In BGP Student Guide version3.2 slide 3-54 you can find this example:
"Requirement: The ISP router will not accept routes with subnet masks longer than /24; subnet masks from class B address space will be no longer than /20."
This is accomplished with the following configuration:
neighbor CUSTOMER prefix-list peer in
ip prefix-list Peer seq 5 permit 18.104.22.168/2 le 20
ip prefix-list Peer seq 10 permit 0.0.0.0/0 le 24
What is the scope of the first prefix-list entry? Even if a class B route with a mask /22 is received it would be permitted by the second entry (seq #20)
The explanation provided by the slide is: "If the route is a class B network both lines match. Testing continues with checking the subnet mask. An upper bound is explicitly indicated , giving a maximum prefix lenght of 20 bits." ==>> I think this is really confusing....
Could someone explain that?