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Beginner

Quick IP Prefix List Question

Hello,

I would like to filter BGP routes using a route map and ip prefix list but am curious what is the best methid to accomplish the following:

I want the prefix list to filter out these networks:

172.39.8.0/24

172.39.8.0/25

172.39.8.0/26

172.39.8.0/27

172.39.8.0/28

172.39.8.0/29

172.39.8.0/30

Which option makes the most sense: (if either)

seq 50 permit 172.39.8.0/24

seq 51 permit 172.39.8.0/24 ge 25

or

seq 50 permit 172.39.8.0/23 ge 24

Thanks,

Bill

2 ACCEPTED SOLUTIONS

Accepted Solutions
Highlighted
Hall of Fame Cisco Employee

Hi Bill,

The most simple way of doing this is by having the prefix-list in the form:

seq 50 permit 172.39.8.0/24 le 30

This says that the network which will be matched must be either the 172.39.8.0/24 or any subnet thereof, and the mask at the same time must be between /24 and /30 inclusive.

Does this meet your needs?

Best regards,

Peter

View solution in original post

Highlighted
Hall of Fame Master

Hello Bill,

first option would be preferred as the second one would match also for any 192.168.9.0/x x >= 24 so it less  specific.

Personally I would use

seq 50 permit 192.168.8.0/24 le 32

to be sure that any prefix within 192.168.8./24 included the base block 192.168.8.0/24 is a match

Hope to help

Giuseppe

View solution in original post

6 REPLIES 6
Highlighted
Hall of Fame Cisco Employee

Hi Bill,

The most simple way of doing this is by having the prefix-list in the form:

seq 50 permit 172.39.8.0/24 le 30

This says that the network which will be matched must be either the 172.39.8.0/24 or any subnet thereof, and the mask at the same time must be between /24 and /30 inclusive.

Does this meet your needs?

Best regards,

Peter

View solution in original post

Highlighted
Hall of Fame Master

Hello Bill,

first option would be preferred as the second one would match also for any 192.168.9.0/x x >= 24 so it less  specific.

Personally I would use

seq 50 permit 192.168.8.0/24 le 32

to be sure that any prefix within 192.168.8./24 included the base block 192.168.8.0/24 is a match

Hope to help

Giuseppe

View solution in original post

Highlighted
Beginner

Thanks guys - didnt even occur to me to use le vice ge for this...guess I need to increase my caffeine intake today

Highlighted
Hall of Fame Cisco Employee

Bill,

A good sleep is better than any amount of coffee Having a good deserved rest is a must. Anyway, we're glad to have helped!

Best regards,

Peter

Highlighted

Lol - I work in the financial sector - sleep is not permitted

Thanks again for the help!

Bill

Highlighted
Hall of Fame Cisco Employee

Bill,

Lol - I work in the financial sector - sleep is not permitted 

I am a teacher - how well I know what you're talking about Take care!

Best regards,

Peter