BGP4 OSPF interations -- draft
"David L. Kensiski" <kensiski@nas.nasa.gov> Mon, 03 August 1992 21:38 UTC
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From: "David L. Kensiski" <kensiski@nas.nasa.gov>
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To: ospf-folks@orville.nas.nasa.gov
>From: "Kannan Varadhan" <kannan@oar.net>
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To: ospf@comet.cit.cornell.edu, iwg@rice.edu
Subject: BGP4 OSPF interations -- draft
Date: Mon, 03 Aug 1992 15:21:17 -0400
Sender: kannan@oar.net
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Here's a copy of the draft, with comments from the last IETF factored in.
Also factored in is updates from comments from the IAB on the earlier
BGP OSPF interactions.
Comments are most desperately sought,
Thanks,
Kannan
valhalla% mkrfc bgp4-ospf.ms
Network Working Group K. Varadhan
Request for Comments: DRAFT OARnet
August 3, 1992
BGP4 OSPF Interaction
Table of Contents
1. Introduction .................................................... 1
2. Reachability Information Exchange ............................... 3
2.1. Exporting OSPF information into BGP ........................... 3
2.2. Importing BGP information into OSPF ........................... 4
3. BGP Identifier and OSPF router ID ............................... 5
4. Setting OSPF tags, BGP ORIGIN and AS_PATH attributes ............ 6
4.1. Semantics of the characteristics bits ......................... 8
4.2. Configuration parameters for setting the OSPF tag ............. 9
4.3. Manually configured tags ...................................... 10
4.4. Automatically generated tags .................................. 11
4.4.1. Destinations with incomplete path information, PathLength =0 . 11
4.4.2. Destinations with incomplete path information, PathLength =1 . 11
4.4.3. Destinations with incomplete path information, PathLength >=1 12
4.4.4. Destinations with complete path information, PathLength =0 ... 12
4.4.5. Destinations with complete path information, PathLength =1 ... 13
4.4.6. Destinations with complete path information, PathLength >=1 .. 14
4.5. Miscellaneous tag settings .................................... 14
4.6. Summary of the TagType field setting .......................... 15
5. Setting OSPF Forwarding Address and BGP NEXT_HOP attribute ...... 15
6. Changes from the BGP 3 - OSPF interactions document ............. 16
7. Security Considerations ......................................... 17
8. Acknowledgements ................................................ 17
9. Bibliography .................................................... 18
10. Author's Address ............................................... 18
Status of this Memo
This memo defines the various criteria to be used when designing an
Autonomous System Border Routers (ASBR) that will run BGP4 with other
ASBRs external to the AS and OSPF as its IGP. Distribution of this
memo is unlimited.
1. Introduction
This document defines the various criteria to be used when designing
an Autonomous System Border Routers (ASBR) that will run BGP4[BGP-4]
with other ASBRs external to the AS, and OSPF[RFC1247] as its IGP.
All future references of BGP in this document will refer to BGP
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version 4, as defined in [BGP-4].
This document defines how the following fields in OSPF and attributes
in BGP are to be set when interfacing between BGP and OSPF at an
ASBR:
BGP MULTI_EXIT_DISC vs. OSPF cost and type
BGP ORIGIN and AS_PATH vs. OSPF tag
BGP NEXT_HOP vs. OSPF Forwarding Address
BGP LOCAL_PREF
For a more general treatise on routing and route exchange problems,
please refer to [ROUTE-LEAKING] and [NEXT-HOP] by Philip Almquist.
This document uses the two terms ``Autonomous System'' and ``Routing
Domain.'' The definitions for the two are below:
The term Autonomous System is the same as is used in the BGP
RFC[RFC1267], given below:
``The use of the term Autonomous System here stresses the fact
that, even when multiple IGPs and metrics are used, the
administration of an AS appears to other ASs to have a single
coherent interior routing plan and presents a consistent picture
of what destinations are reachable through it. From the
standpoint of exterior routing, an AS can be viewed as
monolithic: reachability to destinations directly connected to
the AS must be equivalent from all border gateways of the AS.''
The term Routing Domain was first used in [ROUTE-LEAKING] and is
given below:
``A Routing Domain is a collection of routers which coordinate
their routing knowledge using a single (instance of) a routing
protocol.''
BGP and OSPF have the concept of a set of reachable destinations. |
This set is called NLRI or Network Layer Reachability Information. |
The set can be represented either as an IP address prefix, or an |
address, mask pair. Note that if the mask is contiguous in the |
latter, then the two representations are equivalent. In this |
document, we use the term ``address/mask pair'' in the context of |
OSPF, and ``destination'' or ``set of reachable destinations'' in |
the context of BGP.
This document follows the conventions embodied in the Host
Requirements RFCs [RFC1122, RFC1123], when using the terms "MUST",
"SHOULD," and "MAY" for the various requirements.
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2. Reachability Information Exchange
This section discusses the constraints that must be met to exchange
the set of reachable destinations between an external BGP session
with a peer from another AS and internal OSPF address/mask pairs.
2.1. Exporting OSPF information into BGP
1. The administrator MUST be able to selectively export |
address/mask pairs into BGP via an appropriate filter |
mechanism. |
This filter mechanism MUST support such control with the |
granularity of a single destination. |
This filter mechanism will be the primary method of |
aggregation of OSPF internal and type 1 and type 2 external |
routes within the AS into BGP. |
Additionally, the administrator MUST be able to filter based
on the OSPF tag and the various sub-fields of the OSPF tag.
The settings of the tag and the sub-fields are defined in
section 4 in more detail.
o The default MUST be to export no routes from OSPF into
BGP. A single configuration parameter MUST permit all
OSPF inter-area and intra-area address/mask pairs to be
exported into BGP.
OSPF external address/mask pairs of type 1 and type 2
MUST never be exported into BGP unless they are
explicitly configured.
2. An address/mask pair having a non-contiguous mask MUST not be |
exported to BGP. |
3. When configured to export an address/mask pair from OSPF into |
BGP, the ASBR MUST advertise the route containing the set of |
reachable destinations via BGP as soon as at least one of the |
destinations in the address/mask pair is determined to be |
reachable via OSPF and MUST stop advertising the route |
containing the set of reachable destinations when none of the |
destinations in the address/mask pair is reachable via OSPF. |
4. The network administrator MUST be able to statically |
configure the BGP attribute MULTI_EXIT_DISC attribute to be |
used for any route. |
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o The default MUST be to not include the MULTI_EXIT_DISC |
in the update[BGP-4]. |
5. An implementation of BGP and OSPF on an ASBR MUST have a
mechanism to set up a minimum amount of time that must elapse
between the learning of a new address/mask pair via OSPF and
subsequent advertisement of the address/mask pair via BGP to
the external neighbours.
o The default value for this setting MUST be 0, indicating
that the address/mask pair is to be advertised to the
neighbour BGP peers instantly.
Note that [BGP-4] mandates a mechanism to dampen the
inbound advertisements from adjacent neighbours. See
the variable MinRouteAdvertisementInterval in section
9.2.3.1.
2.2. Importing BGP information into OSPF
1. BGP implementations SHOULD allow an AS to control
announcements of BGP-learned set of reachable destinations
into OSPF. Implementations SHOULD support such control with
the granularity of a single destination. Implementations
SHOULD also support such control with the granularity of an
autonomous system, where the autonomous system may be either
the autonomous system that originated the information or the
autonomous system that advertised the information to the
local system (adjacent autonomous system).
o The default MUST be to import nothing from BGP into
OSPF. Administrators must configure every destination
they wish to import.
A configuration parameter MAY allow an administrator to
configure an ASBR to import all the set of reachable
destinations from BGP into the OSPF routing domain.
2. The administrator MUST be able to configure the OSPF cost and
the OSPF metric type of every destination imported into OSPF.
o The OSPF cost MUST default to 1; the OSPF metric type
MUST default to type 2.
3. Information learned via BGP from peers within the same AS
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MUST not be imported into OSPF.
4. The ASBR MUST never generate a default destination into the
OSPF routing domain unless explicitly configured to do so.
A default destination is a set of all possible destinations.
By convention, it is represented as an IP address 0, with a
prefix 0 or mask of 0 length.
A possible criterion for generating default into an IGP is to
allow the administrator to specify a set of (set of reachable
destinations, AS_PATH, default cost, default type) tuples.
If the ASBR learns of at least one of the destinations in the
set of reachable destinations, with the corresponding
AS_PATH, then it generates a default destination into the
OSPF routing domain, with the appropriate cost and type. The
lowest cost route will then be injected into the OSPF routing
domain.
This is the recommended method for originating default
destinations in the OSPF routing domain. |
5. Note that [RFC1247] requires the network number to be used as |
the Link State ID. This will produce a conflict if the ASBR |
tries to import two destinations, differing only in their |
prefix length. An implementation conforming to [RFC1247] |
MUST, in this case, drop the more specific route, i.e. the |
route corresponding to the longer prefix in the reachability |
information.
3. BGP Identifier and OSPF router ID
The BGP identifier MUST be the same as the OSPF router id at all
times that the router is up. Note that [BGP-4] requires that the BGP |
identifier be an address assigned to the BGP speaker.
This characteristic is required for two reasons.
i Consider the scenario in which 3 ASBRs, RT1, RT2, and RT3,
belong to the same autonomous system.
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+-----+
| RT3 |
+-----+
|
Autonomous System running OSPF
/ \
+-----+ +-----+
| RT1 | | RT2 |
+-----+ +-----+
Both RT1 and RT2 can reach an external destination X and
import this information into the OSPF routing domain. RT3 is
advertising this information about destination X to other
external BGP speakers. RT3 must use the OSPF router ID to
determine whether it is using RT1 or RT2 to forward packets to
destination X and hence build the correct AS_PATH to advertise
to other external speakers.
More precisely, RT3 MUST determine which ASBR it is using to |
reach network X by matching the OSPF router ID for its route |
to network X with the BGP identifier of one of the ASBRs, and |
MUST generate the corresponding network layer reachability |
information for further advertisement to external BGP peers.
ii It will be convenient for the network administrator looking at
an ASBR to correlate different BGP and OSPF information based
on the identifier.
Since the ASBR has to rely on the underlying OSPF infrastructure to |
route transit packets to other networks, it cannot use the |
LOCAL_PREF attribute learned via BGP from neighbours internal to |
the autonomous system. Hence, all ASBRs running BGP and OSPF MUST |
ignore any LOCAL_PREF attributes present in an update.
4. Setting OSPF tags, BGP ORIGIN and AS_PATH attributes
The OSPF external route tag is a ``32-bit field attached to each
external route . . . It may be used to communicate information
between AS boundary routers; the precise nature of such information
is outside the scope of [the] specification.''[RFC1247]
OSPF imports information from various routing protocols at all its
ASBRs. In some instances, it is possible to use protocols other than
EGP or BGP across autonomous systems. It is important, in BGP, to
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differentiate between reachable destinations that are external to the
OSPF routing domain but must be considered internal to the AS, as
opposed to reachable destinations that are external to the AS.
Reachable destinations that are internal to the AS and that may or
may not be external to the OSPF routing domain will not come to the
various BGP speakers from other BGP speakers within the same
autonomous system via BGP. Therefore, ASBRs running BGP must have
knowledge of this class of reachable destinations so that they can
advertise these destinations to the various external AS without
waiting for BGP updates from other BGP speakers within the same
autonomous system about these destinations.
Additionally, in the specific instance of an AS intermixing routers
running EGP and BGP as external gateway routing protocols, using OSPF
as an IGP, then within the autonomous system, it may not be necessary
to run BGP with every ASBR running EGP and not running BGP, if this
information can be carried in the OSPF tag field.
We use the external route tag field in OSPF to intelligently set the
ORIGIN and AS_PATH attributes in BGP. Both the ORIGIN and AS_PATH
attributes are well-known, mandatory attributes in BGP. The exact
mechanism for setting the tags is defined below.
The tag is broken up into sub-fields shown below. The various sub-
fields specify the characteristics of the set of reachable
destinations imported into the OSPF routing domain.
The high bit of the OSPF tag is known as the ``Automatic'' bit. When
this bit is set to 1, the following sub-fields apply:
0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|a|c|p l| ArbitraryTag | AutonomousSystem |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
a is 1 bit called the Automatic bit, indicating that the
Completeness and PathLength bits have been generated
automatically by a router. The meaning of this characteristic
and its setting are defined below.
c is 1 bit of Completeness information. The meaning of this
characteristic and its settings are defined below.
pl are 2 bits of PathLength information. The meaning of this
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characteristic and its setting are defined below.
ArbitraryTag
is 12 bits of tag information, which defaults to 0 but can be
configured to anything else.
AutonomousSystem (or ``AS'')
is 16 bits, indicating the AS number corresponding to the set
of reachable destinations, 0 if the set of reachable
destinations is to be considered as part of the local AS.
local_AS
The term `local_AS' refers to the AS number of the local
OSPF routing domain.
next_hop_AS
`next_hop_AS' refers to the AS number of an external BGP
peer.
When the Automatic bit is set to 0, the following sub-fields apply:
0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|a| LocalInfo |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
a is 1 bit called the Automatic bit, set to 0.
LocalInfo
is 31 bits of an arbitrary value, manually configured by the
network administrator.
The format of the tag for various values of the characteristics
bits is defined below.
4.1. Semantics of the characteristics bits
The Completeness and PathLength characteristics bits define the
characteristic of the set of reachable destinations imported into
OSPF from other ASBRs in the autonomous system. This setting is
then used to set the ORIGIN and NEXT_HOP attributes when re-
exporting these reachable destinations to an external BGP speaker.
o The Automatic characteristic bit is set when the Completeness
and PathLength characteristics bits are automatically set by
a border router.
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For backward compatibility, the Automatic bit must default to
0 and the network administrator must have a mechanism to
enable automatic tag generation. Nothing must be inferred
about the characteristics of the OSPF address/mask pair from
the tag bits, unless the tag has been automatically
generated.
o The Completeness characteristic bit is set when the source of
the incoming route is known precisely, for instance, from an
IGP within the local autonomous system or EGP at one of the
autonomous system's boundaries. It refers to the status of
the path information carried by the routing protocol.
o The PathLength characteristic sub-field is set depending on
the length of the AS_PATH that the protocol could have
carried when importing the reachability information into the
OSPF routing domain. The length bits will indicate whether
the AS_PATH attribute for the length is zero, one, or greater
than one.
Reachable destinations imported from an IGP will usually have
an AS_PATH of length of 0, reachable destinations imported
from an EGP will have an AS_PATH of length 1, BGP and routing
protocols that support complete path information, either as
AS_PATHs or routing domain paths, will indicate a path
greater than 1.
The OSPF tag is not wide enough to carry path information
about reachable destinations that have an associated
PathLength greater than one. Path information about these
destinations will have to be carried via BGP to other ASBRs
with the same autonomous system. Such destinations must not
be exported from OSPF into BGP.
In the following sections, the code YES will have value 1, and the |
code NO will have value 0.
4.2. Configuration parameters for setting the OSPF tag
o There MUST be a mechanism to enable automatic generation of
the tag characteristic bits.
o Configuration of an ASBR running OSPF MUST include the
capability to associate a tag value, for the ArbitraryTag, or
LocalInfo sub-field of the OSPF tag, with each instance of a
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routing protocol.
o Configuration of an ASBR running OSPF MUST include the
capability to associate an AS number with each instance of a
routing protocol.
Associating an AS number with an instance of an IGP is
equivalent to flagging those set of reachable destinations
imported from the IGP to be external destinations outside the
local autonomous system.
Specifically, when the IGP is RIP[RFC1058], it SHOULD be
possible to associate a tag and/or an AS number with every
interface running RIP on the ASBR.
4.3. Manually configured tags
0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0| LocalInfo |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
This tag setting corresponds to the administrator manually
setting the tag bits. Nothing MUST inferred about the
characteristics of the set of reachable destinations
corresponding to this tag setting.
For backward compatibility with existing implementations of
OSPF currently deployed in the field, this MUST be the default
setting for importing destinations into the OSPF routing
domain. There MUST be a mechanism to enable automatic tag
generation for imported destinations.
The OSPF tag to BGP attribute mappings for these reachable
destinations MUST be
Automatic=NO, LocalInfo=Arbitrary_Value =>
ORIGIN=<INCOMPLETE>, AS_PATH=<local_AS>
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4.4. Automatically generated tags
4.4.1. Destinations with incomplete path information, PathLength =
0
0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0|0|0| ArbitraryTag | AutonomousSystem |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
These are reachable destinations imported from routing
protocols with incomplete path information and cannot or may
not carry the neighbour AS or AS path as part of the routing
information.
The OSPF tag to BGP attribute mappings for these destinations
MUST be
Automatic=YES, Completeness=NO, PathLength=00, AS=0 =>
ORIGIN=<EGP>, AS_PATH=<local_AS>
4.4.2. Destinations with incomplete path information, PathLength =
1
0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0|0|1| ArbitraryTag | AutonomousSystem |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
These are reachable destinations imported from routing
protocols with incomplete path information. The neighbour AS
is carried in the routing information.
The OSPF tag to BGP attribute mappings for these destinations
MUST be
Automatic=YES, Completeness=NO, PathLength=01, AS=<next_hop_AS>
=> ORIGIN=<EGP>, AS_PATH=<local_AS, next_hop_AS>
This setting SHOULD be used for importing reachable
destinations from EGP into the OSPF routing domain. This
setting MAY also be used when importing reachable destinations
from BGP whose origin=<EGP> and AS_PATH=<next_hop_AS>; if the
BGP learned route has no other transitive attributes, then its
propagation via BGP to ASBRs internal to the autonomous system
MAY be suppressed.
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4.4.3. Destinations with incomplete path information, PathLength
>= 1
0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0|1|0| ArbitraryTag | AutonomousSystem |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
These are reachable destinations imported from routing
protocols with truncated path information.
The OSPF tag to BGP attribute mappings for these destinations
MUST be
Automatic=YES, Completeness=NO, PathLength=10, AS=don't care
These are imported by a border router, which is running BGP to
a stub domain, and not running BGP to other ASBRs in the same
autonomous system. This causes a truncation of the AS_PATH.
These destinations MUST not be re-exported into BGP at another
ASBR.
4.4.4. Destinations with complete path information, PathLength = 0
0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|1|0|0| ArbitraryTag | AutonomousSystem |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
These are reachable destinations imported from routing
protocols with either complete path information or are known to
be complete through means other than that carried by the
routing protocol.
The OSPF tag to BGP attribute mappings for these destinations
MUST be
Automatic=YES, Completeness=YES, PathLength=00, AS=00
=> ORIGIN=<EGP>, AS_PATH=<local_AS>
This SHOULD be used for importing reachable destinations into
OSPF from an IGP.
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4.4.5. Destinations with complete path information, PathLength = 1
0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|1|0|1| ArbitraryTag | AutonomousSystem |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
These are reachable destinations imported from routing
protocols with either complete path information, or are known
to be complete through means other than that carried by the
routing protocol. The routing protocol also has additional
information about the next hop AS the destination was learned
from.
The OSPF tag to BGP attribute mappings for these destination
MUST be
Automatic=YES, Completeness=YES, PathLength=01, AS=next_hop_AS
=> ORIGIN=<IGP>, AS_PATH=<local_AS, next_hop_AS>
This setting SHOULD be used when the administrator explicitly
associates an AS number with an instance of an IGP. This
setting MAY also be used when importing reachable destinations
from BGP whose origin=<IGP> and AS_PATH=<next_hop_AS>; if the
BGP learned route has no other transitive attributes, then its
propagation via BGP to other ASBRs internal to the autonomous
system MAY be suppressed.
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4.4.6. Destinations with complete path information, PathLength >=1
0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|1|1|0| ArbitraryTag | AutonomousSystem |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
These are reachable destinations imported from routing
protocols with complete path information and carry the AS path
information as part of the routing information.
The OSPF tag MUST be set to
Automatic=YES, Completeness=YES, PathLength=10, AS=don't care
These destinations MUST not be exported into BGP because these
destinations are already imported from BGP into the OSPF RD.
Hence, it is assumed that the BGP speaker will convey this
information to other BGP speakers within the same autonomous
system via BGP. As ASBR learning of such a destination MUST
wait for the BGP update from its internal neighbours before
advertising it to external BGP peers.
Note that an implementation MAY import reachable destinations
from BGP with a path length of 1 and no other transitive
attributes directly into OSPF and not send these routes via BGP
to ASBRs within the same autonomous system. In this situation,
it MUST use tag settings corresponding to 4.4.2, or 4.4.5.
4.5. Miscellaneous tag settings
0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3 0
1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|x|1|1| Reserved for future use |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The value of PathLength=11 is reserved during automatic tag
generation. Routers must not generate such a tag when importing
reachable destinations into the OSPF routing domain. ASBRs must
ignore tags which indicate a PathLength=11.
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4.6. Summary of the tag sub-field setting
The following table summarises the various combinations of
automatic tag settings for the Completeness and PathLength sub-
field of the OSPF tag and the default behaviour permitted for each
setting.
Completeness := 0 | 1
PathLength := 00 | 01 | 10 | 1
ORIGIN := <INCOMPLETE> | <IGP> | <EGP>
AS_PATH := valid AS path settings as defined in BGP [BGP-4]
PathLength ==> 00 01 10 11
Completeness
|| +--------------------------------------------------------------------
vv |
= NO | <EGP> <EGP> never export reserved
| <local_AS> <local_AS,next_hop_AS>
|
= YES | <IGP> <IGP> out of band reserved
| <local_AS> <local_AS,next_hop_AS>
|
The "out of band" in the table above implies that OSPF will not be
able to carry everything that BGP needs in its routing
information. Therefore, some other means must be found to carry
this information. In BGP, this is done by running BGP to other
ASBRs within the same autonomous system.
5. Setting OSPF Forwarding Address and BGP NEXT_HOP attribute
Forwarding addresses are used to avoid extra hops between multiple
routers that share a common network and that speak different routing
protocols with each other on the common network. |
Both BGP and OSPF have equivalents of forwarding addresses. In BGP,
the NEXT_HOP attribute is a well-known, mandatory attribute. OSPF
has a Forwarding address field. We will discuss how these are to be
filled in various situations.
Varadhan [Page 15]
INTERNET DRAFT August 92
Consider the 4 router situation below:
RT1 and RT2 are in one autonomous system, RT3 and RT4 are in another.
RT1 and RT3 are talking BGP with each other.
RT3 and RT4 are talking OSPF with each other.
+-----+ +-----+
| RT1 | | RT2 |
+-----+ +-----+
| | common network
---+-----------------------+--------------------------
<BGP> | |
+-----+ <OSPF> +-----+
| RT3 | | RT4 |
+-----+ +-----+
- Importing a reachable destination into OSPF: |
When importing a destination from BGP into OSPF, RT3 MUST |
always fill the OSPF Forwarding Address with the BGP NEXT_HOP |
attribute for the destination. |
- Exporting a reachable destination into BGP: |
When exporting set of reachable destinations internal to the |
OSPF routing domain from OSPF to BGP, if all the destinations |
in the set of reachable destinations are through RT4, then RT3 |
MAY fill the NEXT_HOP attribute for the set of reachable |
destinations with the address of RT4. This is to avoid |
requiring packets to take an extra hop through RT3 when |
traversing the AS boundary. This is similar to the concept of |
indirect neighbour support in EGP[RFC888, RFC827]. |
6. Changes from the BGP 3 - OSPF interactions document |
o The use of the term "route" has attained a more complicated |
structure in BGP 4. This document follows the constraint in |
the manner shown below: |
- The term set of reachable destinations is called a NLRI |
in [BGP-4]. |
- The term route in the BGP context refers to a set of |
reachable destinations, and the associated attributes for |
the set. |
- The term route in the OSPF context refers to the set of |
reachable destinations, and the cost and the type to |
Varadhan [Page 16]
INTERNET DRAFT August 92
reach destinations. This is to keep the definitions |
consistent in the document. |
o The notion of exchanging reachability information between BGP |
4 and OSPF has been updated to handle variable length net mask |
information. |
o The previous term INTER_AS_METRIC in BGP 3 has now been |
changed to MULTI_EXIT_DISC. |
o BGP 4 requires that the BGP identifier be an address assigned |
to the BGP speaker. This is dealt with in section 3. |
o BGP 4 uses a new attribute, LOCAL_PREF, to indicate to other |
BGP speakers in the autonomous system about preferences for |
exit routers. Section 3 indicates why an autonomous system |
running OSPF should ignore this attribute. |
o Section 5 on setting NEXT_HOP attributes and Forwarding |
Address fields has been updated to account for variable length |
net information. |
o This section, 6, has been added. |
7. Security Considerations
Security considerations are not discussed in this memo.
8. Acknowledgements
I would like to thank Yakov Rekhter, Jeff Honig, John Moy, Tony Li,
Rob Coltun, Dennis Ferguson, and Phil Almquist for their help and
suggestions in writing this document, without which I could not have
written this document. I would also like to thank them for giving me
the opportunity to write this document, and putting up with my
muddlements through various phases of this document.
I would also like to thank the countless number of people from the
OSPF and BGP working groups who have offered numerous suggestions and
comments on the different stages of this document.
Thanks also to Bob Braden, whose suggestions on the earlier BGP-OSPF
document, [BGP-OSPF] were useful even for this one, and have been
carried through.
Varadhan [Page 17]
INTERNET DRAFT August 92
9. Bibliography
[RFC827] Rosen, Eric C., ``Exterior Gateway Protocol (EGP)'', October
1982.
[RFC888] Seamonson, Linda J.; and Rosen, Eric C., ``'STUB' Exterior
Gateway Protocol'', January 1984.
[RFC1058] Hedrick, Charles, L., ``Routing Information Protocol'', June
1988.
[RFC1122] Braden, R.T., ed., ``Requirements for Internet hosts -
communication layers'', October 1989.
[RFC1123] Braden, R.T., ed., ``Requirements for Internet hosts -
application and support'', October 1989.
[RFC1247] Moy, John, ``The OSPF Specification Version 2'', January
1991.
[RFC1338] Fuller, Vince; Li, Tony; Yu, Jessica; Varadhan, Kannan,
``Supernetting: an Address Assignment and Aggregation Strategy'',
June 1992.
[ROUTE-LEAKING] Almquist, Philip, ``Ruminations on Route Leaking'', in
preparation.
[NEXT-HOP] Almquist, Philip, ``Ruminations on the Next Hop'', in
preparation.
[BGP-4] Rekhter, Yakov; and Li, Tony, Editors ``A Border Gateway
Protocol 4 (BGP-4)'', in preparation.
[BGP-OSPF] Varadhan, Kannan; ``BGP OSPF Interaction'', in preparation.
10. Author's Address:
Kannan Varadhan
Internet Engineer, OARnet,
1224, Kinnear Road,
Columbus, OH 43212-1136.
email: kannan@oar.net
Varadhan [Page 18]
valhalla%
-=-
Kannan Varadhan, 1224, Kinnear Road, +1 614 292 4137
Internet Engineer (OARnet) Columbus, OH 43212 +1 614 292 7168 (FAX)
- BGP4 OSPF interations -- draft Kannan Varadhan
- BGP4 OSPF interations -- draft David L. Kensiski
- BGP4 OSPF interations -- draft David L. Kensiski