One document matched: draft-rosen-vpns-ospf-bgp-mpls-01.txt
Differences from draft-rosen-vpns-ospf-bgp-mpls-00.txt
Network Working Group Eric C. Rosen
Internet Draft Peter Psenak
Expiration Date: August 2001 Cisco Systems, Inc.
February 2001
OSPF as the PE/CE Protocol in BGP/MPLS VPNs
draft-rosen-vpns-ospf-bgp-mpls-01.txt
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
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The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.
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Abstract
[VPN] describes a method of providing a VPN service. That method
allows a variety of different protocols to be used as the routing
protocol between the Customer Edge (CE) router and the Provider Edge
(PE) router. However, it does not fully specify the procedures which
must be implemented within the Provider's network when OSPF is used
as the PE/CE routing protocol. This document provides that
specification.
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Table of Contents
1 Specification of Requirements .......................... 2
2 Introduction ........................................... 2
3 Requirements ........................................... 3
4 BGP/OSPF Interaction Procedures for PE routers ......... 5
4.1 Overview ............................................... 5
4.2 Details ................................................ 7
4.2.1 General ................................................ 7
4.2.2 Handling LSAs from the CE .............................. 8
4.2.3 Sham Links ............................................. 10
4.2.4 VPN-IP routes received via BGP ......................... 12
5 Acknowledgments ........................................ 13
6 Authors' Address ....................................... 13
7 Bibliography ........................................... 14
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 a VPN service to customers. In that method, a
customer's edge devices (CE devices) are connected to the provider's
edge routers (PE routers). If the CE device is a router, then the PE
router may become a routing peer of the CE router (in some routing
protocol), and may as a result learn the routes which lead to the
CE's site and which need to be distributed to other PE routers that
attach to the same VPN.
The PE routers which attach to a common VPN use BGP to distribute the
VPN's routes to each other. A CE router can then learn the routes to
other sites in the VPN by peering with its attached PE router in a
routing protocol. CE routers at different sites do not, however,
peer with each other.
It can be expected that many VPNs will use OSPF as their internal
routing protocol. This does not necessarily mean that the PE routers
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need to use OSPF to peer with the CE 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.
Like anything else, the use of OSPF on the PE-CE link has advantages
and disadvantages. The disadvantage to using OSPF on the PE-CE link
is that it gets the PE router involved in a VPN site's IGP, however
peripherally. The advantages though are:
- The administrators of the CE router need not have any expertise
in any routing algorithm other than OSPF.
- The CE routers do not need to have support for any routing
algorithms other than OSPF.
- If a customer is transitioning his network from a traditional
OSPF backbone to the VPN service described in [VPN], the use of
OSPF on the PE-CE link eases the transitional issues.
It seems likely that some SPs and their customers will resolve these
trade-offs in favor of the use of OSPF on the PE-CE link. Thus we
need to specify the procedures which must be implemented by a PE
router in order to make this possible. (No special procedures are
needed in the CE router though; CE routers just run whatever OSPF
implementations they may have.)
3. Requirements
Consider a set of VPN sites which are thought of as a common "OSPF
domain". These will almost certainly be a set of sites which
together constitute an "intranet", and each of which runs OSPF as its
intra-site routing protocol.
Per [VPN], the VPN routes are distributed among the PE routers by
BGP. If the PE uses OSPF to distribute routes to the CE router, the
standard procedures governing BGP/OSPF interactions [OSPF] would
cause routes from one site to be delivered to another as AS-external
routes (in type 5 LSAs). This is undesirable; it would be much
better to deliver such routes in type 3 LSAs (as inter-area routes),
so that they can be distinguished from any "real" AS-external routes
that may be circulating in the VPN. (That is, so that they can be
distinguished by OSPF from routes which really do not come from
within the VPN.) Hence it is necessary for the PE routers to
implement a modified version of the BGP/OSPF interaction procedures.
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In fact, we would like to have a very general set of procedures which
allows a customer to easily replace a legacy private OSPF backbone
with the VPN service. We would like this procedure to meet the
following set of requirements:
- The procedures should not make assumptions about the OSPF
topology. In particular, it should not be assumed that customer
sites are OSPF stub sites or NSSA sites. Nor should it be
assumed that a customer site contains only one OSPF area, or that
it has no area 0 routers.
- If VPN sites A and B are in the same OSPF domain, then routes
from one should be presented to the other as OSPF intra-network
routes. In general, this can be done by presenting such routes
in type 3 LSAs.
Note that this allows two VPN sites to be connected via an "OSPF
backdoor link". That is, one can have an OSPF link between the
two sites which is used only when the VPN backbone is
unavailable. (This would not be possible with the ordinary
BGP/OSPF interaction procedures. The ordinary procedures would
present routes via the VPN backbone as AS-external routes, and
these could never be preferred to intra-network routes.) This
may be very useful during a period of transition from a legacy
OSPF backbone to a VPN backbone.
- It should be possible to make use of an "OSPF backdoor link"
between two sites, even if the two sites are in the same OSPF
area, and neither of the routers attached to the inter-site
backdoor link is an area 0 router. This can also be very useful
during a transition period, and eliminates any need to
reconfigure the sites' routers to be ABRs.
Assuming that it is desired to have the route via the VPN
backbone be preferred to the backdoor route, the VPN backbone
itself must be presented to the CE routers at each site as a link
between the two PE routers to which the CE routers are
respectively attached.
- CE routers, connected to PE routers of the VPN service, may
themselves function as OSPF backbone (area 0) routers. An OSPF
backbone may even consist of several "segments" which are
interconnected themselves only via the VPN service. In such a
scenario, full intercommunication between sites connected to
different segments of the OSPF backbone should still be possible.
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- The transition from the legacy private OSPF backbone to the VPN
service must be simple and straightforward. The transition is
likely to be phased, such that customer sites are migrated one by
one from the legacy private OSPF backbone to the VPN service.
During the transition, any given site might be connected to the
VPN service, to the legacy OSPF backbone, or to both. Complete
connectivity among all such sites must be maintained.
Since the VPN service is to replace the legacy backbone, it must
be possible, by suitable adjustment of the OSPF metrics, to make
OSPF prefer routes which traverse the SP's VPN backbone to
alternative routes which do not.
- The OSPF metric assigned to a given route should be carried
transparently over the VPN backbone.
Routes from sites which are not in the same OSPF domain will appear
as AS-external routes.
We presuppose familiarity with the contents of [OSPF], including the
OSPF LSA types, and will refer without further exegesis to type 1, 2,
3, etc., LSAs. Familiarity with [VPN] is also presupposed.
4. BGP/OSPF Interaction Procedures for PE routers
4.1. Overview
[VPN] defines the notion of a Per-Site Routing and Forwarding Table,
or VRF. A PE router must be capable of running multiple instances of
OSPF, where each instance is associated with a particular VRF.
(Whether these instances are realized as separate processes, or
merely as separate contexts of a common process, is an implementation
matter.)
BGP is used to distribute routes among the set of PE routers that
attach to a single OSPF domain. Per [VPN], these routes are
distributed as VPN-IP routes. Import/export to/from particular VRFs
is governed via Route Targets. To meet the above requirements, a PE
which imports a particular route into a particular VRF needs to know
whether that route comes from the same OSPF domain and the same OSPF
area as the CEs to which it is attached. Our procedure is to encode
this information as BGP Extended Communities attributes [EXT], and
have BGP distribute it along with the VPN-IP route. The OSPF metric
of a route is also carried as a BGP attribute of the route.
If two PEs attach to different VPN sites that are in the same OSPF
area (as indicated by the OSPF area number), the PE/CE links to those
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site MAY be treated as links within that area. In addition, each PE
MAY flood, into that area, a type 1 LSA advertising a link to the
other PE. If this procedure is followed, two VPN sites in the same
OSPF area will see the VPN backbone as a link within that area, a
link between the two PE routers. We refer to this link as a "sham
link". This allows routes from one site to the other to be treated
as intra-area routes. The procedures governing the use of sham links
are specified in Section 4.2.3.
Every PE attached to a particular OSPF network MUST be an OSPF area 0
router. This allows it to distribute inter-area routes to the CE via
Type 3 LSAs. The CE router might or might not be an area 0 router,
and the PE/CE link might or might not be an area 0 link.
If the OSPF network contains area 0 routers (other than the PE
routers), at least one PE router must have an area 0 link to a non-PE
area 0 router in that OSPF network. (The non-PE area 0 router
functions as a CE router.) This ensures that inter-area routes and
AS-external routes can be leaked between the PE routers and the non-
PE OSPF backbone.
When a type 3 LSA is sent over an area 0 link from a PE router to a
CE router, a special bit in the LSA Options field is set. This is
used to ensure that if any CE router sends this type 3 LSA to a PE
router, the PE router will not further redistribute it.
Two sites which are not in the same OSPF area will see the VPN
backbone as being an integral part of the OSPF backbone. However, if
there are area 0 routers which are NOT PE routers, then the VPN
backbone actually functions as a sort of higher level backbone,
providing a third level of hierarchy above area 0. This allows a
legacy OSPF backbone to become disconnected during a transition
period, as long as the various segments all attach to the VPN
backbone.
When a PE router needs to distribute to a CE router a route which
comes from a site outside the latter's OSPF domain, the PE router
presents itself as an ASBR, and distributes the route in a type 5
LSA. OSPF route tagging is used to ensure that a type 5 LSA
generated by a PE router will be ignored by any other PE router that
may receive it. A special OSPF route tag, which we will call the VPN
Route Tag, will be used for this purpose.
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4.2. Details
4.2.1. General
If a PE and a CE are communicating via OSPF, the PE MUST create, and
MUST flood to the CE, a type 1 LSA advertising its link to the CE.
The PE MUST have an OSPF router id which is valid (i.e., unique)
within the OSPF domain. The PE MUST also be configured to know which
OSPF area the link is in. A PE-CE link may be in any area, including
area 0; this is a matter of the OSPF configuration.
Whether or not the PE-CE link is an area 0 link, the PE itself MUST
be an OSPF area 0 router. That is, the link state topology from a
site will not be passed along by the PE.
The PE MUST support at least one OSPF instance for each OSPF domain
to which it attaches. Each instance of OSPF MUST be associated with
a single VRF. If n CEs associated with that VRF are running OSPF on
their respective PE/CE links, then those n CEs are OSPF adjacencies
of the PE in the corresponding instance of OSPF. Generally, though
not necessarily, if the PE attaches to several CEs in the same OSPF
domain, it will associate the interfaces to those PEs with a single
VRF.
If the OSPF domain has any area 0 routers (other than the PE
routers), then at least one of those MUST be a CE router, and MUST
have an area 0 link to at least one PE router. This adjacency MAY be
via an OSPF virtual link.
For each OSPF domain, there MUST be a globally unique identifier
which can be encoded in a BGP Extended Communities attribute. Routes
from a particular OSPF domain will, when distributed in BGP as VPN-
IPv4 routes, carry this attribute. The OSPF Domain will be encoded as
the VPN of Origin Extended Community [VPN, EXT] attribute. (It is
shown in [VPN] how to assign globally unique values to this
attribute.) Each VRF that is associated with an OSPF instance must be
configured with a value of this attribute. All VRFs that correspond
to the same OSPF domain MUST be configured with the same value of
this attribute.
If a particular VRF in a PE is associated with an instance of OSPF,
then it will be configured with a special OSPF route tag value, which
we call the VPN Route Tag. This route tag will be included in any
Type 5 LSAs that the PE originates and sends to any of the attached
CEs. Its value is arbitrary, but must be distinct from any OSPF Route
Tag being used within the OSPF domain. Its value MUST therefore be
configurable.
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If a PE router needs to use OSPF to distribute to a CE router a route
which comes from a site outside the CE router's OSPF domain, the PE
router SHOULD present itself to the CE router as an Autonomous System
Border Router (ASBR), and SHOULD report such routes as AS-external
routes. That is, these PE routers originate Type 5 LSAs reporting
the extra-domain routes as AS-external routes. Each such Type 5 LSA
MUST contain an OSPF route tag whose value is that of the VPN Route
Tag. This tag identifies the route as having come from a PE router.
The VPN Route Tag MUST be used to ensure that a Type 5 LSA originated
by a PE router is not redistributed through the OSPF area to another
PE router.
When a PE/CE link is an area 0 link, the high-order bit of the LSA
Options field (previously unused) is used to distinguish type 3 LSAs
which report routes across the VPN backbone from other VPN sites. We
refer to this as the DN bit. The DN bit is set in a type 3 LSA which
is sent from a PE router to a CE router across an area 0 link. When a
PE router receives, from a CE router, a type 3 LSA with the DN bit
set, the route is ignored. Without this mechanism, the type 3 LSA
could be sent by a PE to a CE, flooded through a number of OSPF
routers, and then sent to another PE; this could cause a loop. This
mechanism prevents such loops.
4.2.2. Handling LSAs from the CE
This section specifies the way in which a PE router handles the OSPF
LSAs it receives from a CE router.
If a route is received in a Type 5 LSA, and if it has an OSPF route
tag value equal to the VPN Route Tag, then that route is ignored.
If a route is received in a Type 3 LSA, and if it has the DN bit set
in its Options Field, then the route is ignored.
Otherwise, there is normal OSPF processing, for the relevant instance
of OSPF in the PE.
Next, the PE must examine the corresponding VRF. For every address
prefix which appears there, the PE must create a VPN-IPv4 route in
BGP. These routes must have the following attributes:
- The VPN of Origin Extended Community attribute must uniquely and
globally identify the OSPF domain, as explained above.
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- OSPF Route Type Extended Community Attribute. This is encoded as
follows:
* Type: 0x8000
* Area Number: 4 bytes, encoding a 32-bit area number. For AS-
external routes, the value is 0. A non-zero value identifies
the route as being internal to the OSPF domain, and as being
within the identified area. Area numbers are relative to a
particular OSPF domain.
* OSPF Route Type: 1 byte, encoded as follows:
** 1 or 2 for intra-area routes (depending on whether the
route came from a type 1 or a type 2 LSA -- however this
difference is not significant to the procedures
specified herein)
** 3 for summary routes
** 5 for external routes (area number must be 0)
** 7 for NSSA routes.
* Options: 1 byte. Currently this is only used if the route
type is 5 or 7. Setting the least significant bit in the
field indicates that the route carries a type 2 metric.
- OSPF Router ID Extended Community. This attribute specifies the
router-id corresponding to the router that is identified in the
BGP Next Hop attribute.
* Type: 0x8001
* Router ID: 4 Byte
* Unused: 2 Byte. Must be zero.
Note that this router-id is an address in the address space of
the OSPF network (i.e., the address space of the VPN), NOT in the
address space of the Service Provider network. That is, it must
be unique only within an OSPF domain.
- MED. By default, this should be set to the value of the OSPF
distance associated with the route, plus 1.
The intention of all this is the following. OSPF Routes from one
site are converted to BGP, distributed across the VPN backbone,
and possibly converted back to OSPF routes before being
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distributed into another site. With these attributes, BGP carries
enough information about the route to enable the route to be
converted back into OSPF "transparently", just as if BGP had not
been involved.
Routes which a PE receives in type 4 LSAs MUST be ignored.
The attributes specified above are in addition to any other
attributes which routes must carry in accord with the [VPN].
The Site of Origin attribute, which is usually required by [VPN], is
OPTIONAL for routes which a PE learns from a CE via OSPF.
Use of the Site of Origin attribute would, in the case of a multiply
homed site (i.e., a site attached to several PE routers), prevent an
intra-site route from being reinjected into a site from the VPN
backbone. Such a reinjection would not harm the routing, because the
route via the VPN backbone would be advertised in a type 3 LSA, and
hence would appear to be an inter-area route; the real intra-area
route would be preferred. But unnecessary overhead would be
introduced. On the other hand, if the Site of Origin attribute is not
used, a partitioned site will find itself automatically repaired,
since traffic from one partition to the other will automatically
travel via the VPN backbone. Therefore the use of a Site of Origin
attribute is optional, so that a trade-off can be made between the
cost of the increased overhead and the value of automatic partition
repair.
4.2.3. Sham Links
Suppose there are two sites in the same OSPF area. Each site is
attached to a different PE router, and there is also an intra-area
OSPF link connecting the two sites.
It is possible to treat these two sites as a single VPN site which
just happens to be multihomed to the backbone. This is in fact the
simplest thing to do, and is perfectly adequate, providing that the
preferred route between the two sites is via the intra-area OSPF link
(a "backdoor link"), rather than via the VPN backbone. There will be
routes between sites that go through the PE routers, but these routes
will appear to be inter-area routes, and OSPF will consider them to
be less preferable than the intra-area routes through the backdoor
link.
If it is desired to have OSPF prefer the routes through the backbone
over the routes through the backdoor link, then the routes through
the backbone must be appear to be intra-area routes. To make a route
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through the backbone appear to be an inter-area route, it is
necessary to make it appear as if there is an intra-area link
connecting the two PE routers. This is what we refer to as a "sham
link". (If the two sites attach to the same PE router, this of
course is not necessary.)
A sham link can be thought of as a relation between two VRFs. To
configure a sham link between two VRFs, at least one of the VRFs has
to be configured to create a sham link to the other, where the
"other" is identified by its router Id. Procedures for single-ended
configuration of the sham link are for further study.
If it were considered desirable to create a full mesh of sham links
among sites that are in the same VPN, and which have the same OSPF
area number, then the creation of the sham links could be fully
automated. It seems more likely that one will want to have more
control over the existence of the sham links, in which case
configuration is needed.
If two VRFs are to be connected by a sham link, each VRF must be
configured with a "sham link endpoint address", a /32 address which
is treated as an address of the PE router containing that VRF. The
sham link endpoint address is an address in the VPN's address space,
not the SP's address space. It is distributed as a VPN-IPv4 address
by BGP, but is not advertised by OSPF. (Each VRF also has an OSPF
router-id, but if the OSPF router-id is itself advertised by OSPF,
then the sham link endpoint address must be different from the
router-id.) To configure a sham link, one must specify the remote
sham link endpoint address.
Sham links MUST be treated by OSPF as OSPF Demand Circuits. This
means that LSAs will be flooded over them, but periodic refresh
traffic is avoided. Note that, as long as the backdoor link is up,
flooding the LSAs over the sham link serves no purpose. However, if
the backdoor link goes down, OSPF does not have mechanisms enabling
the routers in one site to rapidly flush the LSAs from the other
site. Therefore it is still necessary to maintain synchronization
among the LSA databases at the two sites, hence the flooding over the
sham link.
The sham link is an unnumbered point-to-point intra-area link, and is
advertised by means of a type 1 LSA.
Sham links may be created for any area, including area 0.
The OSPF metric associated with a sham link must be configurable (and
there must be a configurable default). Whether traffic between the
sites flows via a backdoor link or via the VPN backbone (i.e., via
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the sham link) depends on the settings of the OSPF link metrics. The
metrics can be set so that the backdoor link is not used unless
connectivity via the VPN backbone fails, for example.
If a PE determines that a particular route traverses the sham link,
then the PE SHOULD NOT redistribute that route into BGP as a VPN-IPv4
route.
4.2.4. VPN-IP routes received via BGP
This section describes how the PE router handles VPN-IP routes
received via BGP.
If a received BGP VPN-IP route is not installed in the VRF, nothing
is reported to the CE. A received route will not be installed into
the VRF if some other route is preferable. (Note that a route which
is not installed in the VRF may still cause the PE to create an OSPF
link to another PE as specified in the previous section.)
Note that according to the usual OSPF route preference rules, intra-
area routes, as computed by the OSPF, will be installed in the VRF in
preference to any other routes received over BGP. This means that the
CE will simply not hear about inter-area or external routes to
address prefixes for which there is an intra-area route.
In the following, we specify what is reported, in OSPF LSAs, by the
PE to the CE, assuming that the PE is not configured to do any
further summarization or filtering of the routing information before
reporting it to the CE.
If ONE of the following conditions holds for a given route that is
received via BGP:
- the route does not have a VPN of Origin attribute (identifying
the OSPF domain), or
- the route's VPN of Origin attribute has a value which is not the
same as that associated with the VRF, or
- the route has no OSPF Route Type attribute
then the route MUST be distributed to the CE in a type 5 LSA with a
type 2 metric. By default, the MED, if present, is converted to a
type 2 metric. If the MED is not present, a default type 2 metric
value is used.
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Otherwise, if the route has an OSPF route type of external route, it
MUST be advertised to the CE in a type 5 LSA. By default, the MED, if
present, is converted to a type 1 or type 2 metric, as determined by
the external route property of the VPN-IPv4 route. If no MED is
present, a default type 2 metric value is used.
Whenever a type 5 LSA is originated by a PE router and sent to a CE
router, the VPN Route Tag MUST be placed in the LSA.
Otherwise, if the route has an OSPF route type of "summary route",
the route should be treated as if it had been received in an OSPF
type 3 LSA. This means that the PE will report the route in a type 3
LSA to the CE. The DN bit is set in this LSA. (Note that this case
is possible even if the VPN-IP route carries an area number identical
to that of the CE router. This means that if an area is
"partitioned" such that the two pieces are connected only via the VPN
backbone, it appears to be two areas, with inter-area routes between
them.)
Note that this way of handling AS-external routes makes every PE
appear to be an ASBR attached to all the AS-external routes. In a
multihomed site, this can result in a number of type 5 LSAs
containing the same information.
5. Acknowledgments
Significant contributions to this work have been made by Derek Yeung
and Yakov Rekhter.
Thanks to Ross Callon and Ajay Singhal for their comments.
6. Authors' Address
Eric C. Rosen
Cisco Systems, Inc.
250 Apollo Drive
Chelmsford, MA, 01824
E-mail: erosen@cisco.com
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Peter Psenak
Parc Pegasus,
De Kleetlaan 6A
1831 Diegem
Belgium
E-mail: ppsenak@cisco.com
7. Bibliography
[EXT] "BGP Extended Communities Attribute", draft-ramachandra-bgp-
ext-communities-04.txt, Ramachandra, S. and Tappan, D. May, 2000.
[OSPF] "OSPF Version 2", RFC 2328, Moy, J., April 1998.
[VPN] "BGP/MPLS VPNs", draft-rosen-rfc2547bis-01.txt, Rosen, E., et.
al., May 2000.
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