prefix-list question

[lgontarsk]

Hi, I'm having trouble understanding the documenation on prefix lists -

examples like this don't make sense to me since you'll never see a network addr like 172.20.10.171/16... why wouldn't this be 172.20.0.0/16 since 172.20.0.0 is the only network addr with the /16 mask?

ipv4 prefix-list list1

10 permit 172.20.10.171/16 le 24

If anyone can point me to a good book or article or white-paper on this I'd really appreciate it.

Thanks, Lisa G

[Harold Ritter · Accepted solution]

Lisa,

This doesn't make sense indeed. If you enter something like 172.20.10.171/16, IOS automatically converts it to 172.20.0.0/16.

Can you please give me the URL, where this combination is used. I will bring it up with out documentation team.

Hope this helps,

[lgontarsk]

Hi,

Here's the url -

http://cisco.com/en/US/docs/ios_xr_sw/iosxr_r2.0/addr_serv/command/reference/3irpref.html

Thx!! Lisa

PS I haven't found any good documentation on prefix-lists (even though I'm pretty sure I understand it now). Do you know of any white papers or url's?

Enke Chen once wrote a little how-to sheet when he created prefix lists which was passed around Cisco but that never made it on the Cisco web site.

LisaG

[Harold Ritter]

Lisa,

Thanks for the URL. I will make sure our documentation folks fix it. I found the paper that Enke had produced back then and I'm attaching it.

Hope this helps,

[lgontarsk]

Hi,

I have one more question about the prefix lists....

if you say permit 10.0.0.0/8 ge /24 le /32

that would permit

10.0.0.0/8 with masks from 255.255.255.0 through 255.255.255.255?

Someone in my group thinks that le /24 means /25 though /32 but I think this is the opposite.

Thanks!!

LisaG

[Harold Ritter · Accepted solution]

Lisa,

You are absolutely correct. Any prefix in 10/8 with a prefix length between 24 and 32 inclusively would match that statement.

Hope this helps,