- Enterasys Security Router User's Guide

Overview
XSR User’s Guide 6-13
• Set community attributes for a specific route with set community
• Set the origin for a specific route with set origin
• Set the MED of a specific route with set metric
• Set the local preference for a specific route with set local-preference
• Set the AS-Path list for a specific route with set as-path
• Set the dampening parameters for a specific route with set dampening
• Set the next hop IP address for a specific route with set ip next-hop
Regular Expressions
Regular expressions commonly notate text string patterns. They specify rules for a set of strings
you may want to match in a search. With BGP, regular expressions search AS paths to match a
particular pattern and are especially useful in building complex policies. Regular expressions are
evaluated from left to right in sequence with binary logic. A number denotes a literal numeral and
AS number. Special characters denote position or operation within a string.
Regular Expression Characters
• 0 through 9 numerals are literals, used in any combination to represent an AS number
• '^' marks the beginning of a path
• '$' marks the end of a path
• '{' marks the beginning of an AS_SET
• '}' marks the end of an AS_SET
• '(' marks the start of an AS_CONFED_SET or AS_CONFED_SEQ
• ')' marks the end of an AS_CONFED_SET or AS_CONFED_SEQ
To match AS numbers in an AS path, use any of the following expressions:
– '.' Matches any valid AS number
– '.*' matches 0 or more sequence or AS numbers
–'.+' matches 1 or more of the sequence of AS numbers
– '_' (underscore) matches 0 or 1 instance of any punctuation character
– [ ] specifies a set of AS numbers or punctuation, for example, “[1234 45 6789]” or “[ {( ]”,
all members of a set must be the same type, i.e. either AS numbers or punctuation
– '-' is used within brackets to specify a range of AS numbers, for example “[23 - 45]”
– '^' when used as the first item within brackets specifies any AS number except the set
specified; for example, to specify any AS number other than 11 or 13 use “[^11 13]”
Regular Expression Examples
The following displays some common examples for matching AS paths using regular expressions
with the
show ip bgp regexp command.
• Display all routes with a single AS number in the AS path:
–
show ip bgp “.”