BGP Extended Communities Attribute
A structured, 8-octet label that BGP routes can carry, so operators can filter and steer routes by type.
Sangli, S., Tappan, D., and Y. Rekhter, "BGP Extended Communities Attribute", RFC 4360, DOI 10.17487/RFC4360, February 2006, <https://www.rfc-editor.org/info/rfc4360>.
@misc{rfc4360,
series = {Request for Comments},
number = 4360,
howpublished = {RFC 4360},
publisher = {RFC Editor},
doi = {10.17487/RFC4360},
url = {https://www.rfc-editor.org/info/rfc4360},
author = {S. Sangli and D. Tappan and Y. Rekhter},
title = {{BGP Extended Communities Attribute}},
pagetotal = 12,
year = 2006,
month = feb,
} RFC text
Contents
Network Working Group S. Sangli
Request for Comments: 4360 D. Tappan
Category: Standards Track Cisco Systems
Y. Rekhter
Juniper Networks
February 2006
BGP Extended Communities Attribute
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
This document describes the "extended community" BGP-4 attribute.
This attribute provides a mechanism for labeling information carried
in BGP-4. These labels can be used to control the distribution of
this information, or for other applications.
1. Introduction
The Extended Community Attribute provides a mechanism for labeling
information carried in BGP-4 [BGP-4]. It provides two important
enhancements over the existing BGP Community Attribute [RFC1997]:
- An extended range, ensuring that communities can be assigned for
a plethora of uses, without fear of overlap.
- The addition of a Type field provides structure for the
community space.
The addition of structure allows the usage of policy based on the
application for which the community value will be used. For example,
one can filter out all communities of a particular type, or allow
only certain values for a particular type of community. It also
allows one to specify whether a particular community is transitive or
non-transitive across an Autonomous System (AS) boundary. Without
structure, this can only be accomplished by explicitly enumerating
all community values that will be denied or allowed and passed to BGP
speakers in neighboring ASes based on the transitive property.
1.1. 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 [RFC2119].
2. BGP Extended Communities Attribute
The Extended Communities Attribute is a transitive optional BGP
attribute, with the Type Code 16. The attribute consists of a set of
"extended communities". All routes with the Extended Communities
attribute belong to the communities listed in the attribute.
Each Extended Community is encoded as an 8-octet quantity, as
follows:
- Type Field : 1 or 2 octets
- Value Field : Remaining 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type high | Type low(*) | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Value |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
(*) Present for Extended types only, used for the Value field
otherwise.
Type Field:
Two classes of Type Field are introduced: Regular type and
Extended type.
The size of Type Field for Regular types is 1 octet, and the
size of the Type Field for Extended types is 2 octets.
The value of the high-order octet of the Type Field determines
if an extended community is a Regular type or an Extended type.
The class of a type (Regular or Extended) is not encoded in the
structure of the type itself. The class of a type is specified
in the document that defines the type and the IANA registry.
The high-order octet of the Type Field is as shown below:
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|I|T| |
+-+-+-+-+-+-+-+-+
I - IANA authority bit
Value 0: IANA-assignable type using the "First Come First
Serve" policy
Value 1: Part of this Type Field space is for IANA
assignable types using either the Standard Action or the
Early IANA Allocation policy. The rest of this Type
Field space is for Experimental use.
T - Transitive bit
Value 0: The community is transitive across ASes
Value 1: The community is non-transitive across ASes
Remaining 6 bits: Indicates the structure of the community
Value Field:
The encoding of the Value Field is dependent on the "type" of
the community as specified by the Type Field.
Two extended communities are declared equal only when all 8 octets of
the community are equal.
The two members in the tuple <Type, Value> should be enumerated to
specify any community value. The remaining octets of the community
interpreted based on the value of the Type field.
3. Defined BGP Extended Community Types
This section introduces a few extended types and defines the format
of the Value Field for those types. The types introduced here
provide "templates", where each template is identified by the high-
order octet of the extended community Type field, and the lower-order
octet (sub-type) is used to indicate a particular type of extended
community.
3.1. Two-Octet AS Specific Extended Community
This is an extended type with Type Field composed of 2 octets and
Value Field composed of 6 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x00 or 0x40 | Sub-Type | Global Administrator |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Local Administrator |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The value of the high-order octet of this extended type is either
0x00 or 0x40. The low-order octet of this extended type is used to
indicate sub-types.
The Value Field consists of two sub-fields:
Global Administrator sub-field: 2 octets
This sub-field contains an Autonomous System number assigned by
IANA.
Local Administrator sub-field: 4 octets
The organization identified by Autonomous System number in the
Global Administrator sub-field can encode any information in
this sub-field. The format and meaning of the value encoded in
this sub-field should be defined by the sub-type of the
community.
3.2. IPv4 Address Specific Extended Community
This is an extended type with Type Field composed of 2 octets and
Value Field composed of 6 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x01 or 0x41 | Sub-Type | Global Administrator |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Global Administrator (cont.) | Local Administrator |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The value of the high-order octet of this extended type is either
0x01 or 0x41. The low-order octet of this extended type is used to
indicate sub-types.
The Value field consists of two sub-fields:
Global Administrator sub-field: 4 octets
This sub-field contains an IPv4 unicast address assigned by one
of the Internet registries.
Local Administrator sub-field: 2 octets
The organization that has been assigned the IPv4 address in the
Global Administrator sub-field can encode any information in
this sub-field. The format and meaning of this value encoded
in this sub-field should be defined by the sub-type of the
community.
3.3. Opaque Extended Community
This is an extended type with Type Field composed of 2 octets and
Value Field composed of 6 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x03 or 0x43 | Sub-Type | Value |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Value (cont.) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The value of the high-order octet of this extended type is either
0x03 or 0x43. The low-order octet of this extended type is used to
indicate sub-types.
This is a generic community of extended type. The value of the sub-
type that should define the Value Field is to be assigned by IANA.
4. Route Target Community
The Route Target Community identifies one or more routers that may
receive a set of routes (that carry this Community) carried by BGP.
This is transitive across the Autonomous System boundary.
The Route Target Community is of an extended type.
The value of the high-order octet of the Type field for the Route
Target Community can be 0x00, 0x01, or 0x02. The value of the low-
order octet of the Type field for this community is 0x02.
When the value of the high-order octet of the Type field is 0x00 or
0x02, the Local Administrator sub-field contains a number from a
numbering space that is administered by the organization to which the
Autonomous System number carried in the Global Administrator sub-
field has been assigned by an appropriate authority.
When the value of the high-order octet of the Type field is 0x01, the
Local Administrator sub-field contains a number from a numbering
space that is administered by the organization to which the IP
address carried in the Global Administrator sub-field has been
assigned by an appropriate authority.
One possible use of the Route Target Community is specified in
[RFC4364].
5. Route Origin Community
The Route Origin Community identifies one or more routers that inject
a set of routes (that carry this Community) into BGP. This is
transitive across the Autonomous System boundary.
The Route Origin Community is of an extended type.
The value of the high-order octet of the Type field for the Route
Origin Community can be 0x00, 0x01, or 0x02. The value of the low-
order octet of the Type field for this community is 0x03.
When the value of the high-order octet of the Type field is 0x00 or
0x02, the Local Administrator sub-field contains a number from a
numbering space that is administered by the organization to which the
Autonomous System number carried in the Global Administrator sub-
field has been assigned by an appropriate authority.
When the value of the high-order octet of the Type field is 0x01, the
Local Administrator sub-field contains a number from a numbering
space that is administered by the organization to which the IP
address carried in the Global Administrator sub-field has been
assigned by an appropriate authority.
One possible use of the Route Origin Community is specified in
[RFC4364].
6. Operations
A BGP speaker may use the Extended Communities attribute to control
which routing information it accepts or distributes to its peers.
The Extended 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 Extended
Communities attribute MAY append this attribute to the route when
propagating it to its peers.
A BGP speaker receiving a route with the Extended Communities
attribute MAY modify this attribute according to the local policy.
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 Extended
Communities path attribute that contains the set union of all the
Extended Communities from all of the aggregated routes. The default
behavior could be overridden via local configuration, in which case
handling the Extended Communities attribute in the presence of route
aggregation becomes a matter of the local policy of the BGP speaker
that performs the aggregation.
If a route has a non-transitivity extended 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 both the BGP Communities attribute, as defined in
[RFC1997]), and the Extended BGP Communities attribute. In this
case, the BGP Communities attribute is handled as specified in
[RFC1997], and the Extended BGP Communities attribute is handled as
specified in this document.
7. IANA Considerations
All the BGP Extended Communities contain a Type field. The IANA has
created a registry entitled, "BGP Extended Communities Type". The
IANA will maintain this registry.
The Type could be either regular or extended. For a regular Type the
IANA allocates an 8-bit value; for an extended Type the IANA
allocates a 16-bit value.
The value allocated for a regular Type MUST NOT be reused as the
value of the high-order octet when allocating an extended Type. The
value of the high-order octet allocated for an extended Type MUST NOT
be reused when allocating a regular Type.
The Type field indicates where the Extended Community is transitive
or not. Future requests for assignment of a Type value must specify
whether the Type value is intended for a transitive or a non-
transitive Extended Community.
Future assignment are to be made using either the Standards Action
process defined in [RFC2434], the Early IANA Allocation process
defined in [RFC4020], or the "First Come First Served" policy defined
in [RFC2434].
The following table summarizes the ranges for the assignment of
Types:
Type Standard Action First Come
Early IANA Allocation First Served
------------------ --------------------- ------------
regular, transitive 0x90-0xbf 0x00-x3f
regular, non-transitive 0xd0-0xff 0x40-0x7f
extended, transitive 0x9000-0xbfff 0x0000-0x3fff
extended, non-transitive 0xd000-0xffff 0x4000-0x7fff
Assignments consist of a name and the value.
The Type values 0x80-0x8f and 0xc0-0xcf for regular Types, and
0x8000-0x8fff and 0xc000-0xcfff for extended Types are for
Experimental use as defined in RFC 3692.
This document defines a class of extended communities called two-
octet AS specific extended community for which the IANA is to create
and maintain a registry entitled "Two-octet AS Specific Extended
Community". All the communities in this class are of extended Types.
Future assignment are to be made using the "First Come First Served"
policy defined in [RFC2434]. The Type values for the transitive
communities of the two-octet AS specific extended community class are
0x0000-0x00ff, and for the non-transitive communities of that class
are 0x4000-0x40ff. Assignments consist of a name and the value.
This document makes the following assignments for the two-octet AS
specific extended community:
Name Type Value
---- ----------
two-octet AS specific Route Target 0x0002
two-octet AS specific Route Origin 0x0003
This document defines a class of extended communities called IPv4
address specific extended community for which the IANA is to create
and maintain a registry entitled "IPv4 Address Specific Extended
Community". All the communities in this class are of extended Types.
Future assignment are to be made using the "First Come First Served"
policy defined in [RFC2434]. The Type values for the transitive
communities of the two-octet AS specific extended community class
are 0x0100-0x01ff, and for the non-transitive communities of that
class are 0x4100-0x41ff. Assignments consist of a name and the
value.
This document makes the following assignments for the IPv4 address
specific extended community:
Name Type Value
---- ----------
IPv4 address specific Route Target 0x0102
IPv4 address specific Route Origin 0x0103
This document defines a class of extended communities called opaque
extended community for which the IANA is to create and maintain a
registry entitled "Opaque Extended Community". All the communities
in this class are of extended Types. Future assignment are to be
made using the "First Come First Served" policy defined in [RFC2434].
The Type values for the transitive communities of the opaque extended
community class are 0x0300-0x03ff, and for the non-transitive
communities of that class are 0x4300-0x43ff. Assignments consist of
a name and the value.
When requesting an allocation from more than one registry defined
above, one may ask for allocating the same Type value from these
registries. If possible, the IANA should accommodate such requests.
8. Security Considerations
This extension to BGP has similar security implications as BGP
Communities [RFC1997].
This extension to BGP does not change the underlying security issues.
Specifically, an operator who is relying on the information carried
in BGP must have a transitive trust relationship back to the source
of the information. Specifying the mechanism(s) to provide such a
relationship is beyond the scope of this document.
9. Acknowledgements
The authors would like to thank John Hawkinson, Jeffrey Haas, Bruno
Rijsman, Bill Fenner, and Alex Zinin for their suggestions and
feedback.
10. Normative References
[BGP-4] Rekhter, Y. and T. Li, "A Border Gateway Protocol 4
(BGP-4)", RFC 4271, January 2006.
[RFC1997] Chandra, 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.
[RFC4020] Kompella, K. and A. Zinin, "Early IANA Allocation of
Standards Track Code Points", BCP 100, RFC 4020,
February 2005.
11. Informative References
[RFC4364] Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private
Networks (VPNs)", RFC 4364, February 2006.
Authors' Addresses
Srihari R. Sangli
Cisco Systems, Inc.
EMail: rsrihari@cisco.com
Dan Tappan
Cisco Systems, Inc.
250 Apollo Drive
Chelmsford, MA 01824
EMail: tappan@cisco.com
Yakov Rekhter
Juniper Networks, Inc.
1194 N. Mathilda Ave
Sunnyvale, CA 94089
EMail: yakov@juniper.net
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AI ELI5
On this page
§1Why Extend BGP Communities
The Extended Community Attribute is a way to label information carried in BGP-4. Those labels can control how the information is distributed, or serve other applications. It improves on the older BGP Community Attribute (RFC 1997) in two ways.
First, it has an extended range, so communities can be assigned for many uses without fear of overlap. Second, it adds a Type field that gives the community space some structure.
Structure matters for policy. With it, a router can filter out all communities of one type, or allow only certain values for one type. It can also say whether a community is transitive or non-transitive across an Autonomous System (AS) boundary. Without structure, the only option is to list every allowed or denied value by hand.
The old labels were like numbered stickers with no categories. The new labels have a category on them, so a router can say 'ignore every sticker of this kind' in one rule.
§2The Attribute and Its Type Field
The Extended Communities Attribute is a transitive optional BGP attribute with Type Code 16. It holds a set of extended communities, and every route carrying it belongs to all the communities listed.
Each extended community is an 8-octet quantity: a Type field of 1 or 2 octets, then a Value field with the remaining octets. There are two classes of type. A Regular type has a 1-octet Type field. An Extended type has a 2-octet Type field. The class is not encoded in the type itself. It is set by the document that defines the type and by the IANA registry.
The high-order octet of the Type field has two flag bits. The I bit is the IANA authority bit. Value 0 means the type is assignable by IANA under the First Come First Served policy. Value 1 means part of that space is assignable under Standards Action or Early IANA Allocation, and the rest is for Experimental use. The T bit is the transitive bit. Value 0 means the community is transitive across ASes, and value 1 means it is non-transitive. The other 6 bits indicate the structure of the community.
Two extended communities are equal only when all 8 octets are equal.
Every label is exactly 8 bytes long. The first byte or two say what kind of label it is and whether it may travel past your network's border. The rest holds the actual message.
§3Three Defined Value Layouts
The RFC defines three extended types as templates. The high-order octet of the Type field picks the template, and the low-order octet is a sub-type that says which particular kind of community it is. Each has a 2-octet Type field and a 6-octet Value field.
- Two-Octet AS Specific (high octet
0x00or0x40): a 2-octet Global Administrator holding an AS number assigned by IANA, and a 4-octet Local Administrator that the organization owning that AS can use for any information. - IPv4 Address Specific (high octet
0x01or0x41): a 4-octet Global Administrator holding an IPv4 unicast address assigned by one of the Internet registries, and a 2-octet Local Administrator that the address holder can use as it likes. - Opaque (high octet
0x03or0x43): a generic type whose 6-octet Value field is defined by a sub-type to be assigned by IANA.
In the first two layouts, the format and meaning of the Local Administrator value should be defined by the sub-type of the community.
There are three label templates. One has your network's number and a note you write yourself. One has your network's address and a shorter note. The third is a blank template for other uses.
| Field | Name | Size | Value | Description |
|---|---|---|---|---|
TYPE-HIGH | Type high | 1 byte | 0x00 or 0x40 | 0x00 or 0x40. |
SUB-TYPE | Sub-Type | 1 byte | Low-order octet of the type; indicates the sub-type. | |
GLOBAL-ADMIN | Global Administrator | 2 bytes | An Autonomous System number assigned by IANA. | |
LOCAL-ADMIN | Local Administrator | 4 bytes | Any information chosen by the organization that owns the AS number. |
| Field | Name | Size | Value | Description |
|---|---|---|---|---|
TYPE-HIGH | Type high | 1 byte | 0x01 or 0x41 | 0x01 or 0x41. |
SUB-TYPE | Sub-Type | 1 byte | Low-order octet of the type; indicates the sub-type. | |
GLOBAL-ADMIN | Global Administrator | 4 bytes | An IPv4 unicast address assigned by one of the Internet registries. | |
LOCAL-ADMIN | Local Administrator | 2 bytes | Any information chosen by the organization that holds the address. |
§4Route Target and Route Origin
Two communities are defined using these templates. Both are of an extended type and both are transitive across the AS boundary.
The Route Target community identifies one or more routers that may receive a set of routes carrying it. Its low-order Type octet is 0x02. The Route Origin community identifies one or more routers that inject a set of routes into BGP. Its low-order Type octet is 0x03.
For both, the high-order octet can be 0x00, 0x01 or 0x02. With 0x00 or 0x02, the Local Administrator holds a number from a numbering space run by the organization that was assigned the AS number in the Global Administrator sub-field. With 0x01, it is a number from a space run by the organization that was assigned the IP address in the Global Administrator sub-field.
The RFC points to RFC 4364 as one possible use of both communities.
One label says 'these routes may be delivered to these routers'. The other says 'these routes started at these routers'. Both are allowed to cross the border between networks.
§5How Routers Handle the Attribute
A BGP speaker may use the attribute to control which routing information it accepts or distributes to its peers. It MUST NOT use it to change best path selection in a way that leads to forwarding loops.
A speaker that receives a route without the attribute MAY add it when passing the route on. A speaker that receives a route with it MAY modify it according to local policy.
For aggregation, by default, when routes are aggregated and the aggregate does not carry the ATOMIC_AGGREGATE attribute, the aggregate should carry the set union of all the extended communities of the aggregated routes. Local configuration can override this.
A non-transitive community SHOULD be removed before a route is advertised across an AS boundary, but SHOULD NOT be removed when crossing a BGP Confederation boundary. A route can carry both the old Communities attribute and the Extended Communities attribute. Each is handled by its own specification.
Routers can add, change or read these labels when they pass routes along. Labels marked 'stay home' get peeled off at the border, and when several routes are merged into one, the merged route keeps all their labels.
§6IANA Registries and Type Ranges
IANA created a registry called "BGP Extended Communities Type". A regular type gets an 8-bit value and an extended type gets a 16-bit value. A value allocated for a regular type MUST NOT be reused as the high-order octet of an extended type, and the reverse also holds. Each request must say whether the type is transitive or non-transitive.
Future assignments use Standards Action, Early IANA Allocation, or First Come First Served. The ranges are split by class and transitivity. For example, regular transitive types use 0x90-0xbf for Standards Action or Early Allocation and 0x00-x3f for First Come First Served. Values 0x80-0x8f and 0xc0-0xcf (regular), and 0x8000-0x8fff and 0xc000-0xcfff (extended), are for Experimental use.
The RFC also creates registries for the three classes: Two-octet AS Specific, IPv4 Address Specific and Opaque, all assigned First Come First Served. It makes four initial assignments, listed below.
A central office keeps the official list of label kinds so nobody reuses the same code for two things. Some codes are reserved for experiments.
- 0x0002Two-octet AS specific Route Target — Routers that may receive the routes.
- 0x0003Two-octet AS specific Route Origin — Routers that inject the routes into BGP.
- 0x0102IPv4 address specific Route Target — Same meaning, with an IPv4 address as the Global Administrator.
- 0x0103IPv4 address specific Route Origin — Same meaning, with an IPv4 address as the Global Administrator.
§7Security Considerations
The RFC says this extension has security implications similar to BGP Communities (RFC 1997) and does not change the underlying security issues.
An operator who relies on information carried in BGP must have a transitive trust relationship back to the source of that information. The RFC leaves the mechanisms that would provide such a relationship out of scope.
If you trust a label, you are trusting everyone who handled the letter before you. This memo does not fix that, it only says so.