BGP Confederations: An Alternative to Full Mesh Internal BGP (IBGP)
iBGP requires a full mesh between all BGP-speaking routers; route reflectors modify iBGP split-horizon rules, and BGP confederations modify iBGP AS-path processing
The full-mesh requirement is relaxed through introduction of member autonomous systems into which the original autonomous system is split.
The addition autonomous system numbers are hidden from the outside world by modified AS-path update procedures.
The intraconfederation segment is removed from the AS path by the egress confederation router prior to prepending the official AS number when sending a BGP update to an external AS.
Intraconfederation eBGP sessions act like eBGP sessions from a session.establishment perspective, and they act like iBGP sessions from the BGP attribute-propagation perspective.
Output after BGP is configured:
from: Cisco IOS IP Routing: BGP Command Reference
Files:
Topology
Configs
BGP Configuration
bgp confederation identifier
To specify a BGP confederation identifier, use the bgp confederation identifier command in router configuration mode. To remove the confederation identifier, use the no form of this command.
bgp confederation identifier autonomous-system-number
no bgp confederation identifier autonomous-system-number
Syntax Description
In Cisco IOS Release 12.0(32)SY8, 12.0(33)S3, 12.2(33)SRE, 12.2(33)XNE, 12.2(33)SXI1, Cisco IOS XE Release 2.4, and later releases, 4-byte autonomous system numbers are supported in the range from 65536 to 4294967295 in asplain notation and in the range from 1.0 to 65535.65535 in asdot notation.Command Default
No BGP confederation identifier is identified.
Command Modes
Router configuration (config-router)
Command History
Usage Guidelines
The bgp confederation identifier command is used to configure a single autonomous system number to identify a group of smaller autonomous systems as a single confederation.
A confederation can be used to reduce the internal BGP (iBGP) mesh by dividing a large single autonomous system into multiple subautonomous systems and then grouping them into a single confederation. The subautonomous systems within the confederation exchange routing information like iBGP peers. External peers interact with the confederation as if it were a single autonomous system.
Each subautonomous system is fully meshed within itself and has a few connections to other autonomous systems within the confederation. Next hop, Multi Exit Discriminator (MED), and local preference information is preserved throughout the confederation, allowing you to retain a single Interior Gateway Protocol (IGP) for all the autonomous systems.
In Cisco IOS Release 12.0(32)SY8, 12.0(33)S3, 12.2(33)SRE, 12.2(33)XNE, 12.2(33)SXI1, Cisco IOS XE Release 2.4, and later releases, the Cisco implementation of 4-byte autonomous system numbers uses asplain—65538 for example—as the default regular expression match and output display format for autonomous system numbers, but you can configure 4-byte autonomous system numbers in both the asplain format and the asdot format as described in RFC 5396. To change the default regular expression match and output display of 4-byte autonomous system numbers to asdot format, use the bgp asnotation dot command followed by the clear ip bgp * command to perform a hard reset of all current BGP sessions.
In Cisco IOS Release 12.0(32)S12, 12.4(24)T, and Cisco IOS XE Release 2.3, the Cisco implementation of 4-byte autonomous system numbers uses asdot—1.2 for example—as the only configuration format, regular expression match, and output display, with no asplain support.
If one member of a BGP confederation is identified using a 4-byte autonomous system number, all other members of a BGP confederation must be upgraded to support 4-byte autonomous system numbers.
Examples
In the following example, the routing domain is divided into autonomous systems 50001, 50002, 50003, 50004, 50005, and 50006 and is identified by the confederation identifier 50007. Neighbor 10.2.3.4 is a peer inside of the routing domain confederation. Neighbor 10.4.5.6 is a peer outside of the routing domain confederation. To external peers and routing domains, the confederation appears as a single autonomous system with the number 50007.
router bgp 50000
bgp confederation identifier 50007
bgp confederation peers 50001 50002 50003 50004 50005 50006
neighbor 10.2.3.4 remote-as 50001
neighbor 10.4.5.6 remote-as 40000
end
In the following example, the routing domain is divided into autonomous systems using 4-byte autonomous system numbers 65538, 65536, and 65550 in asplain format and identified by the confederation identifier 65545. Neighbor 192.168.1.2 is a peer inside of the routing domain confederation. Neighbor 192.168.2.2 is a peer outside of the routing domain confederation. To external peers and routing domains, the confederation appears as a single autonomous system with the number 65545. This example requires Cisco IOS Release 12.0(32)SY8, 12.0(33)S3, 12.2(33)SRE, 12.2(33)XNE, 12.2(33)SXI1, Cisco IOS XE Release 2.4, or a later release.
router bgp 65550
bgp confederation identifier 65545
bgp confederation peers 65538 65536 65550
neighbor 192.168.1.2 remote-as 65536
neighbor 192.168.2.2 remote-as 65547
end
In the following example, the routing domain is divided into autonomous systems using 4-byte autonomous system numbers 1.2 and 1.0 in asdot format and is identified by the confederation identifier 1.9. Neighbor 192.168.1.2 is a peer inside of the routing domain confederation. Neighbor 192.168.2.2 is a peer outside of the routing domain confederation. To external peers and routing domains, the confederation appears as a single autonomous system with the number 1.9. This example requires Cisco IOS Release 12.0(32)S12, 12.4(24)T, or Cisco IOS XE Release 2.3 where asdot notation is the only format for 4-byte autonomous system numbers. This configuration can also be performed using Cisco IOS Release 12.0(32)SY8, 12.0(33)S3, 12.2(33)SRE, 12.2(33)XNE, 12.2(33)SXI1, Cisco IOS XE Release 2.4, or later releases.
router bgp 1.14
bgp confederation identifier 1.9
bgp confederation peers 1.2 1.0
neighbor 192.168.1.2 remote-as 1.0
neighbor 192.168.2.2 remote-as 1.11
end
Related Commands
bgp confederation peers
To configure subautonomous systems to belong to a single confederation, use the bgp confederation peers command in router configuration mode. To remove an autonomous system from the confederation, use the no form of this command.
bgp confederation peers autonomous-system-number [... autonomous-system-number]
no bgp confederation peers autonomous-system-number [... autonomous-system-number]
Syntax Description
Command Default
No BGP peers are configured to be members of a BGP confederation.
Command Modes
Router configuration (config-router)
Command History
Usage Guidelines
The bgp confederation peers command is used to configure multiple autonomous systems as a single confederation. The ellipsis (...) in the command syntax indicates that your command input can include multiple values for the autonomous-system-number argument.
The autonomous system number of the router on which this command is being specified is not allowed in this command (not allowed as a confederation peer). If you specify the local router's autonomous system number in the bgp confederation peers command, the error message "Local member-AS not allowed in confed peer list" will appear.
The autonomous systems specified in this command are visible internally to the confederation. Each autonomous system is fully meshed within itself. Use the bgp confederation identifier command to specify the confederation to which the autonomous systems belong.
In Cisco IOS Release 12.0(32)SY8, 12.0(33)S3, 12.2(33)SRE, 12.2(33)XNE, 12.2(33)SXI1, Cisco IOS XE Release 2.4 , and later releases, the Cisco implementation of 4-byte autonomous system numbers uses asplain—65538 for example—as the default regular expression match and output display format for autonomous system numbers, but you can configure 4-byte autonomous system numbers in both the asplain format and the asdot format as described in RFC 5396. To change the default regular expression match and output display of 4-byte autonomous system numbers to asdot format, use the bgp asnotation dot command followed by the clear ip bgp * command to perform a hard reset of all current BGP sessions.
In Cisco IOS Release 12.0(32)S12, 12.4(24)T, and Cisco IOS XE Release 2.3, the Cisco implementation of 4-byte autonomous system numbers uses asdot—1.2 for example—as the only configuration format, regular expression match, and output display, with no asplain support.
If one member of a BGP confederation is identified using a 4-byte autonomous system number, all other members of a BGP confederation must be upgraded to support 4-byte autonomous system numbers.
Examples
In the following example, autonomous systems 50001, 50002, 50003, 50004, and 50005 are configured to belong to a single confederation under the identifier 50000:
router bgp 50000
bgp confederation identifier 50000
bgp confederation peers 50001 50002 50003 50004 50005
In the following example, the routing domain is divided into autonomous systems using 4-byte autonomous system numbers 65538 and 65536, and is identified by the confederation identifier 65545. Neighbor 192.168.1.2 is a peer inside of the routing domain confederation. Neighbor 192.168.2.2 is a peer outside of the routing domain confederation. To external peers and routing domains, the confederation appears as a single autonomous system with the number 65545. This example requires Cisco IOS Release 12.0(32)SY8, 12.0(33)S3, 12.2(33)SRE, 12.2(33)XNE, 12.2(33)SXI1, Cisco IOS XE Release 2.4, or a later release.
router bgp 65550
bgp confederation identifier 65545
bgp confederation peers 65538 65536
neighbor 192.168.1.2 remote-as 65536
neighbor 192.168.2.2 remote-as 65547
end
In the following example, the routing domain is divided into autonomous systems using 4-byte autonomous system numbers 1.2, 1.0, and 1.14 and is identified by the confederation identifier 1.9. Neighbor 192.168.1.2 is a peer inside of the routing domain confederation. Neighbor 192.168.2.2 is a peer outside of the routing domain confederation. To external peers and routing domains, the confederation appears as a single autonomous system with the number 1.9. This example requires Cisco IOS Release 12.0(32)S12, 12.4(24)T, or Cisco IOS XE Release 2.3 where asdot notation is the only format for 4-byte autonomous system numbers. This configuration can also be performed using Cisco IOS Release 12.0(32)SY8, 12.0(33)S3, 12.2(33)SRE, 12.2(33)XNE, 12.2(33)SXI1, Cisco IOS XE Release 2.4, or later releases.
router bgp 1.14
bgp confederation identifier 1.9
bgp confederation peers 1.2 1.0 1.14
neighbor 192.168.1.2 remote-as 1.0
neighbor 192.168.2.2 remote-as 1.11
end
Related Commands
Files:
Topology
Configs
BGP Configuration
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