> 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 <[email protected]> wrote: > >> From: Ann Kwok <[email protected]> >> Subject: Question about prefix list >> To: [email protected] >> 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) [email protected] | http://mattreath.com Twitter: http://twitter.com/mpreath
