Network Working Group Eric C. Rosen
Internet Draft Peter Psenak
Expiration Date: December 2003 Cisco Systems, Inc.
Padma Pillay-Esnault
Juniper Networks, Inc.
June 2003
OSPF as the PE/CE Protocol in BGP/MPLS VPNs
draft-ietf-l3vpn-ospf-2547-00.txt
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
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Abstract
[VPN] describes a method of providing a VPN service. That method
allows a variety of different protocols to be used as the routing
protocol between the Customer Edge (CE) router and the Provider Edge
(PE) router. However, it does not fully specify the procedures which
must be implemented within the Provider's network when OSPF is used
as the PE/CE routing protocol. This document provides that
specification.
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Table of Contents
1 Specification of Requirements ........................ 2
2 Introduction ......................................... 2
3 Requirements ......................................... 3
4 BGP/OSPF Interaction Procedures for PE routers ....... 5
4.1 Overview ............................................. 5
4.2 Details .............................................. 7
4.2.1 General .............................................. 7
4.2.2 Handling LSAs from the CE ............................ 9
4.2.3 Sham Links ........................................... 12
4.2.3.1 Intra-Area Routes .................................... 12
4.2.3.2 Creating Sham Links .................................. 12
4.2.3.3 Treatment of Sham Links .............................. 14
4.2.4 VPN-IP routes received via BGP ....................... 14
4.2.4.1 Routes from Different Domains ........................ 15
4.2.4.2 Other External Routes ................................ 16
4.2.4.3 Summary Routes ....................................... 16
5 Acknowledgments ...................................... 17
6 Authors' Addresses ................................... 17
7 Normative References ................................. 17
1. 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 RFC 2119.
2. Introduction
[VPN] describes a method by which a Service Provider (SP) can use its
IP backbone to provide a VPN service to customers. In that method, a
customer's edge devices (CE devices) are connected to the provider's
edge routers (PE routers). If the CE device is a router, then the PE
router may become a routing peer of the CE router (in some routing
protocol), and may as a result learn the routes which lead to the
CE's site and which need to be distributed to other PE routers that
attach to the same VPN.
The PE routers which attach to a common VPN use BGP to distribute the
VPN's routes to each other. A CE router can then learn the routes to
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other sites in the VPN by peering with its attached PE router in a
routing protocol. CE routers at different sites do not, however,
peer with each other.
It can be expected that many VPNs will use OSPF as their internal
routing protocol. This does not necessarily mean that the PE routers
need to use OSPF to peer with the CE routers. Each site in a VPN can
use OSPF as its intra-site routing protocol, while using, e.g., BGP
or RIP to distribute routes to a PE router. However, it is certainly
convenient, when OSPF is being used intra-site, to use it on the PE-
CE link as well, and [VPN] explicitly allows this.
Like anything else, the use of OSPF on the PE-CE link has advantages
and disadvantages. The disadvantage to using OSPF on the PE-CE link
is that it gets the PE router involved in a VPN site's IGP, however
peripherally. The advantages though are:
- The administrators of the CE router need not have any expertise
in any routing protocol other than OSPF.
- The CE routers do not need to have support for any routing
protocols other than OSPF.
- If a customer is transitioning his network from a traditional
OSPF backbone to the VPN service described in [VPN], the use of
OSPF on the PE-CE link eases the transitional issues.
It seems likely that some SPs and their customers will resolve these
trade-offs in favor of the use of OSPF on the PE-CE link. Thus we
need to specify the procedures which must be implemented by a PE
router in order to make this possible. (No special procedures are
needed in the CE router though; CE routers just run whatever OSPF
implementations they may have.)
3. Requirements
Consider a set of VPN sites which are thought of as a common "OSPF
domain". These will almost certainly be a set of sites which
together constitute an "intranet", and each of which runs OSPF as its
intra-site routing protocol.
Per [VPN], the VPN routes are distributed among the PE routers by
BGP. If the PE uses OSPF to distribute routes to the CE router, the
standard procedures governing BGP/OSPF interactions [OSPF] would
cause routes from one site to be delivered to another as AS-external
routes (in type 5 LSAs). This is undesirable; it would be much
better to deliver such routes in type 3 LSAs (as inter-area routes),
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so that they can be distinguished from any "real" AS-external routes
that may be circulating in the VPN. (That is, so that they can be
distinguished by OSPF from routes which really do not come from
within the VPN.) Hence it is necessary for the PE routers to
implement a modified version of the BGP/OSPF interaction procedures.
In fact, we would like to have a very general set of procedures which
allows a customer to easily replace a legacy private OSPF backbone
with the VPN service. We would like this procedure to meet the
following set of requirements:
- The procedures should not make assumptions about the OSPF
topology. In particular, it should not be assumed that customer
sites are OSPF stub sites or NSSA sites. Nor should it be
assumed that a customer site contains only one OSPF area, or that
it has no area 0 routers.
- If VPN sites A and B are in the same OSPF domain, then routes
from one should be presented to the other as OSPF intra-network
routes. In general, this can be done by presenting such routes
as inter-area routes, in type 3 LSAs.
Note that this allows two VPN sites to be connected via an "OSPF
backdoor link". That is, one can have an OSPF link between the
two sites which is used only when the VPN backbone is
unavailable. (This would not be possible with the ordinary
BGP/OSPF interaction procedures. The ordinary procedures would
present routes via the VPN backbone as AS-external routes, and
these could never be preferred to intra-network routes.) This
may be very useful during a period of transition from a legacy
OSPF backbone to a VPN backbone.
- It should be possible to make use of an "OSPF backdoor link"
between two sites, even if the two sites are in the same OSPF
area, and neither of the routers attached to the inter-site
backdoor link is an area 0 router. This can also be very useful
during a transition period, and eliminates any need to
reconfigure the sites' routers to be ABRs.
Assuming that it is desired to have the route via the VPN
backbone be preferred to the backdoor route, the VPN backbone
itself must be presented to the CE routers at each site as a link
between the two PE routers to which the CE routers are
respectively attached.
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- CE routers, connected to PE routers of the VPN service, may
themselves function as OSPF backbone (area 0) routers. An OSPF
backbone may even consist of several "segments" which are
interconnected themselves only via the VPN service. In such a
scenario, full intercommunication between sites connected to
different segments of the OSPF backbone should still be possible.
- The transition from the legacy private OSPF backbone to the VPN
service must be simple and straightforward. The transition is
likely to be phased, such that customer sites are migrated one by
one from the legacy private OSPF backbone to the VPN service.
During the transition, any given site might be connected to the
VPN service, to the legacy OSPF backbone, or to both. Complete
connectivity among all such sites must be maintained.
Since the VPN service is to replace the legacy backbone, it must
be possible, by suitable adjustment of the OSPF metrics, to make
OSPF prefer routes which traverse the SP's VPN backbone to
alternative routes which do not.
- The OSPF metric assigned to a given route should be carried
transparently over the VPN backbone.
Routes from sites which are not in the same OSPF domain will appear
as AS-external routes.
We presuppose familiarity with the contents of [OSPF], including the
OSPF LSA types, and will refer without further exegesis to type 1, 2,
3, etc., LSAs. Familiarity with [VPN] is also presupposed.
4. BGP/OSPF Interaction Procedures for PE routers
4.1. Overview
[VPN] defines the notion of a Per-Site Routing and Forwarding Table,
or VRF. A PE router must be capable of running multiple instances of
OSPF, where each instance is associated with a particular VRF.
(Whether these instances are realized as separate processes, or
merely as separate contexts of a common process, is an implementation
matter.)
BGP is used to distribute routes among the set of PE routers that
attach to a single OSPF domain. Per [VPN], these routes are
distributed as VPN-IP routes. Import/export to/from particular VRFs
is governed via Route Targets. To meet the above requirements, a PE
which imports a particular route into a particular VRF needs to know
whether that route comes from the same OSPF domain and the same OSPF
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area as the CEs to which it is attached. Our procedure is to encode
this information as BGP Extended Communities attributes [EXT], and
have BGP distribute it along with the VPN-IP route. The OSPF metric
of a route is also carried as a BGP attribute of the route.
If two PEs attach to different VPN sites that are in the same OSPF
area (as indicated by the OSPF area number), the PE/CE links to those
site MAY be treated as links within that area. In addition, each PE
MAY flood, into that area, a type 1 LSA advertising a link to the
other PE. If this procedure is followed, two VPN sites in the same
OSPF area will see the VPN backbone as a link within that area, a
link between the two PE routers. We refer to this link as a "sham
link". This allows routes from one site to the other to be treated
as intra-area routes. The procedures governing the use of sham links
are specified in Section 4.2.3.
Every PE attached to a particular OSPF network MUST function as an
OSPF area 0 router. This allows it to distribute inter-area routes to
the CE via Type 3 LSAs. The CE router might or might not be an area
0 router, and the PE/CE link might or might not be an area 0 link.
If the OSPF domain has any area 0 routers (other than the PE
routers), then at least one of those MUST be a CE router, and MUST
have an area 0 link to at least one PE router. This adjacency MAY be
via an OSPF virtual link. This is necessary to ensure that inter-area
routes and AS-external routes can be leaked between the PE routers
and the non-PE OSPF backbone.
When a type 3 LSA is sent from a PE router to a CE router, the DN bit
[OSPF-DNbit] in the LSA Options field MUST be set. This is used to
ensure that if any CE router sends this type 3 LSA to a PE router,
the PE router will not further redistribute it.
Two sites which are not in the same OSPF area will see the VPN
backbone as being an integral part of the OSPF backbone. However, if
there are area 0 routers which are NOT PE routers, then the VPN
backbone actually functions as a sort of higher level backbone,
providing a third level of hierarchy above area 0. This allows a
legacy OSPF backbone to become disconnected during a transition
period, as long as the various segments all attach to the VPN
backbone.
When a PE router needs to distribute to a CE router a route which
comes from a site outside the latter's OSPF domain, the PE router
presents itself as an ASBR, and distributes the route in a type 5
LSA. The DN bit [OSPF-DNbit] MUST be set in these LSAs, to ensure
that they will be ignored by any other PE routers that receive them.
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There are deployed implementations which do not set the DN bit, but
instead use OSPF route tagging to ensure that a type 5 LSA generated
by a PE router will be ignored by any other PE router that may
receive it. A special OSPF route tag, which we will call the VPN
Route Tag (see section 4.2.1), is used for this purpose. To ensure
backwards compatibility, all implementations adhering to this
specification MUST by default support the VPN Route Tag procedures
specified in sections 4.2.1 and 4.2.2. Support for the VPN Route Tag
procedures MAY be disabled, when it is no longer necessary.
4.2. Details
4.2.1. General
If a PE and a CE are communicating via OSPF, the PE MUST create, and
MUST flood to the CE, a type 1 LSA advertising its link to the CE.
The PE MUST have an OSPF router id which is valid (i.e., unique)
within the OSPF domain. The PE MUST also be configured to know which
OSPF area the link is in.
A PE-CE link may be in any area, including area 0; this is a matter
of the OSPF configuration.
Whether or not the PE-CE link is an area 0 link, the PE MUST function
as an OSPF area 0 router. That is, the link state topology from a
site will NOT be passed along by the PE.
The PE MUST support at least one OSPF instance for each OSPF domain
to which it attaches. Each instance of OSPF MUST be associated with
a single VRF. If n CEs associated with that VRF are running OSPF on
their respective PE/CE links, then those n CEs are OSPF adjacencies
of the PE in the corresponding instance of OSPF. Generally, though
not necessarily, if the PE attaches to several CEs in the same OSPF
domain, it will associate the interfaces to those PEs with a single
VRF.
If the OSPF domain has any area 0 routers (other than the PE
routers), then at least one of those MUST be a CE router, and MUST
have an area 0 link to at least one PE router. This adjacency MAY be
via an OSPF virtual link.
Each OSPF domain MUST be associated with a Domain Identifier. This
MUST be configurable, and the default value (if none is configured)
SHOULD be NULL. More precisely, each VRF associated with an OSPF
instance will either be configured with one or more Domain
Identifiers, or else will use NULL as its Domain Identifier.
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A VRF MAY be configured with several Domain Identifier values. In
this case, one of these is considered to be the "primary" Domain
Identifier; this MUST be determinable by configuration. If a VRF has
exactly one Domain Identifier configured, this is of course its
primary Domain Identifier. If a VRF is configured with more than one
Domain Identifiers, the NULL Domain Identifier MUST NOT be one of
them.
With respect to the set of VRFs in a given OSPF domain, one of the
following two conditions MUST hold:
1. They all have the NULL Domain Identifier, OR
2. Each VRF is configured with a set of Domain Identifiers, and in
this set is the primary Domain Identifier of every other VRF in
the domain.
A Domain Identifier is an eight-byte quantity which is a valid BGP
Extended Communities attribute, as specified in section 4.2.2. If a
particular VRF has a single non-NULL Domain Identifier, when routes
from that VRF are distributed as by BGP as VPN-IPv4 routes, the
routes MUST carry the corresponding Domain Identifier Extended
Communities attribute. If the VRF has more than one Domain
Identifier, the routes MUST carry the the Domain Identifier Extended
Communities attribute corresponding to the VRF's primary Domain
Identifier. If the VRF's Domain Identifier is NULL, the Domain
Identifier Extended Communities attribute MAY be omitted when routes
from that VRF are distributed; alternatively, a value of the Domain
Identifier Extended Communities attribute which represents NULL (see
section 4.2.2) MAY be carried with the route.
If a particular OSPF domain has the NULL Domain Identifier, care must
be taken to avoid having routes from that OSPF domain and routes from
another OSPF domain imported into the same VRF.
If a particular VRF in a PE is associated with an instance of OSPF,
then by default it MUST be configured with a special OSPF route tag
value, which we call the VPN Route Tag. By default, this route tag
MUST be included in the Type 5 LSAs which the PE originates (as the
result of receiving a BGP-distributed VPN-IP route, see section
4.2.4) and sends to any of the attached CEs.
The configuration and inclusion of the VPN Route Tag is required for
backwards compatibility with deployed implementations that do not set
the DN bit in type 5 LSAs. The inclusion of the VPN Route Tag MAY be
disabled, if it has been determined that it is no longer needed for
backwards compatibility.
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The value of the VPN Route Tag is arbitrary, but must be distinct
from any OSPF Route Tag being used within the OSPF domain. Its value
MUST therefore be configurable. If the Autonomous System number is
two bytes long, the default value SHOULD be an automatically computed
tag based on the Autonomous System number of the VPN backbone:
Tag = <Automatic = 1, Complete = 1, PathLength = 01>
0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 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 0 180
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|1|0|1| ArbitraryTag | AutonomousSystem |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
1 1 0 1 0 0 0 0 0 0 0 0 0 0 0 0 _AS number of the VPN Backbone_
If the Autonomous System number is four bytes long, then a Route Tag
value MUST be configured, and it MUST be distinct from any Route Tag
used within the VPN itself.
If a PE router needs to use OSPF to distribute to a CE router a route
which comes from a site outside the CE router's OSPF domain, the PE
router SHOULD present itself to the CE router as an Autonomous System
Border Router (ASBR), and SHOULD report such routes as AS-external
routes. That is, these PE routers originate Type 5 LSAs reporting
the extra-domain routes as AS-external routes. Each such Type 5 LSA
MUST contain an OSPF route tag whose value is that of the VPN Route
Tag. This tag identifies the route as having come from a PE router.
The VPN Route Tag MUST be used to ensure that a Type 5 LSA originated
by a PE router is not redistributed through the OSPF area to another
PE router.
4.2.2. Handling LSAs from the CE
This section specifies the way in which a PE router handles the OSPF
LSAs it receives from a CE router.
When a PE router receives, from a CE router, any LSA with the DN bit
[OSPF-DN] set, the information from that LSA MUST NOT be used by the
SPF computation. If a Type 5 LSA is received from the CE, and if it
has an OSPF route tag value equal to the VPN Route Tag (see section
4.2.1), then the information from that LSA MUST NOT be used by the
SPF computation.
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Otherwise, the PE must examine the corresponding VRF. For every
address prefix which appears there, the PE must create a VPN-IPv4
route in BGP. Each such route will have some of the following
Extended Community attributes:
- The OSPF Domain Identifier Extended Communities attribute. This
MUST be present, UNLESS the VRF has a NULL Domain Identifier, in
which case it MAY be omitted. This attribute is encoded with a
two-byte type field, and its type is either 0005, 0105, or 0205.
For backwards compatibility, the type 8005 MAY be used as well,
and is treated as if it were 0005. If the VRF has a NULL Domain
Identifier, and the OSPF Domain Identifier Extended Communities
attribute is NOT omitted, then the attribute's value field must
be all zeroes, and its type field may be any of 0005, 0105, 0205,
or 8005.
- OSPF Route Type Extended Communities Attribute. This attribute
MUST be present. It is encoded with a two-byte type field, and
its type is 0306. The type 8000 SHOULD be accepted as well, to
ensure backwards compatibility. The remaining six bytes of the
Attribute are encodes as follows:
* Area Number: 4 bytes, encoding a 32-bit area number. For AS-
external routes, the value is 0. A non-zero value identifies
the route as being internal to the OSPF domain, and as being
within the identified area. Area numbers are relative to a
particular OSPF domain.
* OSPF Route Type: 1 byte, encoded as follows:
** 1 or 2 for intra-area routes (depending on whether the
route came from a type 1 or a type 2 LSA -- however this
difference is not significant to the procedures
specified herein)
** 3 for summary routes
** 5 for external routes (area number must be 0)
** 7 for NSSA routes.
** 129 for Sham Link Endpoint Addresses. See section
4.2.3.2 for the specification of when this value must be
used.
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* Options: 1 byte. Currently this is only used if the route
type is 5 or 7. Setting the least significant bit in the
field indicates that the route carries a type 2 metric.
- OSPF Router Id Extended Communities Attribute. This OPTIONAL
attribute specifies the OSPF Router Id of the system that is
identified in the BGP Next Hop attribute. More precisely, it
specifies the Router Id of the particular VRF from which this
route was exported. This attribute is encoded with a two-byte
type field, and its type is 0107, with the router id itself
carried in the first 4 bytes of the value field. The type 8001
SHOULD be accepted as well, to ensure backwards compatibility,
and should be treated as if it were 0107.
- MED. By default, this SHOULD be set to the value of the OSPF
distance associated with the route, plus 1.
The intention of all this is the following. OSPF Routes from one site
are converted to BGP, distributed across the VPN backbone, and
possibly converted back to OSPF routes before being distributed into
another site. With these attributes, BGP carries enough information
about the route to enable the route to be converted back into OSPF
"transparently", just as if BGP had not been involved.
Routes which a PE receives in type 4 LSAs MUST be ignored.
The attributes specified above are in addition to any other
attributes which routes must carry in accord with the [VPN].
The Site of Origin attribute, which is usually required by [VPN], is
OPTIONAL for routes which a PE learns from a CE via OSPF.
Use of the Site of Origin attribute would, in the case of a multiply
homed site (i.e., a site attached to several PE routers), prevent an
intra-site route from being reinjected into a site from the VPN
backbone. Such a reinjection would not harm the routing, because the
route via the VPN backbone would be advertised in a type 3 LSA, and
hence would appear to be an inter-area route; the real intra-area
route would be preferred. But unnecessary overhead would be
introduced. On the other hand, if the Site of Origin attribute is not
used, a partitioned site will find itself automatically repaired,
since traffic from one partition to the other will automatically
travel via the VPN backbone. Therefore the use of a Site of Origin
attribute is optional, so that a trade-off can be made between the
cost of the increased overhead and the value of automatic partition
repair.
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4.2.3. Sham Links
4.2.3.1. Intra-Area Routes
Suppose there are two sites in the same OSPF area. Each site is
attached to a different PE router, and there is also an intra-area
OSPF link connecting the two sites.
It is possible to treat these two sites as a single VPN site which
just happens to be multihomed to the backbone. This is in fact the
simplest thing to do, and is perfectly adequate, providing that the
preferred route between the two sites is via the intra-area OSPF link
(a "backdoor link"), rather than via the VPN backbone. There will be
routes between sites that go through the PE routers, but these routes
will appear to be inter-area routes, and OSPF will consider them to
be less preferable than the intra-area routes through the backdoor
link.
If it is desired to have OSPF prefer the routes through the backbone
over the routes through the backdoor link, then the routes through
the backbone must be appear to be intra-area routes. To make a route
through the backbone appear to be an intra-area route, it is
necessary to make it appear as if there is an intra-area link
connecting the two PE routers. This is what we refer to as a "sham
link". (If the two sites attach to the same PE router, this of
course is not necessary.)
A sham link can be thought of as a relation between two VRFs. If two
VRFs are to be connected by a sham link, each VRF must be associated
with a "Sham Link Endpoint Address", a /32 address which is treated
as an address of the PE router containing that VRF. The Sham Link
Endpoint Address associated with a VRF MUST be configurable, and it
MAY default to the VRF's router id. The Sham Link Endpoint Address
is an address in the VPN's address space, not the SP's address space.
A VRF needs only a single Sham Link Endpoint Address, no matter how
many sham links it has. It MUST be distributed by BGP as a VPN-IPv4
address, and it MAY be distributed by OSPF.
4.2.3.2. Creating Sham Links
Sham links may be manually configured, or they may be auto-
configured.
Each VRF that is associated with a PE-CE link on which OSPF is
running MUST be configurable as to whether auto-configuration of sham
links to/from that VRF is allowed. The default MUST be "manual
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configuration".
If a VRF is configured for manual configuration of the sham links, it
will never initiate auto-configuration of a sham link, nor will it
ever create a sham link as the result of a remotely initiated auto-
configuration procedure. If a VRF is configured for auto-
configuration of sham links, manual configuration of particular sham
links is still allowed. In any event, no more than one sham link
with the same pair of sham link endpoint addresses will ever be
created.
To manually configure a sham link between two VRFs, each VRF has to
be configured to create a sham link to the other, where the "other"
is identified by its sham link endpoint address. Procedures for
single-ended manual configuration of the sham link are for further
study.
IF a VRF is configured for auto-configuration of sham links, it MUST
distribute, via BGP, a VPN-IP route corresponding to the Sham Link
Endpoint Address. This route MUST have the OSPF Route Type Extended
Community attribute, with the OSPF Route Type field set to "Sham Link
Endpoint Address".
When a PE receives such a route, with a RT value that allows the
route to be imported into a particular VRF, then if
- that route has an OSPF Domain Identifier Extended Communities
attribute which is identical to one of the VRF's Domain
Identifiers, or the route has no OSPF Domain Identifier Extended
Communities attribute and the VRF has a NULL Domain Identifier,
AND
- that route has an OSPF area number which matches that of the VRF,
then a sham link may be created from the local VRF to the VRF
identified by the sham link endpoint address.
When comparing two OSPF Domain Identifier Extended Communities
attributes for equality, all eight bytes of the Extended Communities
attribute must be compared. The two attributes are regarded as equal
only if they are identical in all eight bytes, or if the lower order
six bytes are identical, and one attribute has two high order bytes
of 0005 and the other has two high order bytes of 8005.
If all VRFs are configured for auto-configuration of sham links, a
full mesh of sham links will be created among all the sites which are
in the same OSPF area. This may not be what is desired. To obtain
more control over the set of sham links which are created, some or
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all of the VRFs can be configured to disable auto-configuration of
the sham links.
Note that sham links may be created for any area, including area 0.
4.2.3.3. Treatment of Sham Links
Sham links SHOULD be treated by OSPF as OSPF Demand Circuits. This
means that LSAs will be flooded over them, but periodic refresh
traffic is avoided. Note that, as long as the backdoor link is up,
flooding the LSAs over the sham link serves no purpose. However, if
the backdoor link goes down, OSPF does not have mechanisms enabling
the routers in one site to rapidly flush the LSAs from the other
site. Therefore it is still necessary to maintain synchronization
among the LSA databases at the two sites, hence the flooding over the
sham link.
The sham link is an unnumbered point-to-point intra-area link, and is
advertised by means of a type 1 LSA.
The OSPF metric associated with a sham link must be configurable (and
there must be a configurable default). Whether traffic between the
sites flows via a backdoor link or via the VPN backbone (i.e., via
the sham link) depends on the settings of the OSPF link metrics. The
metrics can be set so that the backdoor link is not used unless
connectivity via the VPN backbone fails, for example.
The default Hello Interval for sham links is 10 seconds, and the
default Router Dead Interval for sham links is 40 seconds.
If a PE determines that a particular route traverses the sham link,
then the PE SHOULD NOT redistribute that route into BGP as a VPN-IPv4
route.
4.2.4. VPN-IP routes received via BGP
This section describes how the PE router handles VPN-IP routes
received via BGP.
If a received BGP VPN-IP route is not installed in the VRF, nothing
is reported to the CE. A received route will not be installed into
the VRF if some other route is preferable. (Note that a route which
is not installed in the VRF may still cause the PE to create an OSPF
link to another PE as specified in the previous section.)
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Note that according to the usual OSPF route preference rules, intra-
area routes, as computed by the OSPF, will be installed in the VRF in
preference to any other routes received over BGP. This means that the
CE will simply not hear about inter-area or external routes to
address prefixes for which there is an intra-area route.
In the following, we specify what is reported, in OSPF LSAs, by the
PE to the CE, assuming that the PE is not configured to do any
further summarization or filtering of the routing information before
reporting it to the CE.
4.2.4.1. Routes from Different Domains
To determine how to process an a received VPN-IP route, it is
necessary to compare the OSPF Domain Identifier Extended Communities
attribute carried by the route (if any) with the OSPF Domain
Identifier Extended Communities attribute(s) with which the VRF has
been configured (if any). In general, when two such attributes are
compared, all eight bytes must be compared. Thus two OSPF Domain
Identifier Extended Communities attributes are regarded as equal if
and only if one of the following three conditions holds:
1. They are identical in all eight bytes.
2. They are identical in their lower order six bytes (value
field), but one attribute has two high order bytes (type field)
of 0005 and the other has two high order bytes (type field) of
8005. (This condition is for backwards compatibility.)
3. The lower order six bytes (value field) of both attributes
consist entirely of zeroes. In this case, the two attributes
are considered to be identical irrespective of their type
fields, and they are regarded as representing the NULL Domain
Identifier.
If one of the following three conditions holds, a VPN-IP route
received via BGP is regarded as being from a different domain than
the VRF into which the route is being installed.
1. The VRF has a non-NULL OSPF Domain Identifier, but either
a) the route has no OSPF Domain Identifier Extended
Communities attribute, or
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b) the route has an OSPF Domain Identifier Extended
Communities attribute whose value field consists of all
zeroes
2. the route has an OSPF Domain Identifier Extended Communities
attribute whose value field does not consist of all zeroes, and
the VRF is configured with the NULL Domain Identifier (or with
any encoding of the NULL Domain Identifier as specified above)
3. the router has an OSPF Domain Identifier Extended Communities
attribute which is not identical to any of the OSPF Domain
Identifiers associated with the VRF.
If any of these three conditions holds, the route is distributed to
the CE in a type 5 LSA. The VPN Route Tag (see section 4.2.1) MUST
be placed in the LSA. By default, a type 2 metric value is included
in the LSA, and by default, its value is taken from the MED attribute
of the VPN-IP route. If the MED is not present, a default metric
value is used.
4.2.4.2. Other External Routes
If the route has an OSPF route type of external route, it MUST be
advertised to the CE in a type 5 LSA. The VPN Route Tag (see section
4.2.1) MUST be placed in the LSA. By default, the MED, if present,
is converted to a type 1 or type 2 metric, as determined by the
external route property of the VPN-IPv4 route. If no MED is present,
a default type 2 metric value is used.
Note that this way of handling AS-external routes makes every PE
appear to be an ASBR attached to all the AS-external routes. In a
multihomed site, this can result in a number of type 5 LSAs
containing the same information.
4.2.4.3. Summary Routes
If the conditions of the previous two sections do not hold, then the
route should be treated as if it had been received in an OSPF type 3
LSA. This means that the PE will report the route in a type 3 LSA to
the CE. (Note that this case is possible even if the VPN-IP route
carries an area number identical to that of the CE router. This
means that if an area is "partitioned" such that the two pieces are
connected only via the VPN backbone, it appears to be two areas, with
inter-area routes between them.)
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5. Acknowledgments
Significant contributions to this work have been made by Derek Yeung
and Yakov Rekhter.
Thanks to Ross Callon and Ajay Singhal for their comments.
6. Authors' Addresses
Eric C. Rosen
Cisco Systems, Inc.
1414 Massachusetts Avenue
Boxborough, MA 01719
E-mail: erosen@cisco.com
Peter Psenak
Parc Pegasus,
De Kleetlaan 6A
1831 Diegem
Belgium
E-mail: ppsenak@cisco.com
Padma Pillay-Esnault
Juniper Networks
1194 N. Mathilda Avenue
Sunnyvale, CA 94089
E-mail: padma@juniper.net
7. Normative References
[EXT] "BGP Extended Communities Attribute", draft-ietf-idr-bgp-ext-
communities-05.txt>, Sangli, S., Tappan, D., Rekhter, Y., May 2002
[OSPF] "OSPF Version 2", RFC 2328, Moy, J., April 1998
[OSPF-DN] "Using an LSA Options Bit to Prevent Looping in BGP/MPLS IP
VPNs", draft-ietf-ospf-2547-dnbit-00.txt, Rosen, Psenak, Pillay-
Rosen, et al. [Page 17]
Internet Draft draft-ietf-ppvpn-ospf-2547-00.txt June 2003
Esnault, June 2003
[VPN] "BGP/MPLS VPNs", draft-ietf-ppvpn-rfc2547bis-03.txt, Rosen,
Rekhter, et. al. May 2003
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