Question about prefix list
matt at mattreath.com
Sat Feb 4 01:40:55 CST 2012
> 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
> --- 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
>> 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
>> Thank you so much
Here is how I look at prefix lists
Lets say I have the following:
ip prefix-list EXAMPLE permit 126.96.36.199/22 le 24
What this essentially means is match any prefixes that match the first 22
bits of 188.8.131.52 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:
Hope that makes sense.
CCIE #27316 (SP)
matt at mattreath.com | http://mattreath.com
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