08-20-2008 07:47 AM - edited 03-03-2019 11:12 PM
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 128.0.0.0/2 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?
08-20-2008 08:14 AM
Marco,
You are correct.
The 2nd entry "ip prefix-list Peer seq 10 permit 0.0.0.0/0 le 24" allows any address class to be longer than /20.
You need to place a deny right after the sequence 5 to explicitly block any prefix longer than /20 for a Class B address space.
ip prefix-list Peer seq 6 deny 128.0.0.0/2 ge 21
HTH,
__
Edison.
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08-20-2008 12:05 PM
Thanks Edison
So it seems there is a mistake in the Student guide. Should I inform Cisco about that? I don't know if there are newer releases of that Course.
08-20-2008 12:07 PM
You should inform the author of the guide, not Cisco itself :)
The guide should contain the contact information and perhaps has a website were erratas can be posted.
__
Edison.
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