User's Manual

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For examples regarding the using of BGP route aggregation, please refer to the section in the bottom of this
chapter “examples of BGP route aggregation”.
3. Configure BGP community attribute
The routing policy that BGP supports is mainly based on one of the 3 values in BGP routing information:
 Network number of routes:
 as_path attribute value of routes:
 The “community” attribute value of routes
Dividing the routes into communities through “community” attribute, and applying the routing policy based on
the community, thereby simplifies the configuration of control of routing information.
Community is a group of routes with the common attributes; each route may belong to several communities.
AS administrators can define a certain route belongs to a certain community.
Community attribute is an optional and transferable global attribute ranging from 1 to 4,294,967,200. The
famous communities pre-defined in the Internet communities include:
 No-export--- Do not advertise this route to EBGP peer (Including the EBGP peers inside
the autonomous system confederation).
 No-advertise---Do not advertise this route to any peer .
 local-as---Do not advertise this route to the exterior of autonomous system (ca send this
route to the other sub-AS peers in the autonomous system confederation.)
When generating, accepting or sending routes, BGP speakers can configure, add or modify the route
community attribute. when aggregating routes, the generated aggregation includes the “community” attributes
from complete communities of all original routes.
By default, “Community” attributes are not sent to the neighbor. Use the following BGP configuration command
to designate sending “community” attribute to the neighbour:
Command Purpose
neighbor
{ip-address | peer-group-name}
send-
community
Designate to send attribute “community” to the
neighbor.
You need to do the following jobs to configure community attribute for the router:
Command Purpose
route-map
map-name sequence-number
{deny
| permit}
Configure route-map.
set community
community-value Configure rule of setting.
router bgp
autonomous-system
Enter into router configuration mode.
neighbor
{ip-address | peer-group-name}
route-
map
access-list-name {
in | out
}
Apply route-map.
To filter routing information based on community attributes, you need to do the following jobs:
Command Purpose
ip community-list
community-list-name {
permit
| deny
}
communtiy-expression
Define community list.
route-map
map-name sequence-number
{deny
| permit}
Configure route-map.