From: Shine Joseph (shinepjoseph@iprimus.com.au)
Date: Sat Apr 05 2008 - 21:38:53 ART
Carols,
Yes, you are right. I have overlooked the 'le'.
The prefix list used in the SG can be used for filtering the current
prefixes advertised by the backbone. But, it is not scalable. I wonder if
you loose points for this task, if you use the SG prefix list.
Regards,
Shine
-----Original Message-----
From: Carlos G Mendioroz [mailto:tron@huapi.ba.ar]
Sent: Sunday, 6 April 2008 8:49 AM
To: Shine Joseph
Cc: 'Athaide, Dwayne'; 'groupstudy'
Subject: Re: Prefix list question
Shine,
"le" means less or equal. "ge" is greater or equal.
If you put the prefix w/o le or ge, it behaves as "eq" <len>,
where <len> is the netmask len in the prefix.
To see it in action, lets say we put 3 statics like:
ip route 28.119.16.0 255.255.254.0 1.1.16.23
ip route 28.119.16.0 255.255.255.0 1.1.16.24
ip route 28.119.17.0 255.255.255.0 1.1.17.24
and we use BGP as a filter test tool, so we:
router bgp 10
redistribute static route-map AF
route-map AF permit 10
match ip address prefix-list FILTER
if we have:
ip prefix-list FILTER seq 10 permit 28.119.16.0/23
then we see (show ip bgp)
*> 28.119.16.0/23 1.1.16.23 0 32768 ?
if we have:
ip prefix-list FILTER seq 10 permit 28.119.16.0/23 le 24
then we have:
*> 28.119.16.0/24 1.1.16.24 0 32768 ?
*> 28.119.16.0/23 1.1.16.23 0 32768 ?
*> 28.119.17.0/24 1.1.17.24 0 32768 ?
but if we have:
ip prefix-list FILTER seq 10 permit 28.119.16.0/23 ge 24 le 24
we get:
*> 28.119.16.0/24 1.1.16.24 0 32768 ?
*> 28.119.17.0/24 1.1.17.24 0 32768 ?
which were the target prefixes.
So I stay with Dwayne here, SG is wrong.
-Carlos
Shine Joseph @ 05/04/2008 15:57 -0300 dixit:
> Athaide,
>
> In the example provided, there is no difference in either prefix list. You
> are essentially saying match the first 23 bits and you need a subnet mask
of
> ge 24 and le 24(which is 24 only).
>
> The solution guide says match the first 23 bits with a subnet mask of le
24.
> Here in this case it can only be 24.
>
> But, look at this example:
>
> ip prefix-list FILTER seq 10 permit 28.119.16.0/22 ge 24 le 24
> Vs
> ip prefix-list FILTER seq 10 permit 28.119.16.0/22 le 24
>
> The first one is saying match the first 22 bits with a subnet mask of 24
> only. In this case the matches are:
> 28.119.16.0/24
> 28.119.17.0/24
> 28.119.18.0/24
> 28.119.19.0/24
>
> Whereas, second one is pointing to match the first 22 bits and the subnet
> mask can be le 24. That means, you the prefixes can have a subnet mask of
> either 23 or 24. In this case the matches are:
> 28.119.16.0/23
> 28.119.16.0/24
> 28.119.17.0/23
> 28.119.17.0/24
> 28.119.18.0/23
> 28.119.18.0/24
> 28.119.19.0/23
> 28.119.19.0/24
>
> HTH,
> Shine
>
>
>
> -----Original Message-----
> From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
> Athaide, Dwayne
> Sent: Sunday, 6 April 2008 2:16 AM
> To: groupstudy
> Subject: Prefix list question
>
> If I want to match the following Subnets in a prefix list
>
> 28.119.16.0/24
>
> 28.119.17.0/24
>
>
>
> My answer
>
> ip prefix-list FILTER seq 10 permit 28.119.16.0/23 ge 24 le 24
>
>
>
> SG
>
> ip prefix-list FILTER seq 10 permit 28.119.16.0/23 le 24
>
>
>
> Please comment
>
> thanks
>
> _______________________________________________________________________
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>
> _______________________________________________________________________
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>
-- Carlos G Mendioroz <tron@huapi.ba.ar> LW7 EQI Argentina
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