L3VPN Working Group P. Marques
Internet-Draft Juniper Networks
Expires: February 24, 2006 R. Raszuk
L. Martini
D. Tappan
Cisco Systems, Inc.
August 23, 2005
Internal BGP as PE-CE protocol
draft-marques-l3vpn-ibgp-01
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Copyright (C) The Internet Society (2005).
Abstract
This document defines protocol extensions and procedures for BGP
PE-CE router iteration in RFC2547bis [4] networks. These have the
objective of making the usage of the RFC2547bis VPN transparent to
the customer network, as far as routing information is concerned.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. IP VPN network as a Route Server . . . . . . . . . . . . . . . 4
3. Path attributes . . . . . . . . . . . . . . . . . . . . . . . 5
4. Carrying internal BGP routes . . . . . . . . . . . . . . . . . 6
5. Next-hop handling . . . . . . . . . . . . . . . . . . . . . . 7
6. Exchanging routes between different VPN customer networks . . 8
7. Security considerations . . . . . . . . . . . . . . . . . . . 10
8. IANA considerations . . . . . . . . . . . . . . . . . . . . . 11
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 12
10. Normative References . . . . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13
Intellectual Property and Copyright Statements . . . . . . . . . . 14
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1. Introduction
In current deployments, when BGP is used as the PE-CE routing
protocol, these peering sessions are typically configured as an
external peering between the VPN provider AS and the customer network
AS. At each External BGP boundary, Path Attributes [1] are modified
as per standard BGP rules. This includes prepending the AS_PATH
attribute with the autonomous system of the originating customer CE
and the automomous system(s) of the provider edge 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 RFC2547bis 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 RFC2547bis 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 which use an
hierarchical design are typically modeled such that the top-level
core or backbone participates in a full iBGP mesh which distributes
routing information between sites via BGP route reflection [2] or
confederations [3]. This will be our service model definition.
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2. IP VPN network 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 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 network as the
equivalent of an Internal BGP Route Server which multiplexes
information from N VPN attachment points.
A route learned by any of the PEs in the IP VPN network, 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 server.
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.
When a route is advertised from PE to CE, if its 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.
A PE router can also act as a route reflector to local CE routers.
Reflection can also be used hierarchically in order to avoid direct
communication between the PE and non-directly connected CEs that may
exist in the site.
This Route Server model can also be used to support a confederation
style abstraction to CE devices. We choose not to describe in detail
the procedures for that mode of operation, at this point.
Confederations are considered to be less common than route reflection
in enterprise environments.
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3. Path attributes
--> push path attributes --> vrf-export --> 2547
VRF route PE-PE route
advertisement
<-- pop path attributes <-- vrf-import <--
The diagram above shows the BGP path attribute stack processing in
relation to existing 2547 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 poped after the Import Target
processing step that identifies the VRF table in which a PE received
route is accepted.
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
domain.
For the purposes of VRF route selection performed at the PE, between
routes received from local CEs and remote PEs, VPN network IGP
metrics should always be considered higher (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 to address this type of situations such as the LOCAL_PREF
attribute.
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
advertisement rules.
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
network prevents the formation of routing loops due to mutual
redistribution between the two networks.
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4. Carrying internal BGP routes
In order to carry the original BGP attributes of a route received
from a CE, 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 the
MP_REACH and MP_UNREACH attributes.
This 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 stack
like attribute when the route is received by the VPN network and
"popped" when the route is advertised in the PE to CE direction.
Using this mechanism isolates the customer network from the
attributes used in the VPN network and vice versa. Attributes as the
route reflection cluster list attribute are segregated such that
customer network cluster identifiers won't be considered by the VPN
network route reflectors and vice-versa.
The autonomous system number present in the ATTR_SET attribute is
designed to prevent a route originating in a given autonomous-system
iBGP to be leaked into a different autonomous-system, without proper
AS_PATH manipulation. It should contain the autonomous system of the
customer network that originates the given set of attributes.
The NEXT_HOP attribute SHOULD NOT be included in an ATTR_SET.
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5. Next-hop handling
When RFC2547bis 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.
An important component of BGP route selection is the IGP distance to
the NEXT_HOP of the route.
When a VPN service is used to provide interconnection between
different sites, since the VPN network runs a different IGP domain,
metrics between the VPN and customer networks are not comparable.
However, the most important component of a metric is the inter-area
metric, which is known to the VPN 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
intra-cluster metrics.
The approach taken by this document is to rewrite the NEXT_HOP
attribute at the PE-CE boundary. PE routers take into account the
PE-PE IGP distance calculated by the VPN 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.
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6. Exchanging routes between different VPN customer networks
A given VPN customer network SHOULD use internal or external BGP
sessions consistently for peering sessions where the same autonomous
system is used.
In scenarios such as what is commonly referred to 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 RFC2547 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 the external peer (CE 2) a BGP
advertisement that would correspond to a situation where AS 1 and 2
have a external BGP session between them.
It order to achieve this goal the following set of rules apply:
When advertising an iBGP originated route to iBGP, a PE router
MUST check that the autonomous-system contained in the ATTR_SET
attribute matches the autonomous system of the CE to which the
route is being advertised.
In case the autonomous-systems do match, the route is advertised
with the attributes contained in the ATTR_SET attribute.
Otherwise, in the case of an autonomous-system mismatch, the set
of attributes to be advertised to the CE in question shall be
constructed as follows:
1. The path attributes are set to the attributes contained in the
ATTR_SET attribute.
2. Internal BGP specific attributes are discarded (LOCAL_PREF,
ORIGINATOR, CLUSTER_LIST, etc).
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3. The autonomous-system 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 destination
ASes.
4. Internal BGP specific attributes corresponding to the
configuration of destination AS (LOCAL_PREF) are added.
When advertising an iBGP originated route to eBGP, a PE router
shall apply steps 1 to 3 defined above and subsequently prepend
its own autonomous-system number to the AS_PATH attribute (i.e.
both the originator and VPN network as numbers are prepended).
When advertising an eBGP originated route to iBGP, a PE router
MUST prepend its own as number before adding iBGP only as-path
attributes (LOCAL_PREF).
In all cases where an iBGP originating route is processed, 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.
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7. 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 a IP VPN provider perspective, this mechanism will assure
separation between the BGP path attributes advertised by the customer
CE router and the BGP attributes used within the provider network,
thus potentially improving security.
Although this behavior is largely implementation dependent, currently
it is 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 out to other PEs.
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 eBGP session (e.g. local-pref), this
does not significantly change the fact that the VPN customer must
trust its service provider to provide it correct routing information.
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8. IANA considerations
This document defines a new BGP path attribute which is part of a
registry space managed by IANA. We request that IANA update its
registry with the value specified above for the ATTR_SET path
attribute.
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9. Acknowledgments
The authors would like to acknowledge the contributions of Yakov
Rekhter, Luyuan Fang and Jan Novak.
10. Normative References
[1] Rekhter, Y. and T. Li, "A Border Gateway Protocol 4 (BGP-4)",
RFC 1771, March 1995.
[2] Bates, T., Chandra, R., and E. Chen, "BGP Route Reflection - An
Alternative to Full Mesh IBGP", RFC 2796, April 2000.
[3] Traina, P., McPherson, D., and J. Scudder, "Autonomous System
Confederations for BGP", RFC 3065, February 2001.
[4] Rosen, E., "BGP/MPLS IP VPNs", draft-ietf-l3vpn-rfc2547bis-03
(work in progress), October 2004.
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Authors' Addresses
Pedro Marques
Juniper Networks
1194 N. Mathilda Ave.
Sunnyvale, CA 94089
US
Email: roque@juniper.net
Robert Raszuk
Cisco Systems, Inc.
170 West Tasman Dr
San Jose, CA 95134
US
Email: rraszuk@cisco.com
Luca Martini
Cisco Systems, Inc.
9155 East Nichols Avenue, Suite 400
Englewood, CO 80112
US
Email: lmartini@cisco.com
Dan Tappan
Cisco Systems, Inc.
300 Beaver Brook Rd.
Boxborough, MA 01719
US
Email: tappan@cisco.com
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