L3VPN WG Hamid Ould-Brahim
draft-ietf-l3vpn-bgpvpn-auto-08.txt Nortel Networks
INFORMATIONAL
Expiration Date: March 2007
Eric C. Rosen
Cisco Systems
Yakov Rekhter
Juniper Networks
(Editors)
September 2006
Using BGP as an Auto-Discovery Mechanism for
VR-based Layer-3 VPNs
Status of this Memo
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Abstract
In any provider-based VPN scheme, the Provider Edge (PE) devices
attached to a common VPN must exchange certain information as a
prerequisite to establish VPN-specific connectivity. The main
purpose of an auto-discovery mechanism is to enable a PE to
dynamically discover the set of remote PEs having VPN members in
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common. The auto-discovery mechanism proceeds by having a PE
advertises to other PEs, at a minimum, its own IP address and the
list of VPN members configured on that PE. Once that information is
received the remote PEs will then identify the list of VPN sites
members of the same VPN, and use the information
carried within the discovery mechanism to establish VPN
connectivity. This draft defines a BGP based auto-discovery
mechanism for Virtual Router-based layer-3 VPNs. This mechanism is
based on the approach used by BGP/MPLS-IP-VPN for distributing VPN
routing information within the service provider(s).
Changes from 07 version (DELETE THIS WHEN IT BECOMES RFC)
- Updated the IANA section to reflect the review from IANA
- Nits from Harald's feedback.
1. Introduction
In any provider-based VPN scheme, the Provider Edge (PE) devices
attached to a common VPN must exchange certain information as a
prerequisite to establish VPN-specific connectivity. An auto-
discovery mechanism allows a PE to dynamically discover the set of
remote PEs having VPN members in common. The auto-discovery
mechanism proceeds by having a PE advertises to other PEs, at a
minimum, its own IP address and the list of VPN sites configured
on that PE. Once that information is received the remote PEs will
then identify the list of VPN sites member of the same VPN with the
advertising PE, and use the information carried within the discovery
mechanism to establish VPN connectivity.
The purpose of this draft is to define a BGP based auto-discovery
mechanism for VR-based VPNs [VPN-VR] solution. This mechanism is
based on the approach used by [BGP/MPLS-IP-VPN] for distributing VPN
routing information within the service provider(s).
Virtual router (VR) addresses must be exchanged, along with the
information needed to enable the PEs to determine which VRs are in
the same VPN ("membership"), and which of those VRs are to have VPN
connectivity ("topology"). Once the VRs are reachable through the
tunnels, routes ("reachability") are then exchanged by running
existing routing protocols per VPN basis.
The BGP-4 multiprotocol extensions are used to carry various
information about VR-based VPNs. VPN-specific information associated
with the NLRI is encoded either as attributes of the NLRI, or as
part of the NLRI itself, or both.
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2. Provider-Provisioned VPN Reference Model
When using BGP as an auto-discovery mechanism, VR-based l3vpns are
using a network reference model as illustrated in figure 1.
PE PE
+--------------+ +--------------+
+--------+ | +----------+ | | +----------+ | +--------+
| VPN-A | | | VPN-A | | | | VPN-A | | | VPN-A |
| Sites |--| |Database /| | BGP route | | Database/| |-| sites |
+--------+ | |Processing| |<----------->| |Processing| | +--------+
| +----------+ | Distribution| +----------+ |
| | | |
+--------+ | +----------+ | | +----------+ | +--------+
| VPN-B | | | VPN-B | | -------- | | VPN-B | | | VPN-B |
| Sites |--| |Database /| |-(Backbones)-| | Database/| |-| sites |
+--------+ | |Processing| | -------- | |Processing| | +--------+
| +----------+ | | +----------+ |
| | | |
+--------+ | +----------+ | | +----------+ | +--------+
| VPN-C | | | VPN-C | | | | VPN-C | | | VPN-C |
| Sites |--| |Database /| | | | Database/| |-| sites |
+--------+ | |Processing| | | |Processing| | +--------+
| +----------+ | | +----------+ |
+--------------+ +--------------+
Figure 1: Network based VPN Reference Model
It is assumed that the PEs can use BGP to distribute information to
each other. This may be via direct IBGP peering, via direct EBGP
peering, via multihop BGP peering, through intermediaries such as
Route Reflectors, through a chain of intermediate BGP connections,
etc.
3. Carrying VR-based VPN information in BGP
The BGP-4 multiprotocol extensions are used to carry various
information about VPNs. VPN-specific information associated with the
NLRI is encoded either as attributes of the NLRI, or as part of the
NLRI itself, or both. The addressing information in the NLRI field
is ALWAYS within the VPN address space, and therefore MUST be unique
within the VPN. The address specified in the BGP next hop attribute,
on the other hand, is in the service provider addressing space.
The NLRI is a VPN-IP address or a labeled VPN-IP address. The NLRI
address prefix is an address of one of the virtual routers
configured on the PE. That address is used by the VRs to establish
routing adjacencies and tunnel to each other [VPN-VR].
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4. Interpretation of VPN Information in the VR Model
4.1 Membership Discovery
The VPN-ID format as defined in [RFC-2685] is used to identify a
VPN. All virtual routers that are members of a specific VPN share
the same VPN-ID. A VPN-ID is carried in the NLRI to make addresses
of VRs globally unique. Making these addresses globally unique is
necessary if one uses BGP for VRs' auto-discovery.
4.2 Encoding of the VPN-ID in the NLRI
For the virtual router model, the VPN-ID is carried within the route
distinguisher (RD) field. In order to hold the 7-bytes VPN-ID, the
first byte of RD type field is used to indicate the existence of the
VPN-ID format. A value of 0x80 in the first byte of RD's type field
indicates that the RD field is carrying the VPN-ID format. In this
case, the type field range 0x8000-0x80ff will be reserved for the
virtual router case.
4.3 VPN-ID Extended Community
A new extended community is used to carry the VPN-ID format. This
attribute is transitive across the Autonomous system boundary. The
type field of the VPN-ID extended community is of regular type to be
assigned by IANA [BGP-COMM]. The remaining 7 bytes hold the VPN-ID
value field as per [RFC-2685]. The BGP UPDATE message will carry
information for a single VPN. It is the VPN-ID Extended Community,
or more precisely route filtering based on the Extended Community
that allows one VR to find out about other VRs in the same VPN.
4.4 VPN Topology Information
A new extended community is used to indicate different VPN topology
values. This attribute is transitive across the Autonomous system
boundary. The value of the type field for extended type is assigned
by IANA. The first two bytes of the value field (of the remaining 6
bytes) are reserved. The actual topology values are carried within
the remaining four bytes. The following topology values are defined:
Value Topology Type
1 "Hub"
2 "Spoke"
3 "Mesh"
Arbitrary values can also be used to allow specific topologies to be
constructed.
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In a hub and spoke topology, spoke VRs (i.e., PE having VRs as
spokes within the VPN) will advertise their BGP information with VPN
topology extended community with value of "2". Spoke VRs will only
be allowed to connect to hub VRs and therefore spoke VR-based PEs
will just import VPN information from BGP that is set of "1". Hub
sites can connect to both hub and spoke sites (i.e., Hub VRs can
import VPN topology of both values "1", "2", or "3". In a mesh
topology, mesh sites connect to each other, each VR will advertise
VPN topology information of "3".
Furthermore, in the presence of both hub and spoke and mesh
topologies within the same VPN, mesh sites can as well connect to
hub sites and vice versa.
5. Tunnel Discovery
Layer-3 VPNs must be implemented through some form of tunneling
mechanism, where the packet formats and/or the addressing used
within the VPN can be unrelated to that used to route the tunneled
packets across the backbone. There are numerous tunneling mechanisms
that can be used by a network based VPN (e.g., IP/IP [RFC-2003], GRE
tunnels [RFC-1701], IPSec [RFC-2401], and MPLS tunnels [RFC-3031]).
Each of these tunnels allows for opaque transport of frames as
packet payload across the backbone, with forwarding disjoint from
the address fields of the encapsulated packets. A provider edge
router may terminate multiple types of tunnels and forward packets
between these tunnels and other network interfaces in different
ways. BGP can be used to carry tunnel endpoint addresses between
edge routers.
The BGP next hop will carry the service provider tunnel endpoint
address. As an example, if IPSec is used as tunneling mechanism, the
IPSec tunnel remote address will be discovered through BGP, and the
actual tunnel establishment is achieved through IPSec signaling
protocol.
When MPLS tunneling is used, the label carried in the NLRI field is
associated with an address of a VR, where the address is carried in
the NLRI and is encoded as a VPN-IP address.
The auto-discovery mechanism should convey minimum information for
the tunnels to be setup. The means of distributing multiplexors must
be defined either via some sort of tunnel-protocol-specific signaling
mechanism, or via additional information carried by the
auto-discovery protocol. That information may or may not be
used directly within the specific signaling protocol. On one end of
the spectrum, the combination of IP address (such as BGP next hop and
IP address carried within the NLRI) and the label and/or VPN-ID
provides sufficient information for a PE to setup per VPN tunnels or
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shared tunnels per set of VPNs. On another end of the spectrum
additional specific tunnel related information can be carried within
the discovery process if needed.
6. Scalability Considerations
In this section, we briefly summarize the main characteristics of
our model with respect to scalability.
Recall that the Service Provider network consists of (a) PE routers,
(b) BGP Route Reflectors, (c) P routers (which are neither PE
routers nor Route Reflectors), and, in the case of multi-provider
VPNs, (d) ASBRs.
A PE router, unless it is a Route Reflector should not retain
VPN-related information unless it has at least one VPN with an
Import Target identical to one of the VPN-related information Route
Target attributes. Inbound filtering should be used to cause such
information to be discarded. If a new Import Target is later added
to one of the PE's VPNs (a "VPN Join" operation), it must then
acquire the VPN-related information it may previously have
discarded.
This can be done using the refresh mechanism described in [BGP-
RFSH].
The outbound route filtering mechanism of [BGP-ORF], [BGP-CONS] can
also be used to advantage to make the filtering more dynamic.
Similarly, if a particular Import Target is no longer present in
any of a PE's VPNs (as a result of one or more "VPN Prune"
operations), the PE may discard all VPN-related information which,
as a result, no longer have any of the PE's VPN's Import Targets as
one of their Route Target Attributes.
Note that VPN Join and Prune operations are non-disruptive, and do
not require any BGP connections to be brought down, as long as the
refresh mechanism of [BGP-RFSH] is used.
As a result of these distribution rules, no one PE ever needs to
maintain all routes for all VPNs; this is an important scalability
consideration.
Route reflectors can be partitioned among VPNs so that each
partition carries routes for only a subset of the VPNs supported by
the Service Provider. Thus no single route reflector is required to
maintain VPN-related information for all VPNs.
For inter-provider VPNs, if multi-hop EBGP is used, then the ASBRs
need not maintain and distribute VPN-related information at all.
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P routers do not maintain any VPN-related information. In order
to properly forward VPN traffic, the P routers need only maintain
routes to the PE routers and the ASBRs.
As a result, no single component within the Service Provider network
has to maintain all the VPN-related information for all the VPNs.
So the total capacity of the network to support increasing numbers
of VPNs is not limited by the capacity of any individual component.
An important consideration to remember is that one may have any
number of INDEPENDENT BGP systems carrying VPN-related information.
This is unlike the case of the Internet, where the Internet BGP
system must carry all the Internet routes. Thus one significant
(but perhaps subtle) distinction between the use of BGP for the
Internet routing and the use of BGP for distributing VPN-related
information, as described in this document is that the former is not
amenable to partition, while the latter is.
7. Security Considerations
This document describes a BGP-based auto-discovery mechanism which
enables a PE router that attaches to a particular VPN to discover
the set of other PE routers that attach to the same VPN. Each PE
router that is attached to a given VPN uses BGP to advertise that
fact. Other PE routers which attach to the same VPN receive these
BGP advertisements. This allows that set of PE routers to discover
each other. Note that a PE will not always receive these
advertisements directly from the remote PEs; the advertisements may
be received from "intermediate" BGP speakers.
It is of critical importance that a particular PE should not be
"discovered" to be attached to a particular VPN unless that PE
really is attached to that VPN, and indeed is properly authorized to
be attached to that VPN. If any arbitrary node on the Internet
could start sending these BGP advertisements, and if those
advertisements were able to reach the PE routers, and if the PE
routers accepted those advertisements, then anyone could add any
site to any VPN. Thus the auto-discovery procedures described here
presuppose that a particular PE trusts its BGP peers to be who they
appear to be, and further that it can trusts those peers to be
properly securing their local attachments. (That is, a PE must
trust that its peers are attached to, and are authorized to be
attached to, the VPNs to which they claim to be attached.).
If a particular remote PE is a BGP peer of the local PE, then the
BGP authentication procedures of RFC 2385 can be used to ensure that
the remote PE is who it claims to be, i.e., that it is a PE that is
trusted.
If a particular remote PE is not a BGP peer of the local PE, then
the information it is advertising is being distributed to the local
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PE through a chain of BGP speakers. The local PE must trust that
its peers only accept information from peers that they trust in
turn, and this trust relation must be transitive. BGP does not
provide a way to determine that any particular piece of received
information originated from a BGP speaker that was authorized to
advertise that particular piece of information. Hence the
procedures of this document should be used only in environments
where adequate trust relationships exist among the BGP speakers.
Some of the VPN schemes which may use the procedures of this
document can be made robust to failures of these trust
relationships. That is, it may be possible to keep the VPNs secure
even if the auto-discovery procedures are not secure. For example,
a VPN based on the VR model can use IPsec tunnels for transmitting
data and routing control packets between PE routers. An
illegitimate PE router which is discovered via BGP will not have the
shared secret which makes it possible to set up the IPsec tunnel,
and so will not be able to join the VPN. Similarly, [IP-GRE]
describes procedures for using IPsec tunnels to secure VPNs based on
the [BGP/MPLS-IP-VPN] model. The details for using IPsec to secure
a particular sort of VPN depend on that sort of VPN and so are out
of scope of the current document.
8. IANA Considerations
IANA has assigned new extended community <TBD> for Topology
values for VR-based L3VPN solution.
IANA has assigned new extended community <TBD> for
carrying VPN-ID format based on RFC2685 format.
IANA has assigned new SAFI number <TBD> for indicating that
the NLRI is carrying information for VR for labeled prefixes.
SAFI number "140" for indicating that the NLRI is carrying
information for VR for non-labeled prefixes.
9. Use of BGP Capability Advertisement
A BGP speaker that uses VPN information as described in this
document with multiprotocol extensions should use the Capability
Advertisement procedures [RFC-3392] to determine whether the speaker
could use Multiprotocol Extensions with a particular peer.
10. Acknowledgement
The authors would like to acknowledge Benson Schliesser, and Thomas
Narten for the constructive and fruitful comments.
11. Normative References
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[BGP-COMM] Ramachandra, Tappan, et al., "BGP Extended Communities
Attribute", RFC4360.
[BGP-MP] Bates, Chandra, Katz, and Rekhter, "Multiprotocol
Extensions for BGP4", February 1998, RFC 2283.
[RFC-3107] Rekhter Y, Rosen E., "Carrying Label Information in
BGP4", January 2000, RFC3107.
[BGP/MPLS-IP-VPN] Rosen E., et al, "BGP/MPLS VPNs", RFC4364.
[RFC-2685] Fox B., et al, "Virtual Private Networks Identifier",
RFC 2685, September 1999.
[RFC-3392] Chandra, R., et al., "Capabilities Advertisement with
BGP-4", RFC3392, May 2002.
[VPN-VR] Knight, P., Ould-Brahim H., Gleeson, B., "Network based IP
VPN Architecture using Virtual Routers", Work in progress.
12. Informative References
[RFC-1701] Hanks, S., Li, T., Farinacci, D. and P. Traina, "Generic
Routing Encapsulation (GRE)", RFC 1701, October 1994.
[RFC-2003] Perkins, C., "IP Encapsulation within IP", RFC 2003,
October 1996.
[RFC-2026] Bradner, S., "The Internet Standards Process -- Revision
3", RFC 2026, October 1996.
[RFC-2401] Kent S., Atkinson R., "Security Architecture for the
Internet Protocol", RFC 2401, November 1998.
[RFC-2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, March 1997.
[IP-GRE] Rosen, E., et al., "Use of PE-PE GRE or IP in BGP/MPLS IP
Virtual Private Networks", draft-ietf-l3vpn-gre-ip-2547-03.txt,
October 2004, Work in Progress.
[BGP-RFSH] Chen, A., "Route Refresh Capability for BGP-4", RFC 2918,
September 2000.
[BGP-ORF] Chen, E., and Rekhter, Y., "Cooperative Route Filtering
Capability for BGP-4", draft-ietf-idr-route-filter-11.txt,
December 2004, Work in Progress.
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[BGP-CONS] Marques, P., et al., "Constrained VPN route distribution"
draft-ietf-l3vpn-rt-constrain-01.txt, September 2004, work in
progress
13. Annex: Auto-Discovery in VR and MPLS-IP-VPN Interworking Scenarios
Two interwoking scenarios are considered when the network is using
both virtual routers and BGP/MPLS-IP-VPN. The first scenario is a
CE-PE relationship between a PE (implementing BGP/MPLS-IP-VPN), and
a VR appearing as a CE to the PE. The connection between the VR, and
the PE can be either direct connectivity, or through a tunnel (e.g.,
IPSec).
The second scenario is when a PE is implementing both architectures.
In this particular case, a single BGP session configured on the
service provider network can be used to advertise either BGP/MPLS-
IP-VPN VPN information or the virtual router related VPN
information. From the VR and the BGP/MPLS-IP-VPN point of view there
is complete separation from data path and addressing schemes.
However the PE's interfaces are shared between both architectures.
A PE implementing only BGP/MPLS-IP-VPN will not import routes from a
BGP UPDATE message containing the VPN-ID extended community. On the
other hand, a PE implementing the virtual router architecture will
not import routes from a BGP UPDATE message containing the route
target extended community attribute.
The granularity at which the information is either BGP/MPLS-IP-VPN
related or VR-related is per BGP UPDATE message. Different SAFI
numbers are used to indicate that the message carried in BGP
multiprotocol extension attributes is to be handled by the VR or
BGP/MPLS-IP-VPN architectures.
14. Contributors
Bryan Gleeson
Nokia
313 Fairchild Drive
Mountain View CA 94043 USA
bryan.gleeson/at/nokia.com
Peter Ashwood-Smith
Nortel Networks
P.O. Box 3511 Station C,
Ottawa, ON K1Y 4H7, Canada
Phone: +1 613 763 4534
Email: petera@nortelnetworks.com
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Luyuan Fang
AT&T
200 Laurel Avenue
Middletown, NJ 07748
Email: Luyuanfang@att.com
Phone: +1 (732) 420 1920
Jeremy De Clercq
Alcatel
Francis Wellesplein 1
B-2018 Antwerpen, Belgium
Phone: +32 3 240 47 52
Email: jeremy.de_clercq@alcatel.be
Riad Hartani
Caspian Networks
170 Baytech Drive
San Jose, CA 95143
Phone: 408 382 5216
Email: riad@caspiannetworks.com
Tissa Senevirathne
Force10 Networks
1440 McCarthy Blvd,
Milpitas, CA 95035.
Phone: 408-965-5103
Email: tsenevir@hotmail.com
15. Author' Addresses
Hamid Ould-Brahim
Nortel Networks
P O Box 3511 Station C
Ottawa, ON K1Y 4H7, Canada
Email: hbrahim@nortelnetworks.com
Eric C. Rosen
Cisco Systems, Inc.
1414 Massachusetts Avenue
Boxborough, MA 01719
E-mail: erosen@cisco.com
Yakov Rekhter
Juniper Networks
1194 N. Mathilda Avenue
Sunnyvale, CA 94089
Email: yakov@juniper.net
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