Setting the AS_Path Attribute
The AS_Path attribute is used to prevent routing loops and control route selection.
Procedure
- Enable a BGP device to accept the routes that contain the local AS number if the number of repetitions in each route is within a configured limit.
Generally, BGP uses AS numbers to detect routing loops. In the Hub and Spoke networking, if EBGP runs between a Hub-PE and a Hub-CE at a Hub site, a route sent from the Hub-PE to the Hub-CE carries the AS number of the Hub-PE. If the Hub-CE sends an Update message that contains the AS number of the Hub-PE to the Hub-PE, the Hub-PE will deny it.
To ensure proper route transmission in the Hub and Spoke networking, configure all the BGP peers on the path, along which the Hub-CE advertises VPN routes to the Spoke-CE, to accept the routes that contain the local AS number if the number of repetitions in each route is within the default limit (1).
- Enable the local device to ignore the AS_Path attribute during route selection.
- Configure a fake AS number.
- Run peer { ipv4-address | group-name } fake-as fake-as-value [ dual-as ] [ prepend-global-as ] [ prepend-fake-as ]
A fake AS number is configured.
The actual AS number can be hidden by using this command.
EBGP peers in other ASs can learn only this fake AS number. The peers in other ASs use the fake AS number as the AS number of the local peer.
- Run peer { ipv4-address | group-name } fake-as fake-as-value [ dual-as ] [ prepend-global-as ] [ prepend-fake-as ]
- Substitute the AS numbers in the AS_Path attribute.
In a BGP/MPLS IP VPN scenario, if the ASs to which two VPN sites belong use private AS numbers, the AS numbers of the two VPN sites may be the same. If a CE in a VPN site sends a VPN route to the connected PE using EBGP and the PE then sends the route to the remote PE, the remote CE will discard the route because the AS number carried by the route is the same as the local AS number. As a result, different sites of the same VPN cannot communicate. The peer substitute-as command can be used on the PE to enable AS number substitution to address this problem. After that, the PE replaces the AS number carried in the VPN route with the local AS number. As a result, the remote CE will not discard the route due to identical AS numbers.
On a BGP public network, two devices have the same AS number and the same EBGP peer. After one of the two devices learns a route of the other device from the EBGP peer, the route is discarded because it carries an AS number that is the same as the local one. To address this problem, run the peer substitute-as command on the EBGP peer.
Configuring the peer substitute-as command may cause a routing loop. Therefore, exercise caution when running this command.
- Enable the device to remove all the private AS numbers from the AS_Path attribute in BGP Update messages.
- Run the peer { ipv4-address | group-name } public-as-only [ force [ replace ] [ include-peer-as ] | limited [ replace ] [ include-peer-as ] ] command to configure the AS_Path attribute in the BGP Update messages to be sent to carry only public AS numbers. Alternatively, run the peer { ipv4-address | group-name } public-as-only import [ force ] command to configure the device to remove all the private AS numbers from the AS_Path attribute in received BGP Update messages.
Generally, AS numbers range from 1 to 4294967295, including public AS numbers, private AS numbers, and reserved AS numbers. Private AS numbers range from 64512 to 65534. The AS numbers 65535 and 4294967295 are reserved for special applications. Other AS numbers are public AS numbers.
If the 4-byte private AS number capability is enabled using the private-4-byte-as enable command, in addition to the range from 64512 to 65534, the private AS number range also includes 4200000000 to 4294967294 (or 64086.59904 to 65535.65534).
Public AS numbers can be used on the Internet, and are assigned and managed by the Internet Assigned Number Authority (IANA). Private AS numbers cannot be advertised to the Internet, and are used only within ASs.
Generally, BGP routes to be advertised to peers carry either public or private AS numbers, or both. In certain cases, private AS numbers do not need to be advertised. In this case, you can run this command to configure the AS_Path attribute to carry only public AS numbers.
- Run the peer { ipv4-address | group-name } public-as-only [ force [ replace ] [ include-peer-as ] | limited [ replace ] [ include-peer-as ] ] command to configure the AS_Path attribute in the BGP Update messages to be sent to carry only public AS numbers. Alternatively, run the peer { ipv4-address | group-name } public-as-only import [ force ] command to configure the device to remove all the private AS numbers from the AS_Path attribute in received BGP Update messages.
- Set the maximum number of AS numbers in the AS_Path attribute.
- Disable the BGP device from checking the first AS number contained in the AS_Path attribute of each Update message received from an EBGP peer.
- Run undo check-first-as
The BGP device is disabled from checking the first AS number in the AS_Path attribute that is carried in each Update message received from an EBGP peer.
Exercise caution when running the undo check-first-as command because use of this command may cause routing loops.
- Run undo check-first-as
- Enable the device to check or disable the device from checking the first AS number in the AS_Path attribute contained in the Update messages received from a specified EBGP peer or peer group.