is described in the following section.

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As defined in [RFC4364], a PE router can be configured with one or
more VRFs. Each VRF configured on the PE corresponds to a customer
VPN and retains the destinations that are reachable within that VPN.
Each VRF may be associated with one or more interfaces, which allows
multiple sites to participate in the same VPN. If OSPFv3 is
instantiated on an interface associated with a VRF, the VRF will be
populated with OSPFv3 routing information.

As OSPFv3 supports multiple instances on a single interface, it is
therefore possible that multiple customer sites can connect to the
same interface of a PE router (e.g., through a Layer 2 switch) using
distinct OSPFv3 instances. A PE interface can be associated with
only one VRF, and all OSPFv3 instances running on the same interface
MUST be associated with the same VRF. Configurations where a PE
interface is associated with multiple VRFs are out of scope for this
document.

4.1.1. Independent OSPFv3 Instances in PEs

Similar to [RFC4577], the PE must associate at least one OSPFv3
instance for each OSPFv3 domain to which it attaches, and each
instance of OSPFv3 MUST be associated with a single VRF.

The support of multiple PE-CE OSPFv3 instances per PE interface does
not change the paradigm that an OSPF instance can be associated with
only a single VRF. Furthermore, for each instance instantiated on
the interface, the PE establishes adjacencies with corresponding CEs
associated with the instance. Note that although multiple instances
may populate a common VRF, they do not leak routes to one another,
unless configured to do so.

4.1.2. OSPFv3 Domain Identifier

The OSPFv3 Domain ID describes the administrative domain of the OSPF
instance that originated the route. It has an AS-wide significance
and is one of the parameters used to determine whether a VPN-IPv6
route should be translated as an Inter-area-prefix LSA or External
LSA. Each OSPFv3 instance MUST have a primary Domain ID that is
transported along with the VPN-IPv6 route in a BGP attribute over the
VPN backbone. Each OSPFv3 instance may have a set of secondary
Domain IDs that applies to other OSPFv3 instances within its
administrative domain.

The primary Domain ID may either be configured or be set to a value
of NULL. The secondary Domain IDs are only allowed if a non-NULL
primary Domain ID is configured. The Domain ID MUST be configured on
a per-OSPFv3 instance basis.

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The Domain ID is used to determine whether an incoming VPN-IPv6 route
belongs to the same domain as the receiving OSPFv3 instance. An
incoming VPN-IPv6 route is said to belong to the same domain if a
non-NULL incoming Domain ID matches either the local primary or one
of the secondary Domain IDs. If the local Domain ID and incoming
Domain ID are NULL, it is considered a match.

4.2. OSPFv3 Areas

Sections 4.1.4 and 4.2.3 of [RFC4577] describe the characteristics of
a PE router within an OSPFv2 domain. The mechanisms and expected
behavior described in [RFC4577] are applicable to an OSPFv3 domain.

4.3. VRFs and Routes

From the perspective of the CE, the PE appears as any other OSPFv3
neighbor. There is no requirement for the CE to support any
mechanisms of IPv6 BGP/MPLS VPNs or for the CE to have any awareness
of the VPNs, thereby enabling any OSPFv3 implementation to be used on
a CE.

Because the export and import policies might cause different routes
to be installed in different VRFs of the same OSPFv3 domain, the VPN
backbone cannot be considered as a single router from the perspective
of the domain's CEs. Rather, each CE should view its connected PE as
a separate router.

The PE uses OSPFv3 to distribute routes to CEs, and MP-BGP [RFC4760]
to distribute VPN-IPv6 routes to other (remote) PE routers as defined
in [RFC4659]. An IPv6 prefix installed in the VRF by OSPFv3 is
changed to a VPN-IPv6 prefix by the addition of an 8-octet Route
Distinguisher (RD) as discussed in Section 2 of [RFC4659]. This VPN-
IPv6 route can then be redistributed into MP-BGP according to an
export policy that adds a Route Target (RT) Extended Communities
attribute to the Network Layer Reachability Information (NLRI)
[RFC4360].

Domain IDs are used to distinguish between OSPFv3 instances. When an
OSPFv3 distributed route is redistributed into MP-BGP, the Domain ID,
OSPFv3 Router ID, Area, OSPFv3 Route Type, and Options fields
(External Route Type) are also carried in Extended Community
Attributes of the MP-BGP route.

A PE receiving a VPN-IPv6 NLRI from MP-BGP uses an import policy to
determine, based on the RT, whether the route is eligible to be
installed in one of its local VRFs. The BGP decision process selects

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which of the eligible routes are to be installed in the associated
VRF, and the selected set of VPN-IPv6 routes are converted into IPv6
routes by removing the RD before installation.

An IPv6 route learned from MP-BGP and installed in a VRF might or
might not be redistributed into OSPFv3, depending on the local
configuration. For example, the PE might be configured to advertise
only a default route to CEs of a particular OSPFv3 instance.
Further, if the route is to be redistributed into multiple OSPFv3
instances, the route might be advertised using different LSA types in
different instances.

If an IPv6 route learned from MP-BGP is to be redistributed into a
particular OSPFv3 instance, the OSPF Domain Identifier Extended
Communities attribute of the VPN-IPv6 route is used to determine
whether the OSPFv3 instance from which the route was learned is the
same as the OSPFv3 instance into which the route is to be
redistributed.

4.3.1. OSPFv3 Routes on PEs

VRFs may be populated by both OSPFv3 routes from a CE or VPN-IPv6
routes from other PEs via MP-BGP. OSPFv3 routes are installed in a
VRF using the OSPFv3 decision process. They may be redistributed
into BGP and disseminated to other PEs participating in the VPN. At
these remote PEs, the VPN-IPv6 routes may be imported into a VRF and
redistributed into the OSPFv3 instance(s) associated with that VRF.

As specified in [RFC4659], routes imported and exported into a VRF
are controlled by the Route Target (RT) Extended Communities
attribute. OSPFv3 routes that are redistributed into BGP are given
an RT that corresponds to the VRF. This RT is examined at remote
PEs. In order to import a route, a VRF must have an import RT that
is identical to the route's RT. For routes that are eligible to be
imported into the VRF, the standard BGP decision process is used to
choose the "best" route(s).

When a route is advertised from a CE to a PE via OSPFv3 and that
route is installed in the VRF associated with the CE, the route is
advertised to other locally attached CEs under normal OSPFv3
procedures.

The route is also redistributed into MP-BGP to be advertised to
remote PEs. The information necessary for accurate redistribution
back into OSPFv3 by the remote PEs is carried in the OSPF Route Type,
OSPF Domain ID, and OSPF Router ID Extended Communities attributes
(Section 4.4). The relevant local OSPFv3 information encoded into
these attributes are as follows:

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The Area ID of the PE-CE link.

The Route Type, as determined by the LSA type from which the route
was learned.

The Options fields (External metric-type).

The Domain ID of the OSPFv3 process. If no Domain ID is
configured, the NULL identifier is used.

The PE's Router ID associated with the OSPFv3 instance.

A Multi-Exit-Discriminator (MED) attribute SHOULD also be set to the
value of the OSPFv3 metric associated with the route plus 1, when the
OSPFv3 route is redistributed into the MP-BGP.

4.3.2. VPN-IPv6 Routes Received from MP-BGP

When a PE receives a valid VPN-IPv6 route from MP-BGP and has
identified an association with a local VRF, it must determine:

Whether a route to the corresponding IPv6 prefix is to be
installed in the VRF;

Whether the installed IPv6 route is to be redistributed to one or
more local OSPFv3 instances; and

What OSPFv3 LSA type is to be used when advertising the route into
each OSPFv3 instance.

An IPv6 route derived from a received VPN-IPv6 route is not installed
in the associated local VRF if:

The BGP decision process identifies a better route to the
destination NLRI; or

A configured import policy prohibits the installation of the
route.

The PE advertises the IPv6 route learned from MP-BGP to attached CEs
via OSPFv3 if:

No configured filtering prohibits redistributing the route to
OSPFv3;

No configured policy blocks the route in favor of a less-specific
summary route; and

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Redistribution of a BGP learned IPv6 route into OSPF is based on
local policy.

The subsequent sections discuss the advertisement of routes learned
from MP-BGP and the rules for determining to which LSA types and to
which CEs to advertise the routes.

When the PE sends an LSA to a CE, it sets the DN-bit in the LSA to
prevent looping. The DN-bit is discussed in Section 4.5.1.

4.3.2.1. OSPF Inter-Area Routes

A PE advertises an IPv6 route using an Inter-Area-Prefix (type
0x2003) LSA under the following circumstances:

The OSPFv3 domain from which the IPv6 route was learned is the
same (as determined by the Domain ID) as the domain of the OSPFv3
instance into which it is to be redistributed; and

The IPv6 route was advertised to a remote PE in an Intra-Area-
Prefix (type 0x2009) OR an Inter-Area-Prefix (type 0x2003) LSA.

Note that under these rules, the PE represents itself as an Area
Border Router (ABR) regardless of whether or not the route is being
advertised into the same area number from which the remote PE learned
it (that is, whether the VPN-IPv6 route carries the same or different
area numbers).

4.3.2.2. OSPF Intra-Area Route

A route is advertised as an intra-area route using an Intra-Area-
Prefix (type 0x2009) LSA only when sham links are used, as described
in Section 5. Otherwise, routes are advertised as either inter-area
(Section 4.3.2.1) or external / Not-So-Stubby Area (NSSA) (Section
4.3.2.3) routes.

4.3.2.3. OSPF External Routes and NSSA Routes

A PE considers an IPv6 route to be external under the following
circumstances:

The OSPFv3 domain from which the route was learned is different
(as determined by the Domain ID) from the domain of the OSPFv3
instance into which it is redistributed; or

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The OSPFv3 domain from which the route was learned is the same as
the domain of the OSPFv3 instance into which it is redistributed,
AND it was advertised to the remote PE in an AS-External-LSA (type
0x4005) or an NSSA-LSA (type 0x2007); or

The route was not learned from an OSPFv3 instance.

To determine if the learned route is from a different domain, the
Domain ID associated with the VPN-IPv6 route (in the OSPF Domain ID
Extended Communities attribute or attributes) is compared with the
local OSPFv3 Domain ID, if configured. Compared Domain IDs are
considered identical if:

1. All 8 bytes are identical; or

2. Both Domain IDs are NULL (all zeroes).

Note that if the VPN-IPv6 route does not have a Domain ID in its
attributes, or if the local OSPFv3 instance does not have a
configured Domain ID (i.e., in either case), the route is considered
to have a NULL Domain ID.

An IPv6 route that is determined to be external might or might not be
advertised to a connected CE, depending on the type of area to which
the PE-CE link belongs and whether there is a configured policy
restricting its advertisement.

If there are multiple external routes to the same prefix, the
standard OSPFv3 decision process is used to select the "best" route.

If the external route is to be advertised and the area type of the
PE-CE link is NSSA, the PE advertises the route in an NSSA-LSA (type
0x2007); otherwise, the external route is advertised in an
AS-External-LSA (type 0x4005).

The DN-bit of the LSA advertising the external route MUST be set, as
described in Section 4.5.1.

If the VPN-IPv6 route indicates a route Type-1 metric, the PE should
advertise the external route with that metric-type; otherwise, the
metric-type of the external IPv6 route is set to Type-2 by default.
Note that, by default, a PE should advertise an external route with a
Type-2 metric if the IPv6 route's Domain ID is different than the
local OSPFv3 instance, unless specified otherwise by local policy.

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4.4. BGP Extended Communities Attributes

OSPFv3 routes from one site are translated and delivered
transparently to the remote site as BGP VPN-IPv6 routes. The
original OSPFv3 routes carry OSPFv3-specific information that needs
to be communicated to the remote PE to ensure transparency. BGP
Extended Communities are used to carry the needed information to
enable the receiving side to reconstruct a database just as in the
OSPFv2 case.

All OSPFv3 routes added to the VRF routing table on a PE router are
examined to create a corresponding VPN-IPv6 route in BGP. Each of
the OSPFv3 routes MUST have the corresponding BGP Extended
Communities Attributes that contain and preserve the OSPFv3
information of the original OSPFv3 route. The BGP Extended
Communities attributes defined in [RFC4577] are reused for
convenience.

OSPF Domain Identifier Extended Communities Attribute

Each OSPFv3 Instance within a VRF MUST have a Domain ID. The Domain
ID is configured per OSPFv3 Instance. The OSPFv3 Domain ID is a
6-byte number, and its default value is 0. This attribute has a
2-byte type field, encoded with a value of 0x0005, 0x0105, or 0x0205.

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type Value | Domain Identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Domain Identifier Cont. |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The OSPF Domain Identifier Extended Communities Attribute

OSPFv3 Domain IDs field : 6 bytes

Each OSPFv3 Instance within a VRF MUST have a Domain ID and its
default value (if none is configured) is 0. The Domain ID is
configured per OSPFv3 Instance.

OSPF Router ID Extended Communities Attribute

The OSPFv3 Router ID is a 32-bit number as in OSPFv2. This attribute
has a 2-byte type field, encoded with a value of 0x0107.

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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type Value | Router ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Router ID Cont. | UNUSED |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The OSPF Router ID Extended Communities Attribute

OSPFv3 Router ID field : 4 bytes

The OSPFv3 Router ID is a 32-bit number as in OSPFv2. Setting
this field is OPTIONAL, and its default value is 0.

OSPF Route Type Extended Communities Attribute

The OSPF Route Type Extended Communities Attribute MUST be present.
It contains a 2-byte type field, encoded with a value of 0x0306. The
remaining 6 bytes are divided into 3 fields, an Area Number, a Route
Type, and an Options field.

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type Value | Area Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Area Number Cont. | Route Type | Options |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The OSPF Route Type Extended Communities Attribute

Area Number : 4 bytes

The area number indicates the 32-bit Area ID to which the route
belongs.

Route Types : 1 byte

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To accommodate OSPFv3 LSA types (as registered by [RFC5340]), the
Route Type field is encoded as follows:

Route Type Route Type LSA Type Description
Code
-----------------------------------------------------------
3 Inter-area-prefix 0x2003 Inter-Area-Prefix-LSA
5 External 0x4005 AS-External-LSA
7 NSSA 0x2007 NSSA-LSA
1 or 2 Intra-area-prefix 0x2009 Intra-Area-Prefix-LSA

Route Type Field Encoding

Options : 1 byte

The Options field indicates the options that are associated with
the OSPFv3 route.

8 7 6 5 4 3 2 1
+---+---+---+---+---+---+---+---+
| | | | | | | | E |
+---+---+---+---+---+---+---+---+

The OSPFv3 Route Options Field

The least significant bit (i.e., bit E) in this field designates
the external metric-type. If the bit is clear, the route carries
a Type-1 external metric; if the bit is set, the route carries a
Type-2 external metric.

4.5. Loop Prevention Techniques

In some topologies, it is possible for routing loops to occur due to
the nature and manner of route reachability propagation. One such
example is the case of a dual-homed CE router connected to two PEs;
those PE routers would receive reachability information both through
their CE and their peer PE. As there is transparent transport of
OSPFv3 routes over the VPN backbone, it is not possible for the PE
routers to determine whether they are within a loop.

The loop scenarios in OSPFv3 topologies are identical to those in the
OSPFv2 topologies described in Sections 4.2.5.1 and 4.2.5.2 of
[RFC4577]. Of the two loop prevention mechanisms described in the
aforementioned sections, only the DN-bit option will be supported in
the OSPFv3 implementation.

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RFC 6565 OSPFv3 as a PE-CE Routing Protocol June 2012

4.5.1. OSPFv3 Down Bit

[RFC4576] describes the usage of the DN-bit for OSPFv2 and is
applicable for OSPFv3 for Inter-area-prefix LSAs, NSSA LSAs, and
External LSAs. Similarly, the DN-bit MUST be set in Inter-area-
prefix LSAs, NSSA LSAs, and AS-External LSAs, when these are
originated from a PE to a CE, to prevent those prefixes from being
re-advertised into BGP. As in [RFC4577], any LSA with the DN-bit set
must not be used for route calculations on PE routers.

The DN-bit MUST be clear in all other LSA types. The OSPFv3 DN-bit
format is described in Appendix A.4.1.1 of [RFC5340].

4.5.2. Other Possible Loops

The mechanism described in Section 4.5.1 of this document is
sufficient to prevent looping if the DN-bit information attached to a
prefix is preserved in the OSPF domain. As described in Section
4.2.5.3 of [RFC4577], caution must be exercised if mutual
redistribution that is performed on a PE causes loss of loop
prevention information.

5. OSPFv3 Sham Links

This section modifies the specification of OSPFv2 sham links (defined
in Section 4.2.7 of [RFC4577]) to support OSPFv3. Support for OSPFv3
sham links is an OPTIONAL feature of this specification.

A sham link enables a VPN backbone to act as an intra-area link. It
is needed when two sites are connected by an intra-area "backdoor"
link and the inter-area VPN backbone route would be less preferable
due to OSPF route preference rules. The figure below shows the
instantiation of a sham link between two VPN sites.

(VPN backbone)
(site-1) <-------- sham link --------> (site-2)
CE1 -------- PE1 -------- P ---------- PE2 -------- CE2
| |
|___________________________________________________|
<------------ backdoor link -------------->
(OSPF intra-area link)

Sham Link

Much of the operation of sham links remains semantically identical to
what was previously specified. There are, however, several
differences that need to be defined to ensure the proper operation of
OSPFv3 sham links.

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One of the primary differences between sham links for OSPFv3 and sham
links as specified in [RFC4577] is for configurations where multiple
OSPFv3 instances populate a VRF. It may be desirable to provide
separate intra-area links between these instances over the same sham
link. To achieve this, multiple OSPFv3 instances may be established
across the PE-PE sham link to provide intra-area connectivity between
PE-CE OSPFv3 instances.

Note that even though multiple OSPFv3 instances may be associated
with a VRF, a sham link is still thought of as a relation between two
VRFs.

Another modification to OSPFv2 sham links is that OSPFv3 sham links
are now identified by 128-bit endpoint addresses. Since sham link
endpoint addresses are now 128 bits, they can no longer default to
the RouterID, which is a 32-bit number. Sham link endpoint addresses
MUST be configured.

Sham link endpoint addresses MUST be distributed by BGP as routeable
VPN IPv6 addresses, each with an IPv6 address prefix that is 128 bits
long. As specified in Section 4.2.7.1 of [RFC4577], these endpoint
addresses MUST NOT be advertised by OSPFv3; if there is no BGP route
to the sham link endpoint address, that address is to appear
unreachable, so that the sham link appears to be down.

If there is a BGP route to the remote sham link endpoint address, the
sham link appears to be up. Conversely, if there is no BGP route to
the sham link endpoint address, the sham link appears to be down.

5.1. Creating a Sham Link

The procedures for creating an OSPFv3 sham link are identical to
those specified in Section 4.2.7.2 of [RFC4577]. Note that the
creation of OSPFv3 sham links requires the configuration of both
local and remote 128-bit sham link endpoint addresses. The local
sham link endpoint address associated with a VRF MAY be used by all
OSPFv3 instances that are attached to that VRF. The OSPFv3 PE-PE
"link" Instance ID in the protocol packet header is used to
demultiplex multiple OSPFv3 instance protocol packets exchanged over
the sham link.

5.2. OSPF Protocol on Sham Link

Much of the operation of OSPFv3 over a sham link is semantically the
same as the operation of OSPFv2 over a sham link, as described in
Section 4.2.7.3 of [RFC4577]. This includes the methodology for
sending and receiving OSPFv3 packets over sham links, as well as

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Hello/Router Dead Intervals. Furthermore, the procedures associated
with the assignment of sham link metrics adhere to those set forth
for OSPFv2. OSPFv3 sham links are treated as on-demand circuits.

Although the operation of the OSPFv3 protocol over the sham link is
the same as OSPFv2, multiple OSPFv3 instances may be instantiated
across this link. By instantiating multiple instances across the
sham link, distinct intra-area connections can be established between
PE-PE OSPFv3 instances associated with the endpoint addresses.

For example, if two OSPFv3 instances (O1, O2) attach to a VRF V1, and
on a remote PE, two other OSPFv3 instances (O3, O4) attach to a VRF
V2, it may be desirable to connect O1 and O3 with an intra-area link,
and O2 and O4 with an intra-area link. This can be accomplished by
instantiating two OSPFv3 instances across the sham link, which
connects V1 and V2. O1 and O3 can be mapped to one of the sham link
OSPFv3 instances; O2 and O4 can be mapped to the other sham link
OSPFv3 instance.

5.3. OSPF Packet Forwarding on Sham Link

The rules associated with route redistribution, stated in Section
4.2.7.4 of [RFC4577], remain unchanged in this specification.
Specifically:

If the next-hop interface for a particular route is a sham link,
then the PE SHOULD NOT redistribute that route into BGP as a VPN-
IPv6 route.

Any other route advertised in an LSA that is transmitted over a
sham link MUST also be redistributed (by the PE flooding the LSA
over the sham link) into BGP.

When redistributing these LSAs into BGP, they are encoded with the
BGP Extended Communities Attributes, as defined in Section 4.4 of
this document.

When forwarding a packet, if the preferred route for that packet has
the sham link as its next-hop interface, then the packet MUST be
forwarded according to the corresponding BGP route (as defined in
[RFC4364] and [RFC4659]).

6. Multiple Address Family Support

The support of multiple address families (AFs) in OSPFv3 is described
in [RFC5838]. [RFC5838] differentiates between AFs by using reserved
ranges of Instance IDs for each AF.

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The architecture described in this document is fully compatible with
[RFC5838]. The OSPFv3 PE-CE protocol can support multiple address
families across a VPN backbone. All AFs redistributed from OSPFv3
into BGP on a PE MUST contain the BGP Extended Communities Attributes
as described in Section 4.4.

7. Security Considerations

The extensions described in this document are specific to the use of
OSPFv3 as the PE-CE protocol and do not introduce any new security
concerns other than those already defined in Section 6 of [RFC4577].

8. IANA Considerations

An early version of this document resulted in the allocation of
OSPFv3 Route Attributes (0x0004) entry in the BGP IPv6 Address
Specific Extended Community. This allocation is no longer required.
IANA has marked the OSPFv3 Route Attributes (0x0004) entry in the BGP
IPv6 Address Specific Extended Community registry as deprecated. The
BGP Extended Communities Attributes in this document have already
been registered by IANA.

9. Acknowledgments

The authors would like to thank Kelvin Upson, Seiko Okano, Matthew
Everett, Dr. Vineet Mehta, Paul Wells, and Marek Karasek for their
support of this work. Thanks to Peter Psenak, Abhay Roy, Acee
Lindem, Nick Weeds, Robert Hanzl, and Daniel Cohn for their Last Call
comments. Special thanks to Stewart Bryant, Stephen Farrel, and Fred
Baker for their thorough review.

10. References

10.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.

[RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328, April 1998.

[RFC4360] Sangli, S., Tappan, D., and Y. Rekhter, "BGP Extended
Communities Attribute", RFC 4360, February 2006.

[RFC4364] Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private
Networks (VPNs)", RFC 4364, February 2006.

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RFC 6565 OSPFv3 as a PE-CE Routing Protocol June 2012

[RFC4576] Rosen, E., Psenak, P., and P. Pillay-Esnault, "Using a
Link State Advertisement (LSA) Options Bit to Prevent
Looping in BGP/MPLS IP Virtual Private Networks (VPNs)",
RFC 4576, June 2006.

[RFC4577] Rosen, E., Psenak, P., and P. Pillay-Esnault, "OSPF as the
Provider/Customer Edge Protocol for BGP/MPLS IP Virtual
Private Networks (VPNs)", RFC 4577, June 2006.

[RFC4659] De Clercq, J., Ooms, D., Carugi, M., and F. Le Faucheur,
"BGP-MPLS IP Virtual Private Network (VPN) Extension for
IPv6 VPN", RFC 4659, September 2006.

[RFC4760] Bates, T., Chandra, R., Katz, D., and Y. Rekhter,
"Multiprotocol Extensions for BGP-4", RFC 4760, January
2007.

[RFC5340] Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF
for IPv6", RFC 5340, July 2008.

[RFC5838] Lindem, A., Ed., Mirtorabi, S., Roy, A., Barnes, M., and
R. Aggarwal, "Support of Address Families in OSPFv3", RFC
5838, April 2010.

10.2. Informative References

[RFC2547] Rosen, E. and Y. Rekhter, "BGP/MPLS VPNs", RFC 2547, March
1999.

Pillay-Esnault, et al. Standards Track [Page 19]

RFC 6565 OSPFv3 as a PE-CE Routing Protocol June 2012

Authors' Addresses

Padma Pillay-Esnault
Cisco Systems
510 McCarty Blvd.
Milpitas, CA 95035
USA

EMail: [email protected]

Peter Moyer
Pollere, Inc.
325M Sharon Park Drive #214
Menlo Park, CA 94025
USA

EMail: [email protected]

Jeff Doyle
Jeff Doyle and Associates
9878 Teller Ct.
Westminster, CO 80021
USA

EMail: [email protected]

Emre Ertekin
Booz Allen Hamilton
5220 Pacific Concourse Drive
Los Angeles, CA 90045
USA

EMail: [email protected]

Michael Lundberg
Booz Allen Hamilton
8283 Greensboro Drive
McLean, VA 22102
USA

EMail: [email protected]

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