What It Is
Conceptually, the Large Community attribute is extremely similar to the Extended Community attribute (but with larger address-spaces). The primary considered use-case for both extended and large communities is to allow network operators to assign descriptors to BGP routes using a type/value pair schema.
While the Extended Community attribute may be either transitive or non-transitive, the Large Community attribute is always a transitive attribute.
While the Extended Comunity attribute’s “type” field is an IANA-managed address-space, the Large Community attribute’s “Local Data Part 1” and “Local Data Part 2” fields are explicitly defined as “operator-defined” values. This provides an operator using Large Communnity attributes with sufficeint space to define up to 2^32 data types, each of which can have up to 2^32 discrete values defined.
The BGP Large Community attribute is defined in RFC 8092, which states that:
This attribute provides a mechanism to signal opaque information within separate namespaces to aid in routing management. The attribute is suitable for use with all Autonomous System Numbers (ASNs) including four-octet ASNs.
The same RFC specifies the structure of the large-community attribute:
Each BGP Large Community value is encoded as a 12-octet quantity, as follows:
- Global Administrator: A four-octet namespace identifier
- Local Data Part 1: A four-octet operator-defined value
- Local Data Part 2: A four-octet operator-defined value
Representation of the large community attribute is also defined in the same RFC:
The canonical representation of BGP Large Communities is three separate unsigned integers in decimal notation in the following order: Global Administrator, Local Data 1, Local Data 2. Numbers MUST NOT contain leading zeros; a zero value MUST be represented with a single zero. Each number is separated from the next by a single colon. For example: 64496:4294967295:2, 64496:0:0
A separate RFC (RFC 8195 provides guidance for the use of this attribute that can be summarized as:
- The first value, the Global Administrator field, should be an AS number.
- Interpreetation of the values of the two remaining fields (“Local Data Part 1” and “Local Data Part 2”) in the large community attribute are to be interpreted as defined by the owner of the AS number in the Global Administrator field.
Application
This reference implementation utilizes the BGP large community type / pairs below to tag BGP route updates with data relevant to the routing policy framework presented in the previous section of this paper. Each Local Data Part 1 value represents a distinct property, and each part 2 value represents
| LD1 Value | LD1 Definition | LD2 2 Values | LD2 Definition |
|---|---|---|---|
| 122881 | SPZ in which the route was originated (“$OSPZ”) | 1-100 | Discrete values for 100 SPZs |
| 122882 | PAG in which the route was originated (“$OPAG”) | 1-100 | Discrete values for 100 PAG |
| 122883 | Number of in-path SPN nodes in the PAG in which the route was originated (“$OSPZ-SPNCOUNT”) | 1-100 | |
| 122884 | PAG of the router propagating the route (“$PPAG”) | 1-100 | |
| 122885 | SPZ of the router propagating the route (“$PSPZ”) | 1-100 | |
| 122886 | SPN property of the router propagating the route ($PSPN) | 0-1 | Boolean (0= not an SPN; 1 = SPN) |
The parenthesized and quoted string (“$OSPZ”, “$PSPS”, etc…) in the “LD1 Definition” colum of the preceding table are the labels that we will use when referring to the data of the Large Community attributes in the following sections.