Refer to the exhibit.
You want to set up an AS path filter that allows networks that originated from AS4, and
autonomous systems that are directly attached to AS4, to enter R1. When you tested the filter, you
noticed that something was wrong with it.
How can this be solved?
A.
Change the regular expressing to ^4_0-9*$.
B.
Change the regular expression to ^4_0-9+$.
C.
Change the regular expression to ^4_0-9.*.
D.
Change the regular expression to ^4_0-9.$.
Only Allow Networks Originated from AS 4, and ASs Directly Attached to AS 4, to Enter Router 1
If you want AS 1 to get networks originated from AS 4 and all directly attached ASs of AS 4, apply the following inbound filter on Router 1.
ip as-path access-list 1 permit ^4_[0-9]*$
router bgp 1
neighbor 4.4.4.4 remote-as 4
neighbor 4.4.4.4 route-map foo in
route-map foo permit 10
match as-path 1
In the ip as-path access-list command, the carat (^) starts the input string and designates “AS”. The underscore (_) means there is a a null string in the string that follows “AS 4”. The [0-9]* specifies that any connected AS with a valid AS number can pass the filter. The advantage of using the [0-9]* syntax is that it gives you the flexibility to add any number of ASs without modifying this command string. For additional information, see AS-Regular Expression.
http://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/13754-26.html