13.4.2 Community

By adding the COMMUNITIES attribute to routing information, you can restrict the range of route advertisements handled by the Switch.

<Structure of this section>

(1) Type of community

The community values supported by the Switch can be divided into two types:

Values (codes) predefined in RFC 1997

When a community whose values are predefined in RFC 1997 is added to the reported routing information, the route is advertised in accordance with those values. For details about the communities defined in RFC1997 that can be used with the Switch, see Table 13-7: Communities that can be used with the Switch.
Values freely specified by the user (that is, not predefined in RFC 1997) in a learned route filter or advertised route filter in the configuration settings

When a community whose values are specified in a learned route filter or advertised route filter in the configuration settings is added to the reported routing information, the configuration settings govern whether that routing information is imported (if a learned route filter) or advertised (if an advertised route filter).

Communities can also be added to the routing information reported by the Switch by using learned route filters and advertised route filters.

The following table describes the communities defined in RFC 1997 and supported by the Switch.

Table 13-7: Communities that can be used with the Switch
Community	Description
no-export	Do not advertise this routing information outside the AS.
no-advertise	Do not advertise this routing information to other peers.
local-AS	Do not advertise this routing information outside the local member AS, including to any other AS.

Note: no-export and local-AS have the same meaning in most configurations.

The following figure shows the range of routes with the COMMUNITIES attribute that can be propagated in a network.

Figure 13-14: Advertising range of routing information with COMMUNITIES attributes

(2) Usage of learned route filtering and COMMUNITIES properties

The following figure shows an example of using learned route filtering and the COMMUNITIES attribute.

Figure 13-15: Example of using learned route filtering and COMMUNITIES properties

In this example, two Switches (A and B) are connected to an external AS. Considering the need to equalize traffic distribution, outbound traffic from Switch C should preferentially be routed through Switch A, and outbound traffic from Switch D should preferentially be routed through Switch B. In this scenario, load balancing can be achieved by setting up the routers as follows:

Add community a to the routing information propagated from Switch A to internal peers.

(You can set an advertised route filter for this purpose.)
Add community b to the routing information propagated from Switch B to internal peers.

(You can set an advertised route filter for this purpose.)
At Switch C, the LOCAL-PREF value is set to x (x > y) if the received route information is tagged with community a, or to y (x > y) if the received route information is tagged with community b. That is, routing information with greater LOCAL-PREF value which was reported from Switch A takes priority.

(You can set a learned route filter for this purpose.)
At Switch D, the LOCAL-PREF value is set to y (x > y) if the received route information is tagged with community a, or to x (x > y) if the received route information is tagged with community b. That is, routing information with a greater LOCAL-PREF value, which was reported from Switch B, takes priority.

(You can set a learned route filter for this purpose.)