BGP Named Community Lists

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A BGP community is a group of routes that share a common property, regardless of their network, autonomous system, or any physical boundaries. In large networks, applying a common routing policy by using prefix lists or access lists requires individual peer statements on each networking device. Using the BGP COMMUNITIES attribute, BGP speakers with common routing policies can implement inbound or outbound route filters based on the community tag, rather than consult long lists of individual permit or deny statements. A COMMUNITIES attribute can contain multiple communities.

A route can belong to multiple communities. The network administrator defines the communities to which a route belongs. By default, all routes belong to the general Internet community.

In addition to numbered communities, there are several predefined (well-known) communities:

• no-export--Do not advertise this route to external BGP peers.
• no-advertise--Do not advertise this route to any peer.
• internet--Advertise this route to the Internet community. All BGP-speaking networking devices belong to this community.
• local-as--Do not send this route outside the local autonomous system.
• gshut--Community of routes gracefully shut down.

The COMMUNITIES attribute is optional, which means that it will not be passed on by networking devices that do not understand communities. Networking devices that understand communities must be configured to handle the communities or else the COMMUNITIES attribute will be discarded. By default, no COMMUNITIES attribute is sent to a neighbor. In order for a COMMUNITIES attribute to be sent to a neighbor, use the neighbor send-community command.

A BGP community list is used to create groups of communities to use in a match clause of a route map. A community list can be used to control which routes are accepted, preferred, distributed, or advertised, for example. You can also use a community list to set, append, or modify the communities of a route.

• Standard community lists are used to specify well-known communities and community numbers.
• Expanded community lists are used to filter communities using a regular expression. Regular expressions are used to specify patterns to match COMMUNITIES attributes.

A BGP named community list allows you to assign a meaningful name to a community list. A named community list can be configured using community numbers, well-known communities, or regular expressions. All the rules of numbered community lists apply to named community lists, except that there is no limit on the number of named community lists that can be configured.

Note	A maximum of 100 standard community lists and 100 expanded community lists can be configured. A named community list does not have this limitation.

Perform this task to filter traffic by creating a BGP community list, referencing the community list within a route map, and then applying the route map to a neighbor, specifying whether the route map applies to incoming or outgoing routes.

In this task, Router B in the figure below is configured with route maps and a community list to control incoming routes.

Figure 1

Topology for Which a Community List Is Configured

1.    enable

2.    configure terminal

3.    router bgp autonomous-system-number

4.    neighbor {ip-address | peer-group-name} remote-as autonomous-system-number

5.    address-family ipv4 [unicast | multicast | vrf vrf-name]

6.    neighbor {ip-address | peer-group-name} route-map route-map-name {in | out}

7.    exit

8.    exit

9.    route-map map-name [permit | deny] [sequence-number]

10.    match community {standard-list-number | expanded-list-number | community-list-name [exact]}

11.    set weight weight

12.    exit

13.    route-map map-name [permit | deny] [sequence-number]

14.    match community {standard-list-number | expanded-list-number | community-list-name [exact]}

15.    set community community-number

16.    exit

17.    ip community-list {standard-list-number | standard list-name {deny | permit} [community-number] [AA:NN] [internet] [local-AS] [no-advertise] [no-export]} | {expanded-list-number | expanded list-name {deny | permit} regular-expression}

18.    Repeat Step 17 to create all the required community lists.

19.    exit

20.    show ip community-list [standard-list-number | expanded-list-number | community-list-name] [exact-match]

Device# show ip community-list 1

Community standard list 1
     permit 100

Device# show ip community-list 2

Community standard list 2
     permit internet

Perform this task to filter traffic by creating an extended BGP community list to control outbound routes.

Figure 2

Topology for Which a Community List Is Configured

In this task, Router B in the figure above is configured with an extended named community list to specify that the BGP peer at 192.168.1.2 is not sent advertisements about any path through or from autonomous system 50000. The IP extended community-list configuration mode is used and the ability to resequence entries is shown.

Note	A sequence number is applied to all extended community list entries by default, regardless of the configuration mode. Explicit sequencing and resequencing of extended community list entries can be configured only in IP extended community-list configuration mode, not in global configuration mode.

1.    enable

2.    configure terminal

3.    ip extcommunity-list {expanded-list-number | expanded list-name | standard-list-number | standard list-name}

4.    [sequence-number] {deny [regular-expression] | exit | permit [regular-expression]}

5.    Repeat Step 4 for all the required permit or deny entries in the extended community list.

6.    resequence [starting-sequence] [sequence-increment]

7.    exit

8.    router bgp autonomous-system-number

9.    neighbor {ip-address | peer-group-name} remote-as autonomous-system-number

10.    Repeat the prior step for all of the required BGP peers.

11.    address-family ipv4 [unicast | multicast | vrf vrf-name]

12.    network network-number [mask network-mask]

13.    end

14.    show ip extcommunity-list [list-name]

Device# show ip extcommunity-list DENY50000

Expanded extended community-list DENY50000
     50 deny _50000_
     150 deny ^50000 .*

This section contains two examples of the use of BGP COMMUNITIES attributes with route maps.

The first example configures a route map named set-community, which is applied to the outbound updates to the neighbor 172.16.232.50. The routes that pass access list 1 are given the well-known COMMUNITIES attribute value no-export. The remaining routes are advertised normally. The no-export community value automatically prevents the advertisement of those routes by the BGP speakers in autonomous system 200.

router bgp 100
 neighbor 172.16.232.50 remote-as 200
 neighbor 172.16.232.50 send-community
 neighbor 172.16.232.50 route-map set-community out
!
route-map set-community permit 10
 match address 1
 set community no-export
!
route-map set-community permit 20
 match address 2

The second example configures a route map named set-community, which is applied to the outbound updates to neighbor 172.16.232.90. All the routes that originate from autonomous system 70 have the COMMUNITIES attribute values 200 200 added to their already existing communities. All other routes are advertised as normal.

route-map bgp 200
 neighbor 172.16.232.90 remote-as 100
 neighbor 172.16.232.90 send-community
 neighbor 172.16.232.90 route-map set-community out
!
route-map set-community permit 10
 match as-path 1
 set community 200 200 additive
!
route-map set-community permit 20
!
ip as-path access-list 1 permit 70$
ip as-path access-list 2 permit .*