From: Silvio Nunes (silvio_98@hotmail.com)
Date: Sun Oct 26 2003 - 21:58:53 GMT-3
Thank4s a lot Brian !!!!
:-)
>From: "Brian McGahan" <bmcgahan@internetworkexpert.com>
>Reply-To: "Brian McGahan" <bmcgahan@internetworkexpert.com>
>To: "'Silvio Nunes'" <silvio_98@hotmail.com>, <ccielab@groupstudy.com>
>Subject: RE: Help - Prefix-list Date: Sun, 26 Oct 2003 17:07:35 -0600
>
>Silvio,
>
> Prefix-lists are used to match on prefix and prefix-length
>pairs. Normal prefix-list syntax is as follows:
>
>Ip prefix-list LIST permit w.x.y.z/len
>
>Where w.x.y.z is your exact prefix
>And where len is your exact prefix-length
>
>"Ip prefix-list LIST permit 1.2.3.0/24" would be an exact match for the
>prefix 1.2.3.0 with a subnet mask of 255.255.255.0. This does not match
>1.2.0.0/24, nor does it match 1.2.3.4/32, nor anything in between.
>
>When you add the keywords "GE" and "LE" to the prefix-list, the "len"
>value changes its meaning. When using GE and LE, the len value
>specifies how many bits of the prefix you are checking, starting with
>the most significant bit.
>
>Ip prefix-list LIST permit 1.2.3.0/24 le 32
>
>This means:
>Check the first 24 bits of the prefix 1.2.3.0
>The subnet mask must be less than or equal to 32
>
>This equates to the access-list syntax:
>Access-list 1 permit host 1.2.3.0
>
>
>Ip prefix-list LIST permit 0.0.0.0/0 le 32
>
>This means:
>Check the first 0 bits of the prefix 0.0.0.0
>The subnet mask must be less than or equal to 32
>This equates to anything
>
>
>Ip prefix-list LIST permit 0.0.0.0/0
>
>This means:
>The exact prefix 0.0.0.0, with the exact prefix-length 0.
>This is matching a default route.
>
>
>Ip prefix-list LIST permit 10.0.0.0/8 ge 21 le 29
>
>This means:
>Check the first 8 bits of the prefix 10.0.0.0
>The subnet mask must be greater than or equal to 21, and less than or
>equal to 29.
>
>
>Ip prefix-list CLASS_A permit 0.0.0.0/1 ge 8 le 8
>
>This matches all class A addresses with classful masks. It means:
>Check the first bit of the prefix, it must be a 0.
>The subnet mask must be greater than or equal to 8, and less than or
>equal to 8. (It is exactly 8)
>
>
>When using the GE and LE values, you must satisfy the condition:
>
>Len < GE <= LE
>
>Therefore:
>Ip prefix-list LIST permit 1.2.3.0/24 ge 8
>Is not a valid list.
>
>
>What you can not do with the prefix-list is match on arbitrary bits like
>you can in an access-list. Prefix-lists cannot be used to check if a
>number is even or odd, nor check if a number is divisible by 15, etc...
>Bit checking in a prefix-list is sequential, starting with the most
>significant (leftmost) bit.
>
>
>HTH,
>
>Brian McGahan, CCIE #8593
>bmcgahan@internetworkexpert.com
>
>Internetwork Expert, Inc.
>http://www.InternetworkExpert.com
>Toll Free: 877-224-8987
>Direct: 708-362-1418 (Outside the US and Canada)
>
>
> > -----Original Message-----
> > From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf
>Of
> > Silvio Nunes
> > Sent: Sunday, October 26, 2003 4:51 PM
> > To: ccielab@groupstudy.com
> > Subject: Help - Prefix-list
> >
> > Hi all,
> >
> > I4m having some dificcults to found any kind of material regarding
> > prefix-list to undertand how it works.
> > I still didn4t understand very well the function and when to use
>ge/le.
> >
> > Do you know some interesting material or tip ?
> >
> > Regards,
> >
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