Question about prefix list

Matthew Reath matt at mattreath.com
Sat Feb 4 01:40:55 CST 2012


> Ann,
> the commas not withstanding, the le/ge operands as applicable to
> prefix-lists simply mean "less-than or equal-to" or greater-than or
> "equal-to" wrt netmasks in CIDR speak.
>
> In you prefix-list below, the le operand means -
> allow following ranges:
>
> /22,/23,/24 deny all else
> for the /21
> it means allow /21 thru /24
>
> Anything without an operand means an exact-match(permit/deny)
>
> Homework for you:
>
> What do the following do:
>
> 1) ip prefix-list foo deny 0.0.0.0/0 le32
> 2) ip prefix-list foo permit 0.0.0/0 le 32
>
> Understand the above and you will understand how operands work in
> prefix-lists.
> ./Randy
>
>
> --- On Wed, 2/1/12, Ann Kwok <annkwok80 at gmail.com> wrote:
>
>> From: Ann Kwok <annkwok80 at gmail.com>
>> Subject: Question about prefix list
>> To: nanog at nanog.org
>> Date: Wednesday, February 1, 2012, 6:32 AM
>> Hi
>>
>> I read this prefix list.
>>
>> Can I know why there is "le 24" after network block in /22
>> and /21
>>
>> Why don't have "le 24" after /24?
>>
>> I also saw another prefix list before. They use "le 32"
>> instead of  "le 24"
>>
>> What are their different?
>>
>> ip prefix-list prefix-filter-as100 seq 10 permit
>> 202,168.136.0/22 le 24
>> ip prefix-list prefix-filter-as100 seq 20 permit
>> 202,22.92.0/22 le 24
>> ip prefix-list prefix-filter-as100 seq 30 permit
>> 202,21.148.0/22 le 24
>> ip prefix-list prefix-filter-as100 seq 40 permit
>> 203,178.88.0/21 le 24
>> ip prefix-list prefix-filter-as100 seq 50 permit
>> 178.88.74.0/24
>>
>> Thank you so much
>>
>
>

Here is how I look at prefix lists 


Lets say I have the following:
ip prefix-list EXAMPLE permit 202.21.148.0/22 le 24

What this essentially means is match any prefixes that match the first 22
bits of 202.21.148.0 with a prefix length less than or equal to /24.

The third octet (148) is 10010100 in binary, the /22 would be at
100101|00. So we would match anything that has the same bits set before
the divider or the /22 mark.

Matching prefixes would be:
202.21.148.0/22
202.21.148.0/23
202.21.150.0/23
202.21.148.0/24
202.21.149.0/24
202.21.150.0/24
202.21.151.0/24

Hope that makes sense.

--
Matt Reath
CCIE #27316 (SP)
matt at mattreath.com | http://mattreath.com
Twitter: http://twitter.com/mpreath




More information about the NANOG mailing list