Network Working Group Eric C. Rosen
Internet Draft Peter Psenak
Expiration Date: March 2004 Cisco Systems, Inc.
Padma Pillay-Esnault
Juniper Networks, Inc.
September 2003
Using an LSA Options Bit to Prevent Looping in BGP/MPLS IP VPNs
draft-ietf-ospf-2547-dnbit-01.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 (i.e., performs the routing
computation) 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 redistribute routes
to these address prefixes into BGP, so that they can 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. Thus routes from different
VPNs can be carried by a single BGP instance, and can be stored in a
common BGP table, without fear of conflict.
If a PE router receives a particular VPN-IP route via BGP, and if
that PE is attached to a CE in the VPN to which the route belongs,
then BGP's decision process may install that route in the BGP route
table. If so, the PE translates the route back into an IP route, and
redistributes it to the routing protocol 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.
If a VPN uses OSPF as its internal routing protocol, it is not
necessarily the case that the CE routers of that VPN 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. In this case, a PE will run a
separate instance of OSPF for each VPN which is attached to the PE;
the PE will in general have multiple VPN-specific OSPF routing
tables.
When OSPF is used on a PE-CE link which belongs to a particular VPN,
the PE router must redistribute to that VPN's OSPF instance certain
routes which have been installed in the BGP routing table.
Similarly, a PE router must redistribute to BGP routes which have
been installed in the VPN-specific OSPF routing tables. Procedures
for this are specified in [VPN-OSPF].
The routes which are redistributed from BGP to OSPF are advertised in
LSAs that are originated by the PE. The PE acts as an OSPF border
router, advertising some of these routes in AS-external LSAs, and
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some in summary LSAs, as specified in [VPN-OSPF].
Similarly, when a PE router receives an LSA from a CE router, it runs
the OSPF routing computation. Any route that gets installed in the
OSPF routing table must be translated into a VPN-IP route and then
redistributed into BGP. BGP will then distribute these routes to the
other PE routers.
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 routing
computation 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
routing computation, 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. Per [OSPF-VPN], each PE router must function as an OSPF area 0
router. If the PE-CE link is an area 0 link, then it is possible for
the PE to receive, over that link, a summary LSA which originated at
another PE router. Thus we need some way of marking a summary LSA to
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indicate that it is carrying information about a path via a PE
router.
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.
+-------------------------------------+
| DN | * | DC | EA | N/P | MC | E | * |
+-------------------------------------+
Options Field with DN Bit
(RFC 2328, Section A.2)
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. We also wish to thank Acee Lindem for his helpful comments.
6. Authors' Addresses
Eric C. Rosen
Cisco Systems, Inc.
1414 Massachusetts Avenue
Boxborough, MA 01719
E-mail: erosen@cisco.com
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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
[OSPF] "OSPF Version 2", RFC 2328, Moy, J., April 1998
[VPN] "BGP/MPLS VPNs", draft-ietf-l3vpn-rfc2547bis-01.txt, Rosen,
Rekhter, et. al., September 2003
[OSPF-VPN] "OSPF as the PE/CE Protocol in BGP/MPLS VPNs", draft-
ietf-l3vpn-ospf-2547-00.txt, Rosen, Psenak, Pillay-Esnault, June 2003
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