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 le 20

ip prefix-list Peer seq 10 permit 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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Overall Rating: 4.5 (2 ratings)
Edison Ortiz Wed, 08/20/2008 - 08:14
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You are correct.

The 2nd entry "ip prefix-list Peer seq 10 permit 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 ge 21




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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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