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Versions: 00 01 02 03
Internet Engineering Task Force A. Lange
Internet-Draft Alcatel-Lucent
Intended status: Standards Track J. Haas
Expires: February 3, 2011 Juniper Networks
K. Patel
Cisco
S. Amante
Level 3
August 2, 2010
Flexible BGP Communities
draft-lange-flexible-bgp-communities-03
Abstract
This document defines a new attribute for BGP called the Flexible
Community attribute. Flexible Communities build on the experience
and utility of the standard BGP community, and the extended BGP
community attributes. This attribute allows operators to associate
structured information with a route or set of routes. This
information can be then be used to execute routing policy. An
enhanced version of communities is necessary to accommodate IPv6,
4-byte ASN's, and introduce a more extensible and flexible policy
expression. This document also introduces the concept of Neighbor
Classes. A Neighbor Class is applied to a group of BGP neighbors who
share certain attributes. For example, the PEER Neighbor Class could
be applied to BGP sessions between ASN X and other networks with
which ASN X has a non-transit peering relationship.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on February 3, 2011.
Copyright Notice
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Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Specification of Requirements . . . . . . . . . . . . . . . . 3
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. The BGP Flexible Community Attribute . . . . . . . . . . . . . 4
3.1. Transitivity Field . . . . . . . . . . . . . . . . . . . . 4
3.2. Structure Field . . . . . . . . . . . . . . . . . . . . . 5
3.3. Type Field . . . . . . . . . . . . . . . . . . . . . . . . 6
3.4. Originating ASN Field . . . . . . . . . . . . . . . . . . 6
3.5. Length Field . . . . . . . . . . . . . . . . . . . . . . . 6
4. Locally Defined Structures and Types . . . . . . . . . . . . . 6
5. Neighbor Classes . . . . . . . . . . . . . . . . . . . . . . . 7
5.1. Locally Defined Neighbor Classes . . . . . . . . . . . . . 8
5.2. Defined Neighbor Classes . . . . . . . . . . . . . . . . . 9
6. Defined Flexible BGP Community Structures . . . . . . . . . . 9
7. Base BGP Flexible Community Type (Opaque Type) . . . . . . . . 11
8. Defined Flexible BGP Community Types . . . . . . . . . . . . . 11
8.1. NO_EXPORT . . . . . . . . . . . . . . . . . . . . . . . . 12
8.2. ONLY_EXPORT . . . . . . . . . . . . . . . . . . . . . . . 13
8.3. ANNOUNCE_WITH . . . . . . . . . . . . . . . . . . . . . . 14
8.4. PREPEND . . . . . . . . . . . . . . . . . . . . . . . . . 15
8.5. The BGP VPN Communities . . . . . . . . . . . . . . . . . 16
9. New Capability Code for Flexible Communities . . . . . . . . . 17
10. Aggregation . . . . . . . . . . . . . . . . . . . . . . . . . 18
11. Operations . . . . . . . . . . . . . . . . . . . . . . . . . . 18
12. Security Considerations . . . . . . . . . . . . . . . . . . . 19
13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19
14. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 20
15. Normative References . . . . . . . . . . . . . . . . . . . . . 20
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 20
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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 [RFC2119].
2. Introduction
This attribute represents the third generation of the BGP community
attribute. The first generation is documented in [RFC1997]. The
second generation, the extended community, is documented in
[RFC4360].
The Flexible Community Attribute provides a number of important
enhancements over the existing BGP Community Attribute and BGP
Extended Community Attribute. These enhancements are:
o Support for IPv6.
o More efficient encoding of lists of data, for example, a list of
ASN's.
o Clean support for a broad range of future data field structures
and interpretations.
o Support for locally defined community structures, and
interpretations.
o Easy extensibility for a range of future applications.
The continuation and expansion of the structure introduced with
Extended Communities allows for policy based on the application where
the community is being used. The separation of structure and
interpretation allows for easier machine and human parsing of
community types which do the same thing with slightly different
input.
This attribute continues the use of the Transitivity community bit,
first introduced in the Extended Community Attribute.
We also define a set of well-known values for this attribute which
can be used to replicate and extend the functionality of the existing
well-known community values.
The concept of Neighbor Classes is introduced. A Neighbor Class is
defined on a BGP peering session. It allows neighbors that share a
set of administrative attributes to be easily grouped together.
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Policy can then be defined through Flexible Communities in reference
to these groupings.
3. The BGP Flexible Community Attribute
The Flexible Community Attribute is a transitive optional BGP
attribute with a Type Code of TBD. The attribute consists of a list
of "flexible communities."
Each Flexible Community is encoded as a variable length quantity.
The encoding scheme is:
o Transitivity Field: 1 bit
o Structure Field: 7 bits
o Type Field: 2 octets
o Length Field: 1 octet
o Value Field: 0-255 octets
0 1 2 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|T| Struct. | Type | Originating |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ASN | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Value Field (0-255 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: Flexible Community Packet Format
3.1. Transitivity Field
The Transitivity Field is one bit. It can take the following values:
o Value 0: The community is transitive across ASes
o Value 1: The community is non-transitive across ASes
It is important to note that the transitivity defined by this field
is different from the general transitivity of a BGP attribute. A
single Flexible Community Attribute, can contain multiple Flexible
Communities, each of which may or may not be transitive. If a route
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originates in an AS with the transitivity bit set, indicating that
the community is non-transitive, then that AS MUST NOT propagate that
community to its peers. However, if a community with the transitive
bit set is applied on an outbound policy expression (e.g., a route-
map), the community will be conveyed to the immediately adjacent
peer. That peer, in turn, will NOT propagate the community to its
peers. The one exception to this is as-confederations. For the
purposes of this attribute, confederation boundaries should be
treated the same as IBGP. In other words, non-transitive flexible
communities should be propagated to other members of the as-
confederation, unless overridden by local policy.
If the community is transitive, then the Value Field MUST contain the
originating ASN. This ASN is encoded as a 4-octet value, occupying
the first 4 octets of the Value Field. Two-octet ASN numbers are
padded out to 4 octets. Any additional information in the Value
Field comes after this origin ASN data.
3.2. Structure Field
The Structure Field's contents modify the Type Field. For example, a
Flexible Community which specifies SPECIFIC_NO_EXPORT in its Type
Field, can be modified by the contents of the Structure Field to let
the receiver know if the list of data on which it must act is a list
of 2 octet or 4 octet ASNs. A set of commonly used Structure values
is defined later in this document.
The Structure field is the latter 7 bits of the first octet. It is
split into two sub-fields.
0
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|T|L| Struct. |
+-+-+-+-+-+-+-+-+
Figure 2: Structure Field
o L - Local bit. The Local bit can take two values:
* Value 0: The Structure is Locally Defined.
* Value 1: The Structure is Well Known.
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3.3. Type Field
The Type Field is two octets. This contents of this field are used
to define an action for the recipient to take on the route, or to
define and attribute that is related to that route. An example of
the former would be a Type which requests that a route be
ONLY_EXPORTed to a specific set of peers. An example of the latter
would be a Type that defines the LINK_BANDWIDTH associated with a
certain NLRI.
Like the Structure Field the Type Field is split into two Sub-Fields:
1 2
8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|L| Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: Type Field
The Local bit can take two values:
o Value 0: The Type is Locally Defined.
o Value 1: The Type is Well Known.
An implementation MUST allow an operator to filter out entire Types
of Flexible Communities from their peering sessions if they so
choose.
3.4. Originating ASN Field
This field contains the ASN that added the flexible community to the
route. It is 4 octets long. In the case of a 2-byte ASN, the first
2 octets are set to zero, as padding.
3.5. Length Field
The Length Field specifies the length, in octets, of the Value Field.
4. Locally Defined Structures and Types
The Local bit allows the operator of the network to define Structures
and Types that are relevant only within that ASN's boundaries. The
definition the term "local" used throughout this document is: "A
value used by ad hoc agreement or convention outside the scope of
standardization, which has meaning only between the parties using the
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Flexible Community in question." This typically means that the Local
value only has meaning within an AS or set of ASes controlled by a
single entity.
A Locally Defined Structure or Type will have a syntax for
interpretation defined on the routers that need to interpret it. If
a router receives a community with a Locally Defined Structure or
Type that it does not recognize, then it should ignore the contents
and process the route based on the information in the route that it
does understand. This includes obeying the transitivity bit, in the
Flexible Community. If the community is set to non-transitive, even
if the router does not understand the rest of the Structure or Type
of the community, that community should not be forwarded outside the
AS.
In order to prevent collisions with other operators' Locally Defined
values, Flexible Communities containing Locally Defined Structures or
Types MUST be non-transitive (have their Transitivity Field set to
1).
5. Neighbor Classes
A Neighbor Class is a value which represents a certain class, or
group, of BGP neighbors. Each BGP peering session can be configured
with zero or more Neighbor Classes. This value will allow a general
classification of what sort of relationship the BGP session
represents. With the sort of session defined, it becomes easier to
apply policy to only that class of neighbors. Neighbor Classes make
the expression of policy through flexible communities much easier.
There are a number of examples in the sections on defined values.
Neighbor Classes can be used in two main ways. First they can be
used in a passive manner, where the configuration acts as a policy
expression for communities matching it. An example of this would be
configuring a neighbor with the PEER neighbor class, and having a
community that, say is set to NO_EXPORT for the NEIGHBOR CLASS PEER.
In this configuration, all the neighbors that are configured as PEERs
would match and filter out the route carrying the NO_EXPORT, NEIGHBOR
CLASS PEER community. This is the standard way of using neighbor
classes.
A second, optional, way to use Neighbor Classes would be to allow
inbound community tagging. In this usage, routes traversing the
session would be automatically tagged with a flexible community with
the appropriate neighbor class value. This eases configuration. To
extend the example above, our neighbor designated PEER would add a
community with NEIGHBOR CLASS PEER to routes traversing the BGP
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session. This community could then be used further down the line to
just announce PEER routes to a particular customer. This usage is
configurable on a per-neighbor basis.
A Neighbor Class is encoded as a 2-octet value with 2 parts:
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|L| Neighbor Class |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: Neighbor Class
The Local bit can take two values:
o Value 0: The Neighbor Class is Locally Defined.
o Value 1: The Neighbor Class is Well Known.
5.1. Locally Defined Neighbor Classes
If a router receives a Flexible Community containing a Neighbor Class
that it does not recognize, then it should ignore the contents and
process the route based on the information in the route that it does
understand. If the Transitivity Field of the Flexible Community with
the Locally Defined Structure or Type is set to 1 (the community is
non-transitive) then the router MUST NOT forward the Flexible
Community. Similarly, if the Transitivity Field is set to 0 (the
community is transitive) the router MUST forward the community along
with the NLRI.
Using Locally Defined Neighbor Classes an operator could easily
define a set that is locally useful. This set could be used in a
flat name space, for example, one could say that "31" would be "Asian
Public Peers", and "34" would be European Private Peers.
Also, a given BGP Neighbor can be part of multiple Neighbor Classes.
This would allow for a hierarchical or additive name space. For
example, a neighbor could be part of both "PEER", and locally defined
"ASIAN" and "PUBLIC PEER" Classes. The logical matching
functionality available is left implementation-dependent. However,
the default in such as case is logical OR functionality for matching
this neighbor class. In the case where routes are being tagged
inbound, then by default a single community with all configured
neighbor classes in a list is added. Implementation dependent knobs
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are suggested to allow more fine grained control.
5.2. Defined Neighbor Classes
This document defines Neighbor Class values for common BGP neighbor
groupings:
o ALL NEIGHBORS
* This class is the default Neighbor Class for all BGP peers.
* This class is represented by a value of 0 (0x8000).
o PEER
* This class is typically applied to sessions where a transit-
free relationship exists between the two providers.
* This class is represented by a value of 1 (0x8001).
o CUSTOMER
* This class is typically applied to sessions where the remote
end of the session is operated by a customer.
* This class is represented by a value of 2 (0x8002).
o UPSTREAM
* This class is typically applied to sessions where the remote
end of the session is operated by a network from which you
receive transit routes.
* This class is represented by a value of 3 (0x8003).
o CONFEDERATION PEER
* This class is typically applied to sessions where the remote
end of the session is part of a confederation.
* This class is represented by a value of 4 (0x8004).
6. Defined Flexible BGP Community Structures
This section defines a number of Structure values which different
Type values can inherit.
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Summary of the Defined Values:
o opaque/variable (0x40 or 0xC0)
o list of ASN's (0x41 or 0xC1)
o list of IPv4 addresses (0x42 or 0xC2)
o list of IPv6 addresses (0x43 or OxC3)
o list of neighbor_classes (0x44 or 0xC4)
Defined Structure can be transitive or non-transitive, they are well
known.
Opaque/Variable Structure
This sort of structure defers interpretation of the community
and value field to the Type value. Typically this structure
value will be used when the Type value does not have a lot of
variations, but rather one structure for the Value Field.
This structure is represented by the value 0x00.
List of ASN's
This structure value means that the Type Field's action is
qualified by a list of ASN's, contained in the Value Field. In
the case of 2 byte ASN's the value is padded to 4 bytes by
inserting 2 octets worth of zeros in the leftmost portion of
the value.
This structure is represented by the value 0x01.
List of IPv4 Addresses
This structure value means that the Type Field's action is
qualified by a list of IPv4 addresses, contained in the Value
Field.
This structure is represented by the value 0x02.
List of IPv6 Addresses
This structure value means that the Type Field's action is
qualified by a list of IPv6 addresses, contained in the Value
Field.
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This structure is represented by the value 0x03.
List of Neighbor Classes
This structure value means that the Type Field's action is
qualified by a list of neighbor classes, contained in the Value
Field.
This structure is represented by the value 0x04.
7. Base BGP Flexible Community Type (Opaque Type)
This section defines the base BGP community specification. Since the
root value of communities is the ability to tag a route with
arbitrary information, and then create a system to give that
information meaning, the base flexible community type (type 0), is
very simple. This base type could also be described as the "opaque
type." All actions can be replicated with a well-thought out,
provider dependent, implementation of a scheme using this opaque
type.
Like all flexible communities, this type can be transitive or non-
transitive. This is a well known type.
0 1 2 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x00,0xC0 | 0x0000 | Originating |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ASN | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Value Field (0-255 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5: Opaque Type
8. Defined Flexible BGP Community Types
The Type Field specifies the subgroup that a set of communities
belongs to. Typically this subgroup represents an action to be taken
on the data. A variety of well-known Type Values follow.
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8.1. NO_EXPORT
This grouping of well-known communities specify a list of ASNs, Peer
IPs, or Neighbor Classes NOT to announce a route to.
Name: NO_EXPORT
Type Code: 0x0001
Can Take Structures:
o Ox01 (ASN)
o 0x02 (IPv4)
o 0x03 (IPv6)
o 0x04 (neighbor-class)
Transitive: Non-Transitive
Min Length of Value Field: 2 octets
Max Length of Value Field: 254 octets
Behavior:
All routes received with this community MUST NOT be advertised
to the list of ASNs, Peer IPs, or Neighbor Classes contained in
the Value Field.
Notes:
GLOBAL_NO_EXPORT is accomplished by sending a NO_EXPORT Flexible
Community with the Neighbor Class of 0x00 (ALL NEIGHBORS).
GLOBAL_NO_EXPORT's NO_EXPORT behavior is defined as: All routes
received with this community MUST NOT be advertised outside a BGP
confederation boundary (a stand-alone autonomous system that is
not part of a confederation should be considered a confederation
itself.)[RFC1997]
This is analogous to the NO_EXPORT community defined in [RFC1997].
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8.2. ONLY_EXPORT
This grouping of well-known communities specify a list of ASNs, Peer
IPs, or Neighbor Classes to announce a route to.
Name: ONLY_EXPORT
Type Code: 0x0002
Can Take Structures:
o Ox01 (ASN)
o 0x02 (IPv4)
o 0x03 (IPv6)
o 0x04 (neighbor-class)
Transitive: Non-Transitive
Min Length of Value Field: 0 octets
Max Length of Value Field: 254 octets
Behavior:
The Value Field contains a list of ASNs, neighbor IP addresses,
or Neighbor Classes to which the route should be advertised.
The default behavior of a route carrying this community is the
same as the GLOBAL_NO_EXPORT behavior, except for the ASNs,
IPs, or Neighbor Classes listed in the Value Field.
Notes:
This community can be used to replicate the NO_ADVERTISE
functionality from [RFC1997]. To do so, simply announce
ONLY_EXPORT with a Structure of 0x03 or 0x04 (one of the IP
address Structures), but with no IP address in the list. This
will tell the receiving router that you wish to ONLY_EXPORT this
route to NO peer IPs.
This community can also be use to replicate the
NO_EXPORT_SUBCONFED functionality from [RFC1997]. To do so,
simply announce ONLY_EXPORT with a Neighbor Class of CONFEDERATION
PEER (4, 0x8004). This will tell the receiving router that you
wish to ONLY_EXPORT this route to Confederation Peers.
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8.3. ANNOUNCE_WITH
This group of well-known communities allows a network to announce a
community to an ASN beyond those that it directly peers with,
assuming its direct peers allow it to transit the community value.
This community group has a lot of flexibility, and could be used to
nest another ANNOUNCE_WITH community to gain reach greater than 2
ASN-hops away. If this is a good idea or not is unknown and is left
to further study. The only theoretical restriction to the amount of
nesting is that the community cannot exceed the maximum size for the
Value Field.
Since true transitivity can be obtained by simply setting a bit, this
community is mainly useful for propagating NO_EXPORT or ONLY_EXPORT
(which are non-transitive) to your neighbor's neighbors.
In effect, this community, if allowed by the BGP neighbors in the
chain, can be used for an originating network to very specifically
control the distribution of its routes. This community type does
contain a LOT of rope, and should be used with care. In the end,
though, a mistake should only effect the person originating the
route.
Name: ANNOUNCE_WITH
Type Code: 0x0003
Can Take Structures:
o Ox01 (ASN)
o 0x02 (IPv4)
o 0x03 (IPv6)
o 0x04 (neighbor-class)
Transitive: Non-Transitive
Min Length of Value Field: 8 octets
Max Length of Value Field: 254 octets
Behavior: This community's Value Field is split into two sections.
* The first section is a variable length field that contains the
full community value that you wish to announce.
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* The second section is a variable length field that contains the
list of, ASN's, IP addresses, or neighbor_classes you wish to
propagate this community to. If you wish to propagate to all
peers, use the ALL NEIGHBORS neighbor class.
When a router receives this community value, it should strip the
ANNOUNCE_WITH community and announce the underlying community value
to its neighbor.
8.4. PREPEND
This community can be used to ask a BGP peer to prepend its own ASN
to its peers.
Name: PREPEND
Type Code: 0x0004
Can Take Structures:
o Ox01 (ASN)
o 0x02 (IPv4)
o 0x03 (IPv6)
o 0x04 (neighbor-class)
Transitive: Non-Transitive
Min Length of Value Field: 3 octets
Max Length of Value Field: 254 octets
Behavior: This community has 2 sections:
* The first section: Is a one-octet value which specifies the
number of times that the ASN should prepend its ASN. It is
recommended that operators constrain this value to no more than
3. Implementations MUST offer the ability for an operator to
set a maximum bound for this field. The suggested default is
also 3.
* The second section: Contains a list of ASNs, peer IPs, or
Neighbor Classes to which the originator of this community
wishes its peer to prepend its ASN.
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8.5. The BGP VPN Communities
These communities are used mostly for BGP MPLS VPN's. Please see
[RFC2547] for more detail on how these VPNs are constructed.
Name: ROUTE_TARGET
Type Code: 0x0005
Can Take Structures:
o 0x02 (IPv4)
o 0x03 (IPv6)
Transitive: Transitive or Non-Transitive
Min Length of Value Field: 4 octets
Max Length of Value Field: 254 octets
Behavior: The Value Field of this community represents a list of
the IP addresses where this route is to be announced.
Name: ROUTE_ORIGIN
Type Code: 0x0006
Can Take Structures:
o 0x02 (IPv4)
o 0x03 (IPv6)
Transitive: Transitive or Non-Transitive
Min Length of Value Field: 4 octets
Max Length of Value Field: 16 octets
Behavior: The Value Field of this community represents a list of
the IP address where this route is originated. This community can
only contain one IP address.
Name: LINK_BANDWIDTH
Type Code: 0x0007
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Can Take Structures:
o 0x00 (opaque)
o Ox01 (ASN)
Transitive: Transitive or Non-Transitive
Min Length of Value Field: 4 octets
Max Length of Value Field: 6 octets
Behavior: This community consists of two parts:
* The first part represents the bandwidth of the link in bits-
per- second, encoded in IEEE floating point format. This part
is 4 octets long.
* The second part consists of a list of ASNs of the peer whose
link bandwidth you wish to propagate.
9. New Capability Code for Flexible Communities
To ensure compatibility between implementations that may or may not
implement this protocol extension, this document defines a new
capability.
Capability Code: TBD
Capability Length: 1
Capability Value:
* 0x00 for unsupported
* 0x01 for supported
Capability negotiation is especially important for this attribute
because we are creating a transitivity action within an optional,
transitive attribute. If an implementation sends a flexible
community with the non-transitive bit set within the community to a
router that does not support flexible communities, that router will
send the community on to its peers when it should not do so.
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10. Aggregation
Aggregation behaves the same as with other community types.
By default if a range of routes is to be aggregated and the resultant
aggregates path attributes do not carry the ATOMIC_AGGREGATE
attribute, then the resulting aggregate should have an Flexible
Communities path attribute which contains the set union of all the
Flexible Communities from all of the aggregated routes. The default
behavior could be overridden via local configuration, in which case
handling the Flexible Communities attribute in the presence of route
aggregation becomes a matter of the local policy of the BGP speaker
that performs the aggregation.
11. Operations
Flexible Communities are handled operationally in a manner very
similar to other community values.
A BGP speaker may use the Flexible Communities attribute to control
which routing information it accepts or distributes to its peers.
The Flexible Community attribute MUST NOT be used to modify the BGP
best path selection algorithm in a way that leads to forwarding
loops.
A BGP speaker receiving a route that doesn't have the Flexible Commu-
nities attribute MAY append this attribute to the route when
propagating it to its peers.
A BGP speaker receiving a route with the Flexible Communities
attribute MAY modify this attribute according to the local policy.
If a route has a non-transitivity flexible community, then before
advertising the route across the Autonomous system boundary the
community SHOULD be removed from the route. However, the community
SHOULD NOT be removed when advertising the route across the BGP
Confederation boundary.
A route may carry the BGP Communities attribute as defined in
[RFC1997], the Extended BGP Communities attribute as defined in
[RFC4360], and the Flexible Communities attribute. In this case the
BGP Communities attribute is handled as specified in [RFC1997], the
Extended BGP Communities attribute is handled as specified in
[RFC4360] and the Flexible Communities attribute is handled as
specified in this document.
If older community types are present in addition to the flexible
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community there is the possibility that the information contained may
be redundant. Although effort should be made to avoid this
situation, if it does occur the BGP Speaker shall prefer the flexible
community first, then the extended community second and then the base
community.
12. Security Considerations
This extension to BGP does not change the underlying security issues.
13. IANA Considerations
The values for the Transitivity Field (1 or 0) are completely defined
in this document.
The assignment policy for the Structure Field is:
o The "L" bit's usage is completely defined in this document.
o Values of the Structure Field where the "L" bit is "0" are to be
assigned in accordance with the Private Use policy outlined in
[RFC2434].
o Values of the Structure Field where the "L" bit is "1" defined in
this document are: 0-4 (0x40-0x44 and 0xC1-0xC4). Remaining
values in this range are to be assigned using the IETF Consensus
policy outlined in [RFC2434].
The assignment policy for the Type Field is:
o The "L" bit's usage is completely defined in this document.
o Values of the Type Field where the "L" bit is "0" are to be
assigned in accordance with the Private Use policy outlined in
[RFC2434].
o Values of the Type Field where the "L" but is "1" defined in this
document are: 0-7 (0x0000-0x0007). Remaining values in this range
are to be assigned using the IETF Consensus policy outlined in
[RFC2434].
The assignment policy for Neighbor Classes is:
o The "L" bit's usage is completely defined in this document.
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o Values of the Type Field where the "L" bit is "0" are to be
assigned in accordance with the Private Use policy outlined in
[RFC2434].
o Values of the Type Field where the "L" but is "1" defined in this
document are: 0-4 (0x8000-0x8004). Remaining values in this range
are to be assigned using the IETF Consensus policy outlined in
[RFC2434].
14. Acknowledgements
I would like to thank Tom Barron, Dave Ward and especially Hal
Peterson for their valuable comments and feedback. I would also like
to thank Brian Haberman.
15. Normative References
[RFC1771] Rekhter, Y. and T. Li, "A Border Gateway Protocol 4
(BGP-4)", RFC 1771, March 1995.
[RFC1997] Chandrasekeran, R., Traina, P., and T. Li, "BGP
Communities Attribute", RFC 1997, August 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 2434,
October 1998.
[RFC2547] Rosen, E. and Y. Rekhter, "BGP/MPLS VPNs", RFC 2547,
March 1999.
[RFC2842] Chandra, R. and J. Scudder, "Capabilities Advertisement
with BGP-4", RFC 2842, May 2000.
[RFC4360] Sangli, S., Tappan, D., and Y. Rekhter, "BGP Extended
Communities Attribute", RFC 4360, February 2006.
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Authors' Addresses
Andrew Lange
Alcatel-Lucent
Email: andrew.lange@alcatel-lucent.com
Jeffrey Haas
Juniper Networks
Email: jhaas@pfrc.org
Keyur Patel
Cisco
Email: keyupate@cisco.com
Shane Amante
Level 3
Email: shane@castlepoint.net
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