From: Marco P. Rodrigues (marco@rodrigues.ca)
Date: Sun Oct 26 2003 - 23:58:29 GMT-3
.:Thank you so much for such clear explanation. I have never seen such good
.:interpretation before in terms of ip prefix-list . Thanks.
.:
.:I just don't understand the digit 1 after 0.0.0.0/ in your example below.
.:Could you please explain it a bit more? Thanks indeed,
It's currently only checking the first bit of the prefix being
announced. The first bit must be zero in order to be a Class A
classful prefix. If you remember: 0 - Class A , 10 - Class B
110 - Class C. It's just checking the LSB to make sure it's zero
(suggesting it is a class A network). The GE and LE 8 make sure it's
not a CIDR block falling within the Class A. Just full Class A
addresses.
.:
.:========================================
.: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)
.:=========================================
.:
.:Cheers,
.:
.:++++
.:MAC
.:++++
.:
.:----- Original Message -----
.:From: "Brian McGahan" <bmcgahan@internetworkexpert.com>
.:To: "'Silvio Nunes'" <silvio_98@hotmail.com>; <ccielab@groupstudy.com>
.:Sent: Monday, October 27, 2003 10:07 AM
.:Subject: RE: Help - Prefix-list
.:
.:
.:> 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,
.:> >
.:> > _________________________________________________________________
.:> > MSN Messenger: converse com os seus amigos online.
.:> > http://messenger.msn.com.br
.:> >
.:> >
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