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
Hi Ann, The le parameter can be included to match all more-specific prefixes within a par ten prefix up to a specified length. FE: 202.168.136.0/22 le 25 will match 202.168.136.0/22 and all prefixes contained therein with a length of 24 or less. They appear to be blocking everything with a length longer dan /24 (so /25 /26 etc etc.) the last line doesn't have this because it's only 1 /24 subnet. Regards, Wouter On Feb 1, 2012, at 15:32 , Ann Kwok wrote:
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
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 ^
randy
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@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 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
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@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 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@mattreath.com | http://mattreath.com Twitter: http://twitter.com/mpreath
participants (5)
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Ann Kwok -
Matthew Reath -
Randy -
Randy Bush -
Wouter van der Vaart