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Table of Contents
1. Introduction ....................................................22. Requirements Language ...........................................33. IP VPN as a Route Server ........................................34. Path Attributes .................................................55. BGP Customer Route Attributes ...................................66. Next-Hop Handling ...............................................77. Exchanging Routes between Different VPN Customer Networks .......88. Deployment Considerations ......................................109. Security Considerations ........................................1210. IANA Considerations ...........................................1211. Acknowledgments ...............................................1212. References ....................................................1312.1. Normative References .....................................1312.2. Informative References ...................................131. Introduction
In current deployments, when BGP is used as the Provider/Customer
Edge routing protocol, these peering sessions are typically
configured as an external peering between the VPN provider autonomous
system (AS) and the customer network autonomous system. At each
External BGP boundary, BGP path attributes [RFC4271] are modified as
per standard BGP rules. This includes prepending the AS_PATH
attribute with the autonomous-system number of the originating
Customer Edge (CE) router and the autonomous-system number(s) of the
Provider Edge (PE) router(s).
In order for such routes not to be rejected by AS_PATH loop
detection, a PE router advertising a route received from a remote PE
often remaps the customer network autonomous-system number to its
own. Otherwise, the customer network can use different autonomous-
system numbers at different sites or configure their CE routers to
accept routes containing their own AS number.
While this technique works well in situations where there are no BGP
routing exchanges between the client network and other networks, it
does have drawbacks for customer networks that use BGP internally for
purposes other than interaction between CE and PE routers.
In order to make the usage of BGP/MPLS VPN services as transparent as
possible to any external interaction, it is desirable to define a
mechanism by which PE-CE routers can exchange BGP routes by means
other than External BGP.
One can consider a BGP/MPLS VPN as a provider-managed backbone
service interconnecting several customer-managed sites. While this
model is not universal, it does constitute a good starting point.
Independently of the presence of VPN service, networks often use a
hierarchical design utilizing either BGP route reflection [RFC4456]
or confederations [RFC5065]. This document assumes that the IP VPN
service interacts with the customer network following a similar
2. Requirements Language
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 [RFC2119].
3. IP VPN as a Route Server
In a typical backbone/area hierarchical design, routers that attach
an area (or site) to the core use BGP route reflection (or
confederations) to distribute routes between the top-level core
Internal BGP (iBGP) mesh and the local area iBGP cluster.
To provide equivalent functionality in a network using a provider-
provisioned backbone, one can consider the VPN as the equivalent of
an Internal BGP Route Server that multiplexes information from _N_
VPN attachment points.
A route learned by any of the PEs in the IP VPN is available to all
other PEs that import the Route Target used to identify the customer
network. This is conceptually equivalent to a centralized route
In a PE router, PE-received routes are not advertised back to other
PEs. It is this split-horizon technique that prevents routing loops
in an IP VPN environment. This is also consistent with the behavior
of a top-level mesh of route reflectors (RRs).
In order to complete the Route Server model, it is necessary to be
able to transparently carry the Internal BGP path attributes of
customer network routes through the BGP/MPLS VPN core. This is
achieved by using a new BGP path attribute, described below, that
allows the customer network attributes to be saved and restored at
the BGP/MPLS VPN boundaries.
When a route is advertised from PE to CE, if it is advertised as an
iBGP route, the CE will not advertise it further unless it is itself
configured as a route reflector (or has an External BGP session).
This is a consequence of the default BGP behavior of not advertising
iBGP routes back to iBGP peers. This behavior is not modified.
On a BGP/MPLS VPN PE, a CE-received route MUST be advertised to other
VPN PEs that import the Route Targets that are associated with the
route. This is independent of whether the CE route has been received
as an external or internal route. However, a CE-received route is
not re-advertised back to other CEs unless route reflection is
explicitly configured. This is the equivalent of disabling client-
to-client reflection in BGP route reflection implementations.
When reflection is configured on the PE router, with local CE routers
as clients, there is no need to internally mesh multiple CEs that may
exist in the site.
This Route Server model can also be used to support a confederation-
style abstraction to CE devices. At this point, we choose not to
describe in detail the procedures for that mode of operation.
Confederations are considered to be less common than route reflection
in enterprise environments.
4. Path Attributes
--> push path attributes --> vrf-export --> BGP/MPLS IP VPN
VRF route PE-PE route
<-- pop path attributes <-- vrf-import <--
The diagram above shows the BGP path attribute stack processing in
relation to existing BGP/MPLS IP VPN [RFC4364] route processing
procedures. BGP path attributes received from a customer network are
pushed into the stack, before adding the Export Route Targets to the
BGP path attributes. Conversely, the stack is popped following the
Import Target processing step that identifies the VPN Routing and
Forwarding (VRF) table in which a PE-received route is accepted.
When the advertising PE performs a "push" operation at the
"vrf-export" processing stage, it SHOULD initialize the attributes of
the BGP IP VPN route advertisement as it would for a locally
originated route from the respective VRF context.
When a PE-received route is imported into a VRF, its IGP metric, as
far as BGP path selection is concerned, SHOULD be the metric to the
remote PE address, expressed in terms of the service provider metric
For the purposes of VRF route selection performed at the PE, between
routes received from local CEs and remote PEs, customer network IGP
metrics SHOULD always be considered higher (and thus least preferred)
than local site metrics.
When backdoor links are present, this would tend to direct the
traffic between two sites through the backdoor link for BGP routes
originated by a remote site. However, BGP already has policy
mechanisms, such as the LOCAL_PREF attribute, to address this type of
When a given CE is connected to more than one PE, it will not
advertise the route that it receives from a PE to another PE unless
configured as a route reflector, due to the standard BGP route
When a CE reflects a PE-received route to another PE, the fact that
the original attributes of a route are preserved across the VPN
prevents the formation of routing loops due to mutual redistribution
between the two networks.
5. BGP Customer Route Attributes
In order to transparently carry the BGP path attributes of customer
routes, this document defines a new BGP path attribute:
ATTR_SET (type code 128)
ATTR_SET is an optional transitive attribute that carries a set of
BGP path attributes. An attribute set (ATTR_SET) can include any
BGP attribute that can occur in a BGP UPDATE message, except for
the MP_REACH and MP_UNREACH attributes.
The ATTR_SET attribute is encoded as follows:
| Attr Flags (O|T) Code = 128 |
| Attr. Length (1 or 2 octets) |
| Origin AS (4 octets) |
| Path Attributes (variable) |
The Attribute Flags are encoded according to RFC 4271 [RFC4271]. The
Extended Length bit determines whether the Attribute Length is one or
The attribute value consists of a 4-octet "Origin AS" value followed
by a variable-length field that conforms to the BGP UPDATE message
path attribute encoding rules. The length of this attribute is 4
plus the total length of the encoded attributes.
The ATTR_SET attribute is used by a PE router to store the original
set of BGP attributes it receives from a CE. When a PE router
advertises a PE-received route to a CE, it will use the path
attributes carried in the ATTR_SET attribute.
In other words, the BGP path attributes are "pushed" into this
attribute, which operates as a stack, when the route is received by
the VPN and "popped" when the route is advertised in the PE-to-CE
Using this mechanism isolates the customer network from the
attributes used in the customer network and vice versa. Attributes
such as the route reflection cluster list attribute are segregated
such that customer network cluster identifiers won't be considered by
the customer network route reflectors and vice versa.
The Origin autonomous-system number is designed to prevent a route
originating in a given autonomous-system iBGP from being leaked into
a different autonomous system without proper AS_PATH manipulation.
It SHOULD contain the autonomous-system number of the customer
network that originates the given set of attributes. The value is
encoded as a 32-bit unsigned integer in network byte order,
regardless of whether or not the originating PE supports 4-octet AS
The AS_PATH and AGGREGATOR attributes contained within an ATTR_SET
attribute MUST be encoded using 4-octet AS numbers [RFC4893],
regardless of the capabilities advertised by the BGP speaker to which
the ATTR_SET attribute is transmitted. BGP speakers that support the
extensions defined in this document MUST also support RFC 4893
[RFC4893]. The reason for this requirement is to remove ambiguity
between 2-octet and 4-octet AS_PATH attribute encoding.
The NEXT_HOP attribute SHOULD NOT be included in an ATTR_SET. When
present, it SHOULD be ignored by the receiving PE. Future
applications of the ATTR_SET attribute MAY define meaningful
semantics for an included NEXT_HOP attribute.
The ATTR_SET attribute SHALL be considered malformed if any of the
o Its length is less than 4 octets.
o The original path attributes carried in the variable-length
attribute data include the MP_REACH or MP_UNREACH attribute.
o The included attributes are malformed themselves.
An UPDATE message with a malformed ATTR_SET attribute SHALL be
handled as follows. If its Partial flag is set and its
Neighbor-Complete flag is clear, the UPDATE is treated as a route
withdraw as discussed in [OPT-TRANS-BGP]. Otherwise (i.e., Partial
flag is clear or Neighbor-Complete is set), the procedures of the
BGP-4 base specification [RFC4271] MUST be followed with respect to
an Optional Attribute Error.
6. Next-Hop Handling
When BGP/MPLS VPNs are not in use, the NEXT_HOP attribute in iBGP
routes carries the address of the border router advertising the route
into the domain. The IGP distance to the NEXT_HOP of the route is an
important component of BGP route selection.
When a BGP/MPLS VPN service is used to provide interconnection
between different sites, since the customer network runs a different
IGP domain, metrics between the provider and customer networks are
However, the most important component of a metric is the inter-area
metric, which is known to the customer network. The intra-area
metric is typically negligible.
The use of route reflection, for instance, requires metrics to be
configured so that inter-cluster/area metrics are always greater than
The approach taken by this document is to rewrite the NEXT_HOP
attribute at the VRF import/export boundary. PE routers take into
account the PE-PE IGP distance calculated by the customer network
IGP, when selecting between routes advertised from different PEs.
An advantage of the proposed method is that the customer network can
run independent IGPs at each site.
7. Exchanging Routes between Different VPN Customer Networks
In the traditional model, where External BGP sessions are used
between the BGP/MPLS VPN PE and CE, the PE router identifies itself
as belonging to the customer network autonomous system.
In order to use Internal BGP sessions, the PE router has to identify
itself as belonging to the customer AS. More specifically, the VRF
that is used to interconnect to that customer site is assigned to the
customer AS rather than the VPN provider AS.
The Origin AS element in the ATTR_SET path attribute conveys the
AS number of the originating VRF. This AS number is used in a
receiving PE in order to identify route exchanges between VRFs in
In scenarios such as what is commonly referred to as an "extranet"
VPN, routes MAY be advertised to both internal and external VPN
attachments belonging to different autonomous systems.
| PE1 |-----------------| PE2 |
/ \ |
+-----+ +-----+ +-----+
| CE1 | | CE2 | | CE3 |
+-----+ +-----+ +-----+
AS 1 AS 2 AS 1
Consider the example given above, where (PE1, CE1) and (PE2, CE3)
sessions are iBGP. In BGP/MPLS VPNs, a route received from CE1 above
may be distributed to the VRFs corresponding to the attachment points
for CEs 2 and 3.
The desired result in such a scenario is to present the internal peer
(CE3) with a BGP advertisement that contains the same BGP path
attributes received from CE1, and to present the external peer (CE2)
with a BGP advertisement that would correspond to a situation where
AS 1 and AS 2 have an External BGP session between them.
In order to achieve this goal, the following set of rules applies:
When importing a VPN route that contains the ATTR_SET attribute
into a destination VRF, a PE router MUST check that the "Origin
AS" number contained in the ATTR_SET attribute matches the
autonomous system associated with the VRF.
In case the autonomous-system numbers do match, the route is
imported into the VRF with the attributes contained in the
ATTR_SET attribute. Otherwise, in the case of an autonomous-
system number mismatch, the set of attributes to be associated
with the route SHALL be constructed as follows:
1. The path attributes are set to the attributes contained in the
2. iBGP-specific attributes are discarded (LOCAL_PREF,
ORIGINATOR, CLUSTER_LIST, etc).
3. The "Origin AS" number contained in the ATTR_SET attribute
is prepended to the AS_PATH following the rules that would
apply to an External BGP peering between the source and
4. If the autonomous system associated with the VRF is the same
as the VPN provider autonomous system and the AS_PATH
attribute of the VPN route is not empty, it SHALL be prepended
to the AS_PATH attribute of the VRF route.
When advertising the VRF route to an External BGP peer, a PE
router SHALL apply steps 1 to 4 defined above and subsequently
prepend its own autonomous-system number to the AS_PATH attribute.
For example, if the route originated in a VRF that supports
Internal BGP peering and the ATTR_SET attribute and is advertised
to a CE that is configured in the traditional External BGP mode,
then the originator AS, the VPN AS_PATH segment, and the customer
network AS are prepended to the AS_PATH.
When importing a route without the ATTR_SET attribute to a VRF
that is configured in a different autonomous system, a PE router
MUST prepend the VPN provider AS number to the AS_PATH.
In all cases where a route containing the ATTR_SET attribute is
imported, attributes present on the VPN route other than the NEXT_HOP
attribute are ignored, both from the point of view of route selection
in the VRF Adj-RIB-In and route advertisement to a CE router. In
other words, the information contained in the ATTR_SET attribute
overrides the VPN route attributes on "vrf-import".
8. Deployment Considerations
It is RECOMMENDED that different VRFs of the same VPN (i.e., in
different PE routers) that are configured with iBGP PE-CE peering
sessions use different Route Distinguisher (RD) values. Otherwise
(in the case where the same RD is used), the BGP IP VPN
infrastructure may select a single BGP customer path for a given IP
Network Layer Reachability Information (NLRI) without access to the
detailed path information that is contained in the ATTR_SET
As mentioned previously, the model for this service is a "Route
Server" where the IP VPN provides the customer network with all the
BGP paths known by the CEs. This effectively implies the use of
unique RDs per VRF.
The stated goal of this extension is to isolate the customer network
from the BGP path attribute operations performed by the IP VPN and
conversely isolate the service provider network from any attributes
injected by the customer. For instance, BGP communities can be used
to influence the behavior of the IP VPN infrastructure. Using this
extension, the service provider network can transparently carry these
attributes without interfering with its operations.
Another example of unwanted interaction between customer and IP VPN
BGP attributes is a scenario where the same service provider
autonomous-system number is used to provide Internet service as well
as the IP VPN service. In this case, it is not uncommon to have a
VPN customer route contain the AS number of the service provider.
The IP VPN should work transparently in this case as in all others.
This protocol extension is designed to behave such that each PE VRF
operates as a router in the configured AS. Previously, VRFs operated
in the provider network AS only. The VPN backbone provides
interconnection between VRFs of the same AS, as well as
interconnection between different ASes (subject to the appropriate
policies). When interconnecting VRFs in the same AS, the VPN
backbone operates as a top-level route reflection mesh. When
interconnecting VRFs in different ASes, the provider network provides
an implicit peering relationship between the ASes that originate and
import a specific route.
This extension is also applicable to scenarios where the VPN backbone
spans multiple ASes. When the VPN backbone Inter-AS operation
follows option b) or c) as defined in Section 10 of [RFC4364], the
provider networks are able to influence the route attributes and
route selection of the VPN routes while providing a transparent
service to the customer AS. Either Internal BGP connectivity or
extranets can be provided to the customer AS.
When VPN provider networks interconnect via option a), there is no
possibility of providing a fully transparent service. By definition,
option a) implies that each autonomous-system border router (ASBR)
has a VRF associated with the customer VPN that is configured to
operate in the respective provider AS. These ASBR VRFs then
communicate via External BGP with their peer provider ASes.
In this case, it is still possible to have all the customer VRFs with
one provider network be configured in the same customer AS. This
customer AS will then peer with the provider AS implicitly at the
ASBR, which will in turn peer explicitly with a second provider AS.
This is not, however, a scenario in which transparency to the
customer AS is possible.
9. Security Considerations
It is worthwhile to consider the security implications of this
proposal from two independent perspectives: the IP VPN provider and
the IP VPN customer.
From an IP VPN provider perspective, this mechanism will assure
separation between the BGP path attributes advertised by the CE
router and the BGP attributes used within the provider network, thus
potentially improving security.
Although this behavior is largely implementation dependent, it is
currently possible for a CE device to inject BGP attributes (extended
communities, for example) that have semantics on the IP VPN provider
network, unless explicitly disabled by configuration in the PE.
With the rules specified for the ATTR_SET path attribute, any
attribute that has been received from a CE is pushed into the stack
before the route is advertised to other PEs.
As with any other field based on values received from an external
system, an implementation must consider the issues of input
validation and resource management.
From the perspective of the VPN customer network, it is our opinion
that there is no change to the security profile of PE-CE interaction.
While having an iBGP session allows the PE to specify additional
attributes not allowed on an External BGP session (e.g., LOCAL_PREF),
this does not significantly change the fact that the VPN customer
must trust its service provider to provide it with correct routing
10. IANA Considerations
This document defines a new BGP path attribute that is part of a
registry space managed by IANA. IANA has updated its BGP Path
Attributes registry with the value specified above (128) for the
ATTR_SET path attribute.
The authors would like to thank Stephane Litkowski and Bruno Decraene
for their comments.
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