mailing list archives
Re: Question about prefix list
From: "Matthew Reath" <matt () mattreath com>
Date: Sat, 4 Feb 2012 01:40:55 -0600
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 () gmail com> wrote:
From: Ann Kwok <annkwok80 () gmail com>
Subject: Question about prefix list
To: nanog () 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
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 22.214.171.124/22 le 24
What this essentially means is match any prefixes that match the first 22
bits of 126.96.36.199 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 () mattreath com | http://mattreath.com