< Free Open Study >

Syntax Description:

• ip-address— Neighbor’s IP address.

• peer-group-name— Name of the peer group. See section 8-19.

Purpose: By default, BGP community attributes are not advertised to peers. The neighbor send-community command enables the sending of BGP community attributes to BGP peers. Routing policies can be based on a neighbor address, a peer group name, or AS path information. Situations might arise in which you need to apply policies to routes that do not have any of the previously mentioned attributes in common. A community value is a numerical value or set of values that can be attached to a BGP route. Routing policies can then be applied to routes that contain a particular community value or attribute.

Cisco IOS Software Release: 10.3. The peer-group-name option was added in Release 11.0.

Configuration Example: NO-EXPORT Community Value

Two well-known community values are NO-EXPORT and NO-ADVERTISE. If a route carries the NO-EXPORT community value, the route is not advertised outside the AS. The behavior of the NO-EXPORT community value is illustrated in Figure 8-29. The NO-ADVERTISE community value prevents a router from advertising the route to any peer, as shown in Figure 8-30. To configure a COMMUNITY attribute, we will use an outbound route map on Router A, as shown in the following configuration. For a complete discussion of route maps, see Appendix C.

Figure 8-29. NO-EXPORT Community

Figure 8-30. NO-ADVERTISE Community

Router A

router bgp 65530

 network 172.16.0.0 mask 255.255.255.0

 network 172.16.1.0 mask 255.255.255.0

 network 172.16.2.0 mask 255.255.255.0

 network 172.16.3.0 mask 255.255.255.0

 neighbor 10.1.1.2 remote-as 1

 neighbor 10.1.1.2 send-community

 neighbor 10.1.1.2 route-map setnoexport out

!

access-list 1 permit 172.16.1.0 0.0.0.255

!

route-map setnoexport permit 10

 match ip address 1

 set community no-export

route-map setnoexport permit 20

__________________________________________________________________________

Router B

router bgp 1

 neighbor 10.1.1.1 remote-as 65530

 neighbor 10.1.2.2  remote-as 2

__________________________________________________________________________

Router C

router bgp 2

 neighbor 10.1.2.1 remote-as 1

For this configuration, Router B should receive the four 172.16 prefixes from Router A. Because the 172.16.1.0 prefix has the NO_EXPORT community value, Router B should not advertise this route to Router C.

Verify the effect of the NO_EXPORT community value by checking the BGP tables on Routers B and C:

rtrB#show ip bgp

BGP table version is 41, local router ID is 10.1.1.2

Status codes: s suppressed, d damped, h history, * valid, > best, i - internal

Origin codes: i - IGP, e - EGP, ? - incomplete



   Network          Next Hop            Metric LocPrf Weight Path

*> 172.16.0.0/24    10.1.1.1                 0             0 65530 i

*> 172.16.1.0/24    10.1.1.1                 0             0 65530 i

*> 172.16.2.0/24    10.1.1.1                 0             0 65530 i

*> 172.16.3.0/24    10.1.1.1                 0             0 65530 i

__________________________________________________________________________

rtrC#show ip bgp

BGP table version is 41, local router ID is 10.1.2.2

Status codes: s suppressed, d damped, h history, * valid, > best, i - internal

Origin codes: i - IGP, e - EGP, ? - incomplete



   Network          Next Hop            Metric LocPrf Weight Path

*> 172.16.0.0/24    10.1.2.1                 0             0 1 65530 i

*> 172.16.2.0/24    10.1.2.1                 0             0 1 65530 i

*> 172.16.3.0/24    10.1.2.1                 0             0 1 65530 i

The community values of routes can be checked using the show ip bgp community command:

rtrB#show ip bgp community no-export

BGP table version is 41, local router ID is 10.1.1.2

Status codes: s suppressed, d damped, h history, * valid, > best, i - internal

Origin codes: i - IGP, e - EGP, ? - incomplete



   Network          Next Hop            Metric LocPrf Weight Path

*> 172.16.1.0/24    10.1.1.1                 0             0 65530 i

Troubleshooting

1. Verify that the BGP neighbors are in the Established state using the show ip bgp neighbors command.

   If the neighbor relationship is not in the Established state, see section 8-23.

2. Verify that you have configured the neighbor send-community command.

3. Check the syntax of the route map used to set the community attribute.

4. Use the command show ip bgp community community-value in order to verify that the community value has been set and received.

< Free Open Study >