apply as-path

Function

The apply as-path command sets the action for changing the AS_Path attribute of BGP routes in a routing policy.

The undo apply as-path command restores the default setting.

By default, the action for changing the AS_Path attribute of BGP routes is not set in a routing policy.

Product	Support
S5720-EI, S5720-HI, S5720I-SI, S5720S-SI, S5720-SI, S5735-S, S5735S-S, S5735-S-I, S5730-HI, S5730S-EI, S5730-SI, S5731-H, S5731-S, S5731S-H, S5731S-S, S5732-H, S6720-EI, S6720-HI, S6720S-EI, S6720S-SI, S6720-SI, S6730-H, S6730S-H, S6730-S, and S6730S-S	Supported.
S2720-EI, S5720-LI, S5735-L, S5735S-L, S5735S-L-M, S5720S-LI, S6720-LI, and S6720S-LI	Not supported.

Format

apply as-path { { as-number-plain | as-number-dot } &<1-10> { additive | overwrite } | none overwrite }

undo apply as-path

Parameters

Parameter	Description	Value
as-number-dot	Specifies an AS number in dotted notation to be added to the AS_Path list or to replace the existing AS_Path list. A maximum of 10 AS numbers can be specified in one command.	The value is in the format of x.y, where x and y are integers that range from 1 to 65535 and from 0 to 65535, respectively.
additive	Adds the specified AS number to the original AS_Path attribute.	-
overwrite	Replaces the original AS_Path with the specified AS number.	-
none	Clears the original AS_Path list.	-

Views

Route-Policy view

Default Level

2: Configuration level

Usage Guidelines

Usage Scenario

To change the AS_Path attribute of BGP routes BGP for selecting the optimal route, you can apply a routing policy containing the apply as-path command.

AS_Path is a private attribute of BGP and records all ASs that a route passes through from the local end to the destination address. Using the AS_Path attribute controls route selection and prevents routing loops. If multiple routes are destined for the same destination address, BGP compares the AS_Path lists of these routes and considers the route with the shortest AS_Path list as the optimal route.

After this command is configured, the AS_Path list for matched BGP routes will change. Assume that the original AS-Path is (30, 40, 50) and the BGP route matching condition is met. In this case:

If the apply as-path 60 70 80 additive command is run, the AS-Path list is changed to (60, 70, 80, 30, 40, 50). This configuration change is generally used to make the BGP route not preferentially selected.
If the apply as-path 60 70 80 overwrite command is run, the AS-Path list is changed to (60, 70, 80). There are many application scenarios for changing the AS-Path list, and the major application scenarios are as follows:
- Hide the real path information of routes. For example, after the AS-Path list is changed to (60, 70, 80), the AS-Path information of the route (30, 40, 50) is lost.
- Implement load balancing. For example, a router receives two routes with the same destination IP address 10.1.0.0/16. The AS_Path list of one route is (60, 70, 80) and that of the other route is (30, 40, 50). In this case, you can change the AS_Path list (30, 40, 50) to (60, 70, 80), and load balancing then may be implemented on the two routes.
- Shorten the AS-Path list to prevent the route from being discarded. If the as-path-limit command is configured, whether the number of AS numbers in the AS-Path list of the incoming route exceeds the maximum value needs to be checked. If the number exceeds the maximum value, the route is discarded. Therefore, before receiving a route with a long AS-Path list, replace the AS-Path list with a shorter AS-Path list. For example, if the original AS-Path list is (60, 70, 80, 65001, 65002, 65003), run the apply as-path 60 70 80 overwrite command to change the AS-Path list to (60, 70, 80). In this manner, the length of the AS-Path is shortened, preventing the route from being discarded.
- Shorten the AS-Path list to make the route preferentially selected and traffic directed to the local AS.
If the apply as-path none overwrite command is run, the AS-Path list is changed to be vacant. In BGP route selection, if the AS-Path list is vacant, the length of the AS-Path list is considered as 0. Therefore, clearing the AS-Path list cannot only hide the real path information, but also make the route preferentially selected and traffic directed to the local AS because the AS-Path list is shortened.

Prerequisites

The apply as-path command can be used only after the route-policy command is used.

Precautions

When a routing policy takes effect, it affects BGP route selection.

Running the apply as-path command changes the path through which network traffic passes. Use this command only when you are familiar with the network topology and impact of the command on services.

Example

# Change the AS number in the original AS_Path attribute to 200, 10.10.

<HUAWEI> system-view

[HUAWEI] route-policy policy permit node 10

[HUAWEI-route-policy] apply as-path 200 10.10 additive