BGP Extended Communities Attribute

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.¶

This document obsoletes [RFC4360].¶

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.¶

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.¶

• A BGP speaker sends the community over both internal and external BGP sessions.¶

Value 1:

The community is non-transitive.¶

• A BGP speaker sends the community over internal BGP sessions.¶
• If the BGP speaker is attaching the community, it also sends the community over external BGP sessions.¶
• If the BGP speaker is not attaching the community, it MUST NOT send the community over external BGP sessions. The BGP speaker SHALL remove the community before announcing the corresponding route over external BGP sessions.¶
• When a BGP speaker receives the community over external BGP sessions, it SHOULD NOT remove the community by default. This default behavior MAY be configurable. The BGP speaker SHOULD also allow local policies to match against or remove the community.¶

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.¶

 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                       |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

 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        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

 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.)                         |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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 (as defined in Section 3.1), 0x01 (as defined in Section 3.2), or 0x02 (as defined in [RFC5668]). 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 (as defined in Section 3.1), 0x01 (as defined in Section 3.2), or 0x02 (as defined in [RFC5668]). 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-transitive 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 [RFC8126], the Early IANA Allocation process defined in [RFC7120], or the "First Come First Served" policy defined in [RFC8126].¶

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 [RFC3692].¶

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 [RFC8126]. 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 [RFC8126]. The Type values for the transitive communities of the IPv4 Address 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 [RFC8126]. 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.¶

10. References