ip aspath-list configuration command directory

ip as-path access-list

show ip as-path-list

 

ip as-path access-list

Establish as-path list regulation to match BGP route. Use command “no” to delete as-path list configured.

ip as-path access-list  <name>  <deny | permit>  <regexp>

no ip as-path access-list  <name>  [deny | permit]  [regexp]

Parameter:

<name>             name of as-path-list

<deny | permit>       character of as-path-list regulation

<regexp>         as-path property regular expression

Default:

Default refuses all as-path expression except for those with clearly stated permit.

Command mode:

global configuration mode

Explanation:

AS path list is used to filter BGP routes according to routes’s AS_PATH attribute. AS_PATH attribute of BGP routes are series of numbers (indicating autonomous system number). They are usually expressed in character strings. The number on the most right side is the number of autonomous system originating this route. Then there are numbers of autonomous systems passed through. For example, 22 23 98, means that this route originated by autonomous system 98, then passes through autonomous systems 23 and 22, and reaches this autonomous system.

AS path list is indicted with names. The total number of AS PATH list in the system is only constrained by system resource. Several matching rules can be configured under one as-path list. The process to apply the as-path list is to check in turn according to configuration orders. Once a rule is matched successfully, stop the following check and return the attribute of this rule (deny/permit). If all rules cannot be matched successfully, return deny. Every rule is organized according to configuration orders. The “as path” expression is expressed in regular expressionwith the following commonly used special characters of expression:

Character

Symbol

Meaning

Period

.

To match any single character including blank character.

Star

*

Matching 0 or more series in the mode.

Plus

+

Matching 1 or more series in the mode.

Question mark

!

Matching 0 or 1 time appearance in the mode (the same to “?”).

Adding symbol

^

Matching the beginning of entering character strings.

Symbol $

$

Matching the end of entering character strings.

Underline

_

Matching comma, left big bracket, right big bracket, left bracket, right bracket, beginning of entering character strings, ending of entering character strings, or a space.

Brackets

[Range]

Means the range of single character mode.

Slash

-

Separate a range.

Use regular expression with right expression of path attribute will establish powerful as path list. The following are some examples:

    .*         Means any as path attributes.

    ^$         Means empty path attribute.

    ^22$       Means attribute of path only containing autonomous system 22.

    ^22_       Means path attribute beginning with 22, such as 22, 22 33.

    _22$       Means path attribute ending with 22, such as 22, 34 2299 45 22.

    _22_       Means path attribute containing 22, such as 23 22 45442 22 23 44.

The as-path list can be used together with command match as-path in route-map and command filter-list of BGP.  

Example:

The as-path list hell defined in the following example will only permit path attribute beginning with 23 or containing 22:

ip as-path access-list hell permit ^23

ip as-path access-list hell permit _22_

And:

ip as-path access-list guangzhou deny ^300

ip as-path access-list guangzhou deny _300_

ip as-path access-list guangzhou permit .*

All as-path attributes beginning with 300 of containing 300 are refused. Others are passed. The result will be different if the order of defining is different. In the following example, all as-path attributes will pass.

ip as-path access-list guangzhou permit .*

ip as-path access-list guangzhou deny ^300

ip as-path access-list guangzhou deny _300_

Relevant command:

match as-path 3

neighbor filter-list

show ip aspath-list

Show as path list configured in the system. Designating names can show designated as path list information.

show ip as-path-list <name>

Parameter:

<name>      name of as path-list

Default:

none

Command mode:

Supervisor mode

Explanation:

If name is not designated, all information of as path list configured in the system.

Example:

In the following example, all as path lists will be showed:

show ip as-path-list

Relevant command:

ip as-path access-list 1