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prefix-list question

lgontarsk
Level 1
Level 1

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

2 Accepted Solutions

Accepted Solutions

Harold Ritter
Cisco Employee
Cisco Employee

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,

Harold Ritter
Sr Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
México móvil: +52 1 55 8312 4915
Cisco México
Paseo de la Reforma 222
Piso 19
Cuauhtémoc, Juárez
Ciudad de México, 06600
México

View solution in original post

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,

Harold Ritter
Sr Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
México móvil: +52 1 55 8312 4915
Cisco México
Paseo de la Reforma 222
Piso 19
Cuauhtémoc, Juárez
Ciudad de México, 06600
México

View solution in original post

5 Replies 5

Harold Ritter
Cisco Employee
Cisco Employee

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,

Harold Ritter
Sr Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
México móvil: +52 1 55 8312 4915
Cisco México
Paseo de la Reforma 222
Piso 19
Cuauhtémoc, Juárez
Ciudad de México, 06600
México

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

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,

Harold Ritter
Sr Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
México móvil: +52 1 55 8312 4915
Cisco México
Paseo de la Reforma 222
Piso 19
Cuauhtémoc, Juárez
Ciudad de México, 06600
México

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

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,

Harold Ritter
Sr Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
México móvil: +52 1 55 8312 4915
Cisco México
Paseo de la Reforma 222
Piso 19
Cuauhtémoc, Juárez
Ciudad de México, 06600
México
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