This is one of the CCIE Flash Cards Q, whats the difference between A and D ?
What is the difference between the regular expressions _200_ and _200$?
A.) The first expression refers to routes that have the substring 200 in their AS paths; the second
expression refers only to routes that originated in AS 200.
B.) The first expression refers to routes that have the substring 200 in their AS paths; the second
expression refers only to routes that are directly connected to AS 200.
C.) The first expression refers to routes that are directly connected to AS 200; the second expression
refers to routes that originated in AS 200.
D.) The first expression refers to routes that go through AS 200; the second expression refers to routes
that originated in AS 200.
As I understand,
_40$ means the Originating AS is 40.
^40$ means ..only AS 40 which is directly connected and is the originator also.
_40_ means 40 can be anywhere on the AS_PATH.
http://www.cisco.com/en/US/customer/tech/tk365/technologies_tech_note09186a0080094a92.shtml says
ip as-path access-list 1 permit ^4_[0-9]*$ will allow Networks Originated from AS 4, and ASs Directly Attached to AS 4
How?
The ^ means the routes are being received from AS 4, and not being originated from AS4 ?