02-17-2009 09:52 PM - edited 03-04-2019 03:37 AM
Hi Guys,
Can someone point me a good tutorial for Prefix-list? After reading couple of doc it's not very clear!
I have a prefix-list as follows:
ip prefix-list routes_in seq 10 permit 172.24.0.0/16 ge 22
ip prefix-list routes_in seq 15 permit 192.168.0.0/16 ge 24
ip prefix-list routes_in seq 20 permit 192.2.0.0/16 ge 24
ip prefix-list routes_in seq 25 permit 192.3.0.0/16 ge 24
ip prefix-list routes_in seq 30 permit 10.80.0.0/16 ge 24
ip prefix-list routes_in seq 35 permit 10.90.0.0/16 ge 24
ip prefix-list routes_in seq 40 permit 10.100.0.0/16 ge 24
ip prefix-list routes_in seq 45 permit 10.120.0.0/16 ge 24
ip prefix-list routes_in seq 50 permit 10.222.0.0/16 ge 24
ip prefix-list routes_in seq 55 permit 10.223.0.0/16 ge 24
ip prefix-list routes_in seq 60 permit 10.227.0.0/16 ge 24
ip prefix-list routes_in seq 65 permit 10.228.0.0/16 ge 24
ip prefix-list routes_in seq 70 permit 10.229.0.0/16 ge 24
!
ip prefix-list routes_out seq 75 permit 172.20.0.0/16 ge 22
ip prefix-list routes_out seq 80 permit 176.20.0.0/16 ge 22
ip prefix-list routes_out seq 85 permit 192.168.0.0/16 ge 24
ip prefix-list routes_out seq 90 permit 172.24.23.0/24
I need to block 192.168.40.0/24 in the outbound, how do I do this? as 192.168.0.0/16 ge 24 already in place??
Thanks.
Solved! Go to Solution.
02-17-2009 10:26 PM
Hi Shawkat,
You need to insert into the prefix-list the following line:
ip prefix-list routes_out seq 83 deny 192.168.40.0/24
So the whole route_out prefix-list will look like this:
ip prefix-list routes_out seq 75 permit 172.20.0.0/16 ge 22
ip prefix-list routes_out seq 80 permit 176.20.0.0/16 ge 22
ip prefix-list routes_out seq 83 deny 192.168.40.0/24
ip prefix-list routes_out seq 85 permit 192.168.0.0/16 ge 24
ip prefix-list routes_out seq 90 permit 172.24.23.0/24
Prefix-lists are evaluated in the order of the given sequence numbers.
ip prefix-list routes_out seq 83 deny 192.168.40.0/24 states a more specific prefix than ip prefix-list routes_out seq 85 permit 192.168.0.0/16 ge 24.
Therefore line 83 must have a lower sequence number to block 192.168.40.0/24, before line 85 permits 192.168.0.0/16 ge 24 (which embraces 192.168.40.0/24 as well).
For more info, see this thread on my explanation on how prefix-lists work.
Cheers:
Istvan
02-17-2009 10:26 PM
Hi Shawkat,
You need to insert into the prefix-list the following line:
ip prefix-list routes_out seq 83 deny 192.168.40.0/24
So the whole route_out prefix-list will look like this:
ip prefix-list routes_out seq 75 permit 172.20.0.0/16 ge 22
ip prefix-list routes_out seq 80 permit 176.20.0.0/16 ge 22
ip prefix-list routes_out seq 83 deny 192.168.40.0/24
ip prefix-list routes_out seq 85 permit 192.168.0.0/16 ge 24
ip prefix-list routes_out seq 90 permit 172.24.23.0/24
Prefix-lists are evaluated in the order of the given sequence numbers.
ip prefix-list routes_out seq 83 deny 192.168.40.0/24 states a more specific prefix than ip prefix-list routes_out seq 85 permit 192.168.0.0/16 ge 24.
Therefore line 83 must have a lower sequence number to block 192.168.40.0/24, before line 85 permits 192.168.0.0/16 ge 24 (which embraces 192.168.40.0/24 as well).
For more info, see this thread on my explanation on how prefix-lists work.
Cheers:
Istvan
02-18-2009 01:43 PM
Thanks a lot Istvan. Very good explanation. This one worked perfect.
cheers.
02-18-2009 10:01 PM
You're welcome Shawkat,
It's good to hear that I was helpful.
Istvan
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