BGP AS Paths

AS Path access lists entries determine if networks are allowed or denied in specific BGP configuration contexts. They are primarily used in BGP route maps, but also can be used in other areas of BGP configuration which accept AS Path lists as parameters.

The order of entries inside an AS Path list is important, and this order is determined by a sequence number. As with other access lists, AS Path access lists implicitly deny anything not matched.

AS Path lists are managed at Services > FRR BGP on the AS Paths tab.

The order of entries inside an AS Path list is important, and this order is determined by a sequence number.

BGP AS Path Configuration

The AS Paths tab contains a list of current AS Path lists, if any, and controls to manage the entires (e.g. edit, delete). The Add button creates a new AS Path list.

When creating or editing an AS Path list, the following options are available:

Name:

The name of this BGP AS Path list.

Description:

A text description of this list (e.g. its purpose)

AS Path Entries:

A list of AS paths to match in this list. Click Add to create additional rules on the same list.

Sequence:

The sequence number for this rule, which controls the order in which rules are matched inside this AS Path list. Each rule must have a unique sequence number. Best practice is to leave gaps in the sequence to allow for adding rules in the future. For example, use 10, 20, 30, rather than 1, 2, 3.

Action:

The action taken when this AS Path rule is matched, either permit or deny.

Regular Expression:

A regular expression pattern which will match on the AS number.

Regular expression patterns support common pattern special characters for matching, but also a special _ character. The _ character matches common AS delimiters such as start of line, end of line, space, comma, braces, and parenthesis. The _ character can be used on either side of an AS number to match it exactly, such as _65534_.