BGP prefix list .... need help

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Aug 20th, 2008
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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?


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Edison Ortiz Wed, 08/20/2008 - 08:14
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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,


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Marcofbbr Wed, 08/20/2008 - 12:05
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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.



Edison Ortiz Wed, 08/20/2008 - 12:07
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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.


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