|Title||Use of Provider Edge to Provider Edge (PE-PE) Generic Routing
Encapsulation (GRE) or IP in BGP/MPLS IP Virtual Private Networks
|Author||Y. Rekhter, R. Bonica, E. Rosen
Network Working Group Y. Rekhter
Request for Comments: 4797 R. Bonica
Category: Informational Juniper Networks
Cisco Systems, Inc.
Use of Provider Edge to Provider Edge (PE-PE)
Generic Routing Encapsulation (GRE) or IP
in BGP/MPLS IP Virtual Private Networks
Status of This Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright (C) The IETF Trust (2007).
This document proposes an automated mechanism for establishing
tunnels between provider-edge routers in a VPN, but does not provide
an automated mechanism for establishing security associations for
these tunnels. Without such a mechanism, this document is not
appropriate for publication on the Internet standards track.
This document describes an implementation strategy for BGP/MPLS IP
Virtual Private Networks (VPNs) in which the outermost MPLS label
(i.e., the tunnel label) is replaced with either an IP header or an
IP header with Generic Routing Encapsulation (GRE).
The implementation strategy described herein enables the deployment
of BGP/MPLS IP VPN technology in networks whose edge devices are MPLS
and VPN aware, but whose interior devices are not.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions Used In This Document . . . . . . . . . . . . . . . 4
3. Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Specification . . . . . . . . . . . . . . . . . . . . . . . . . 5
4.1. MPLS-in-IP/MPLS-in-GRE Encapsulation by Ingress PE . . . . 5
4.2. MPLS-in-IP/MPLS-in-GRE Decapsulation by Egress PE . . . . . 6
5. Implications on Packet Spoofing . . . . . . . . . . . . . . . . 7
6. Security Considerations . . . . . . . . . . . . . . . . . . . . 7
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 7
8. Normative References . . . . . . . . . . . . . . . . . . . . . 8
A "conventional" BGP/MPLS IP VPN  is characterized as follows:
Each Provider Edge (PE) router maintains one or more Virtual
Routing and Forwarding (VRF) instances. A VRF instances is a VPN-
specific forwarding table.
PE routers exchange reachability information with one another
using BGP  with multi-protocol extensions .
MPLS Label Switching Paths (LSPs)  connect PE routers to one
In simple configurations, the VPN service is offered by a single
Autonomous System (AS). All service provider routers are contained
by a single AS and all VPN sites attach to that AS. When an ingress
PE router receives a packet from a VPN site, it looks up the packet's
destination IP address in a VRF that is associated with the packet's
ingress attachment circuit. As a result of this lookup, the ingress
PE router determines an MPLS label stack, a data link header, and an
output interface. The label stack is prepended to the packet, the
data link header is prepended to that, and the resulting frame is
queued for the output interface.
The innermost label in the MPLS label stack is called the VPN route
label. The VPN route label is significant and visible to the egress
PE router only. It controls forwarding of the packet by the egress
The outermost label in the MPLS label stack is called the tunnel
label. The tunnel label causes the packet to be delivered to the
egress PE router that understands the VPN route label. Specifically,
the tunnel label identifies an MPLS LSP that connects the ingress PE
router to the egress PE router. In the context of BGP/MPLS IP VPNs,
this LSP is called a tunnel LSP.
The tunnel LSP provides a forwarding path between the ingress and
egress PE routers. Quality of service (QoS) information can be
mapped from the VPN packet to the tunnel LSP header so that required
forwarding behaviors can be maintained at each hop along the
Sections 9 and 10 of reference  define more complex configurations
(i.e., carriers' carrier and multi-AS backbones) in which service
providers offer L3VPN services across multiple autonomous systems.
In these configurations, VPN route labels can be stitched together
across AS boundaries. Within each AS, tunnel LSPs carry VPN packets
from network edge to network edge.
In most configurations, tunnel LSPs never traverse AS boundaries.
The tunnel LSP is always contained within a single AS. In one
particular configuration (i.e., Inter-provider Option C), tunnel LSPs
may traverse AS boundaries.
This memo describes procedures for creating an MPLS packet that
carries the VPN route label, but does not carry the tunnel label.
Then, using either GRE or IP encapsulation, the ingress PE router
sends the MPLS packet across the network to the egress PE router.
That is, a GRE or IP tunnel replaces the tunnel LSP that was present
in "conventional" BGP/MPLS IP VPNs. Like the tunnel LSP, the GRE/IP
tunnel provides a forwarding path between the ingress and egress PE
routers. QoS information can be copied from the VPN packet to the
GRE/IP tunnel header so that required forwarding behaviors can be
maintained at each hop along the forwarding path. However, because
the GRE/IP tunnel lacks traffic engineering capabilities, any traffic
engineering features provided by the tunnel LSP are lost.
2. Conventions Used In This Document
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 .
"Conventional" BGP/MPLS IP VPNs require an MPLS Label Switched Path
(LSP) between a packet's ingress PE router and its egress PE router.
This means that a BGP/MPLS IP VPN cannot be implemented if there is a
part of the path between the ingress and egress PE routers that does
not support MPLS.
In order to enable BGP/MPLS IP VPNs to be deployed even when there
are non-MPLS routers along the path between the ingress and egress PE
routers, it is desirable to have an alternative, which allows the
tunnel label to be replaced with either an IP or (IP + GRE) header.
The encapsulation header would have the address of the egress PE
router in its destination IP address field, and this would cause the
packet to be delivered to the egress PE router.
In this procedure, the ingress and egress PE routers themselves must
support MPLS, but that is not an issue, as those routers must
necessarily have BGP/MPLS IP VPN support, whereas the transit routers
need not support MPLS or BGP/MPLS VPNs.
In short, the technical approach specified here is:
1. Continue to use MPLS to identify a VPN route, by continuing to
add an MPLS label stack to the VPN packets. Between the ingress
and egress PE router, the outermost member of the label stack
will represent the VPN route label.
2. An MPLS-in-GRE or MPLS-in-IP  encapsulation will be used to
turn the MPLS packet, described above, back into an IP packet.
This, in effect, creates a GRE or an IP tunnel between the
ingress PE router and the egress PE router.
The net effect is that an MPLS packet gets sent through a GRE or an
Service providers must protect the above-mentioned IP or GRE tunnel
as recommended in Section 8.2 of reference . As stated in that
"If the tunnel lies entirely within a single administrative
domain, address filtering at the boundaries can be used to ensure
that no packet with the IP source address of a tunnel endpoint or
with the IP destination address of a tunnel endpoint can enter the
domain from outside.
However, when the tunnel head and the tunnel tail are not in the
same administrative domain, this may become difficult, and
filtering based on the destination address can even become
impossible if the packets must traverse the public Internet.
Sometimes only source address filtering (but not destination
address filtering) is done at the boundaries of an administrative
domain. If this is the case, the filtering does not provide
effective protection at all unless the decapsulator of an
MPLS-in-IP or MPLS-in-GRE validates the IP source address of the
packet. This document does not require that the decapsulator
validate the IP source address of the tunneled packets, but it
should be understood that failure to do so presupposes that there
is effective destination-based (or a combination of source-based
and destination-based) filtering at the boundaries."
4.1. MPLS-in-IP/MPLS-in-GRE Encapsulation by Ingress PE
The following description is not meant to specify an implementation
strategy; any implementation procedure that produces the same result
When an ingress PE router receives a packet from a Customer Edge (CE)
router, it looks up the packet's destination IP address in a VRF that
is associated with the packet's ingress attachment circuit. This
enables the (ingress) PE router to find a VPN-IP route. The VPN-IP
route will have an associated VPN route label and an associated BGP
Next Hop. The label is pushed on the packet. Then an IP (or IP+GRE)
encapsulation header is prepended to the packet, creating an
MPLS-in-IP (or MPLS-in-GRE) encapsulated packet. The IP source
address field of the encapsulation header will be an address of the
ingress PE router itself. The IP destination address field of the
encapsulation header will contain the value of the associated BGP
Next Hop; this will be an IP address of the egress PE router. QoS
information can be copied from the VPN packet to the GRE/IP tunnel
header so that required forwarding behaviors can be maintained at
each hop along the forwarding path.
The IP address of the remote tunnel endpoints MAY be inferred from
the Network Address of the Next Hop field of the MP_REACH_NLRI BGP
attribute . Note that the set of Next Hop Network Addresses is
not known in advance, but is learned dynamically via the BGP
distribution of VPN-IP routes. Assuming a consistent set of tunnel
capabilities exist between all the PEs and Autonomous System Border
Routers (ASBRs), no a priori configuration of the remote tunnel
endpoints is needed. The entire set of PE and ASBRs MUST have the
same tunnel capabilities if the dynamic creation of IP (or GRE)
tunnels is desired. The preference to use an IP (or GRE) tunnel MUST
be configured. A set of PEs using two or more tunnel mechanisms
(i.e., LSP, GRE, IP, etc.) MUST determine the tunnel type on a per-
peer basis. The automatic association of tunnel capabilities on a
per-peer basis is for future study. Note that these tunnels SHOULD
NOT be IGP-visible links, and routing adjacencies SHOULD NOT be
supported across these tunnel.
4.2. MPLS-in-IP/MPLS-in-GRE Decapsulation by Egress PE
Every egress PE is also an ingress PE, and hence has the ability to
decapsulate MPLS-in-IP (or MPLS-in-GRE) packets. After
decapsulation, the packets SHOULD be delivered to the routing
function for ordinary MPLS switching.
As stated above, if the service provider deploys source-based
filtering at network edges to protect the IP/GRE tunnel (instead of
destination-based filtering), the decapsulator must validate the IP
source address of the tunneled packets.
5. Implications on Packet Spoofing
It should be noted that if the tunnel MPLS labels are replaced with
an unsecured IP encapsulation, like GRE or IP, it becomes more
difficult to protect the VPNs against spoofed packets. This is
because a Service Provider (SP) can protect against spoofed MPLS
packets by the simple expedient of not accepting MPLS packets from
outside its own boundaries (or more generally, by keeping track of
which labels are validly received over which interfaces, and
discarding packets that arrive with labels that are not valid for
their incoming interfaces).
By contrast, protection against spoofed IP packets requires all SP
boundary routers to perform filtering; either (a) filtering packets
from "outside" the SP, which are addressed to PE routers, or (b)
filtering packets from "outside" the SP, which have source addresses
that belong "inside" and, in addition, filtering on each PE all
packets that have source addresses that belong "outside" the SP.
The maintenance of these filter lists can be management intensive.
Furthermore, depending upon implementation, these filter lists can be
performance affecting. However, such filters may be required for
reasons other than protection against spoofed VPN packets, in which
case the additional maintenance overhead of these filters to protect
(among other things) against spoofing of VPN packets may be of no
practical significance. Note that allocating IP addresses used for
GRE or IP tunnels out of a single (or a small number of) IP block
could simplify maintenance of the filters.
6. Security Considerations
Security considerations in reference  apply here as well.
Additional security issues are discussed in the previous section
"Implications on Packet Spoofing".
Thanks to Robert Raszuk and Scott Wainner for their contributions to
8. Normative References
 Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
 Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private Networks
(VPNs)", RFC 4364, February 2006.
 Rekhter, Y., Li, T., and S. Hares, "A Border Gateway Protocol 4
(BGP-4)", RFC 4271, January 2006.
 Bates, T., Chandra, R., Katz, D., and Y. Rekhter, "Multiprotocol
Extensions for BGP-4", RFC 4760, January 2007.
 Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol Label
Switching Architecture", RFC 3031, January 2001.
 Worster, T., Rekhter, Y., and E. Rosen, "Encapsulating MPLS in
IP or Generic Routing Encapsulation (GRE)", RFC 4023,
1194 N. Mathilda Ave.
Sunnyvale, CA 94089
Ronald P. Bonica
2251 Corporate Park Drive
Herndon, VA 20171
Eric C. Rosen
Cisco Systems, Inc.
1414 Massachusetts Avenue
Boxborough, MA 01719
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