BGP Community Properties

BGP Communities Properties

The community property of BGP is an optional transitive property, and routers that do not support this attribute pass the community value to the downstream BGP neighbor intact (provided that the delivery is configured Community property).

the community attribute of BGP simplifies the matching of routers in the network to the same routing entries, and if all routers in a large network need to match the same route entry, if the ACL or prefix list is used to match, The workload is relatively large, so you can on a router for the corresponding route entry on the community attribute value, on other routers can match the Community attribute value of the way indirectly to match the route entry, This simplifies the work of matching routing entries.

category of the Community property:

can be divided into private Community property values and well-known Community property values.

the value of the private Community property is usually represented by a number of 4 bytes (which can be either a three -digit number or a two - digit number, such as : 1)

in the CISCO routers, by default, are displayed in the number of digits, and if you want to convert to two digits , you need to convert by command ((config) #ip bgp-community New-format), the display format in the BGP table and the display format in the configuration fileare converted at the same time .

The IP bgp-community new-format command is valid only for this router and is not valid for the neighbor router.

the most well known Community attribute values are:

No-advertise: do not advertise to any BGP neighbors (most stringent).

No-export: do not advertise to any EBGP neighbor, if there is a BGP federation, will be passed between EBGP neighbors within the federation.

Local-as : Passed only within as, if there is a BGP federation,

Internet : Represents all routes and can match all routing entries by matching Internet properties

when the router accepts a route entry with a well-known Community attribute value, it must make the appropriate action (different depending on the value of the Community property)

It can also be divided into standard Community property values and extended Community property values.

Extended Community attribute values are generally not involved in MPLS VPN networks.

Note:

a route prefix can carry multiple community values, and can carry both private community values and well-known community values.

in any one The community value can be overwritten, appended, and removed on the BGP router .

The community property value of BGP is not automatically passed to the neighbor and must be passed for a specific neighbor configuration.

(config-router) #neighbor 192.168.12.2 send-community # # Pass the community property when sending routes for a particular neighbor .

Configuration:

Add a Community property value for a route entry

1.use ACL or prefix list to match the route entry that needs to add Community attribute value, try to use prefix list match .

(config) #ip prefix-list 1 Permit 1.1.1.1/32

2, define a route-map , match to the corresponding route entry, and append or overwrite the community attribute value .

(config) #route-map 1 Permit 10

(config-route-map) # match IP address prefix-list 1 # # matches the route entry that needs to add the community attribute

(CONFIG-ROUTE-MAP) # Set Community 126:1 # # hit a private community BGP community Property for the matching route entry