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Filtering BGP
The following section describes the methods used to filter the updates received from BGP neighbors.
Following are the filtering methods of BGP updates:
● Filtering using IP prefix lists
● Filtering using route maps
● Filtering using AS-PATH information
● Filtering using community lists
Regular Expressions as Filters
Regular expressions are used to filter AS paths or community lists. A regular expression is a special character used to define a
pattern that is then compared with an input string.
For an AS-path access list, as shown in the previous commands, if the AS path matches the regular expression in the access list,
the route matches the access list.
The following lists the regular expressions accepted in Dell EMC Networking OS.
Regular
Expression
Definition
^ (caret) Matches the beginning of the input string. Alternatively, when used as the first character within brackets
[^ ], this matches any number except the ones specified within the brackets.
$ (dollar) Matches the end of the input string.
. (period) Matches any single character, including white space.
* (asterisk) Matches 0 or more sequences of the immediately previous character or pattern.
+ (plus) Matches 1 or more sequences of the immediately previous character or pattern.
? (question) Matches 0 or 1 sequence of the immediately previous character or pattern.
( ) (parenthesis) Specifies patterns for multiple use when one of the multiplier metacharacters follows: asterisk *, plus sign
+, or question mark ?
[ ] (brackets) Matches any enclosed character and specifies a range of single characters.
- (hyphen) Used within brackets to specify a range of AS or community numbers.
_ (underscore) Matches a ^, a $, a comma, a space, or a {, or a }. Placed on either side of a string to specify a literal and
disallow substring matching. You can precede or follow numerals enclosed by underscores by any of the
characters listed.
| (pipe) Matches characters on either side of the metacharacter; logical OR.
As seen in the following example, the expressions are displayed when using the show commands. To view the AS-PATH
ACL configuration, use the show config command in CONFIGURATION AS-PATH ACL mode and the show ip as-path-
access-list command in EXEC Privilege mode.
For more information about this command and route filtering, refer to Filtering BGP Routes.
The following example applies access list Eagle to routes inbound from BGP peer 10.5.5.2. Access list Eagle uses a regular
expression to deny routes originating in AS 32. The first lines shown in bold create the access list and filter. The second lines
shown in bold are the regular expression shown as part of the access list filter.
Example of Using Regular Expression to Filter AS Paths
DellEMC(config)#router bgp 99
DellEMC(conf-router_bgp)#neigh AAA peer-group
DellEMC(conf-router_bgp)#neigh AAA no shut
DellEMC(conf-router_bgp)#show conf
!
router bgp 99
neighbor AAA peer-group
Border Gateway Protocol (BGP)
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