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
Using an LSA Options Bit to Prevent Looping in BGP/MPLS IP VPNs
draft-ietf-ospf-2547-dnbit-00.txt
Status of this Memo
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Abstract
[VPN] describes a method by which a Service Provider (SP) may provide
an "IP VPN" service to its customers. In VPNs of that sort, a
Customer Edge (CE) Router and a Provider Edge Router become routing
peers, and the customer routes are sent to the SP. BGP is then used
to carry the customer routes across the SP's backbone to other PE
routers, and the routes are then sent to other CE routers. Since CE
routers and PE routers are routing peers, it is customary to run a
routing protocol between them. [VPN] allows a number of different
PE-CE protocols. If OSPF is used as the PE-CE routing protocol, the
PE must execute additional procedures not specified in [VPN]; these
procedures are specified in [OSPF-VPN]. These additional procedures
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translate customer OSPF routes from a CE router into BGP routes. The
BGP routes are sent to the other PE routers, which translate them
back into OSPF routes, and then distribute them to CE routers.
During this translation, some of the information needed to prevent
loops may be lost. The procedures specified in this document remedy
this situation by specifying that one of the OSPF options bits be
used to ensure that when a VPN route is sent from a PE to a CE, the
route will be ignored by any PE which receives it back from a CE.
Table of Contents
1 Specification of Requirements ........................ 2
2 Introduction ......................................... 2
3 Information Loss and Loops ........................... 4
4 Using the LSA Options to Prevent Loops ............... 5
5 Acknowledgments ...................................... 5
6 Authors' Addresses ................................... 5
7 Normative References ................................. 6
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 an "IP VPN" service to customers. In that
sort of service, a customer's edge devices (CE devices) are connected
to the provider's edge routers (PE routers). Each CE device is in a
single VPN. Each PE device may attach to multiple CEs, of the same
or of different VPNs. A VPN thus consists of a set of "network
segments" connected by the SP's backbone.
A CE exchanges routes with a PE, using a routing protocol that is
jointly agreed to by the customer and the SP. The PE runs that
routing protocol's decision process to determine the set of IP
address prefixes for which the following two conditions hold:
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- each address prefix in the set can be reached via that CE
- the path from that CE to each such address prefix does NOT
include the SP backbone (i.e., does not include any PE routers).
The PE routers which attach to a particular VPN then "leak" routes to
these address prefixes into BGP, and use BGP to distribute the VPN's
routes to each other. BGP carries these routes in the "VPN-IP
address family", so that they are distinct from ordinary Internet
routes. (The VPN-IP address family also extends the IP addresses on
the left so that address prefixes from two different VPNs are always
distinct to BGP, even if both VPNs use the same piece of the private
RFC1918 address space.) The routes of a particular VPN are carried
only to the PE routers which attach to that VPN.
If a PE router receives a VPN-IP route via BGP, and that PE is
attached to a CE in the VPN to which the route belongs, the PE will
translate the route back to IP, and leak it into the routing
algorithm which is running on the link to that CE.
This methodology provides a "peer model"; CE routers peer with PE
routers, but CE routers at different sites do not peer with each
other.
When a VPN uses OSPF as its internal routing protocol this does not
necessarily mean that the CE routers of that VPN must use OSPF to
peer with the PE 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.
When this is done, the PE routers must convert between BGP routes and
OSPF routes. Procedures for this are specified in [VPN-OSPF]. PE
routers act like OSPF border routers. PE routers will sometimes
convert BGP routes to OSPF AS-external routes, and will sometimes
convert BGP routes to OSPF summary routes. In either case, the PE
will originate an LSA, and distribute it to any CE routers (in the
appropriate VPN) with which it maintains an OSPF adjacency.
Similarly, when a PE router receives a summary LSA or an AS-external
LSA from a CE router, it may convert those LSAs to BGP routes for
distribution to other PEs.
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3. Information Loss and Loops
A PE, say PE1, may learn a route to a particular VPN-IP address
prefix via BGP. This may cause it to generate a summary LSA or an
AS-external LSA in which it reports that address prefix. This LSA
may then be distributed to a particular CE, say CE1. The LSA may
then be distributed throughout a particular OSPF area, reaching
another CE, say CE2. CE2 may then distribute the LSA to another PE,
say PE2.
As stated in the previous section, PE2 must run the OSPF decision
process to determine whether a particular address prefix, reported in
an LSA from CE2, is reachable from CE2 via a path which does not
include any PE router. Unfortunately, there is no standard way to do
this. The OSPF LSAs do not necessarily carry the information needed
to enables PE2 to determine that the path to address prefix X in a
particular LSA from CE2 is actually a path that includes, say, PE1.
If PE2 then leaks X into BGP as a VPN-IP route, then PE2 is violating
one of the constraints for loop-freedom in BGP, viz., that routes
learned from a particular BGP domain not be redistributed back into
that BGP domain. This could cause a routing loop to be created.
It is therefore necessary to have a means by which an LSA may carry
the information that a particular address prefix has been learned
from a PE router. Any PE router which receives an LSA with this
information would omit the information in this LSA from its OSPF
decision process, and thus would not leak the information back into
BGP.
When a PE generates an AS-external LSA, it could use a distinct tag
value to indicate that the LSA is carrying information about an
address prefix for whom the path includes a PE router. However, this
method is not available in the case where the PE generates a Summary
LSA. (The reader will note that loops can only induced by Summary
LSAs when the PE-CE links are area 0 links; however, this is an
important case to handle correctly.) Thus one needs a more generally
applicable method of indicating that an LSA contains information
about a path via a PE router.
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4. Using the LSA Options to Prevent Loops
The high-order bit of the LSA Options field (a previously unused bit)
is used to solve the problem described in the previous section. We
refer to this bit as the DN bit. When an LSA is sent from a PE to a
CE, the DN bit MUST be set.
When the PE receives, from a CE router, an LSA with the DN bit set,
the information from that LSA MUST NOT be used during the OSPF route
calculation. As a result, the LSA is not translated into a BGP
route.
This prevents routes learned via BGP from being redistributed to BGP.
5. Acknowledgments
The idea of using the high-order options bit for this purpose is due
to Derek Yeung. Thanks to Yakov Rekhter for his contribution to this
work.
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
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Padma Pillay-Esnault
Juniper Networks
1194 N. Mathilda Avenue
Sunnyvale, CA 94089
E-mail: padma@juniper.net
7. Normative References
[OSPF] "OSPF Version 2", RFC 2328, Moy, J., April 1998
[VPN] "BGP/MPLS VPNs", draft-ietf-ppvpn-rfc2547bis-04.txt, Rosen,
Rekhter, et. al., May 2003
[OSPF-VPN] "OSPF as the PE/CE Protocol in BGP/MPLS VPNs", draft-
ietf-ppvpn-ospf-2547-00.txt, Rosen, Psenak, Pillay-Esnault, June 2003
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