Community Attribute
A community is a set of destination addresses with the same characteristics. It is expressed as a list in the unit of four bytes. In the device, the community is in the format of aa:nn or the community number.
- aa:nn: The value of aa or nn ranges from 0 to 65535. The administrator can set a specific value as required. aa indicates the AS number and nn indicates the community identifier defined by the administrator. For example, if a route is from AS 100, and its community identifier defined by the administrator is 1, the format of the community is 100:1.
- Community number: It is an integer that ranges from 0 to 4294967295. As defined in RFC 1997, numbers from 0 (0x00000000) to 65535 (0x0000FFFF) and from 4294901760 (0xFFFF0000) to 4294967295 (0xFFFFFFFF) are reserved.
The community is used to simplify the application, maintenance, and management of routing policies. With the community, a group of BGP devices in multiple ASs can share the same routing policy. The community is a route attribute. It is transmitted between BGP peers and is not restricted by the AS. Before advertising a route with the community to peers, a BGP device can change the original community of this route.
The peer group allows a group of peers to share the same policy while the community allows a group of BGP routes to share the same policy.
Besides the well-known communities, you can use a community filter to filter self-defined extended communities to control routing policies in a more flexible manner.
Well-known Community
Table 1 lists the well-known communities of BGP routes.
| Community Name | Community Identifier | Description |
|---|---|---|
| Internet | 0(0x00000000) | By default, all routes belong to the Internet community. A route with this attribute can be advertised to all BGP peers. |
| No_Export | 4294967041(0xFFFFFF01) | A route with this attribute cannot be advertised outside the local AS. If a confederation is defined, the route with this attribute cannot be advertised to the ASs outside the confederation, but to other sub-ASs in the confederation. |
| No_Advertise | 4294967042(0xFFFFFF02) | A route with this attribute cannot be advertised to any other BGP peers. |
| No_Export_Subconfed | 4294967043(0xFFFFFF03) | A route with this attribute cannot be advertised outside the local AS or to other sub-ASs in the confederation. |
Networking Applications
As shown in Figure 1, EBGP connections are established between FW_B and FW_A, and between FW_B and FW_C. With the community attribute of No_Export configured on FW_A, the routes from AS10 advertised to AS20 are not advertised to other ASs by AS20.
