Boundaries: Delineating Literals

The underscore (_) is used when you want to specify a string of literals and must specify their separation. Suppose, for example, that you want to match on the specific AS_PATH (5610, 148, 284, 13). The filter is as follows :

  ip as-path access-list 30 permit ^5610_148_284_13$  

The underscore matches a beginning of line, an end of line, a comma, or a space. Notice the difference between the preceding filter and this filter:

  ip as-path access-list 31 permit _5610_148_284_13_  

Because the first filter specified the beginning and end of the line, only AS_PATH (5610, 148, 284, 13) matches. In this second filter, the specified sequence must be included in the AS_PATH, but it is not necessarily the only AS numbers in the attribute. So, AS_PATHs (5610, 148, 284, 13), (23, 15, 5610, 148, 284, 13), and (5610, 148, 284, 13, 3005) all match.