OSPF IP Prefix-List

jskfan
jskfan used Ask the Experts™
on


R4 has Loopback 0 4.4.4.4 in area 0
other physical interfaces as shown in the topology they are in area 1,2,3


I have configured on R4:
R4#sh run | beg ip prefix
ip prefix-list INTO-AREA3 seq 5 deny 2.2.2.2/32
ip prefix-list INTO-AREA3 seq 10 permit 0.0.0.0/0 le 32
ip prefix-list INTO-AREA3 seq 15 deny 192.168.14.0/24
ip prefix-list INTO-AREA3 seq 20 deny 192.168.24.0/24

router ospf 1
 area 3 filter-list prefix INTO-AREA3 in

when I go to R3 I see the routes below still there when they should be filtered out:
192.168.14.0/24
192.168.24.0/24

I have changed the sequence number of this command to sequence 25:
ip prefix-list INTO-AREA3 seq 25 permit 0.0.0.0/0 le 32

and now I see the filtering worked. When I go to R3 which is in area 3. I do not see the routes:

192.168.14.0/24
192.168.24.0/24

Any Expert to explain the logic that the route has used in regard to the  ip prefix-list sequences. ?

Thank you
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JustInCase
Distinguished Expert 2018

Commented:
ip prefix-list INTO-AREA3 seq 10 permit 0.0.0.0/0 le 32
This one is equivalent of permit ip any any - after deny 2.2.2.2/32 all routes are permitted. Prefix-list is ordered just as any other ACL.

Statements
ip prefix-list INTO-AREA3 seq 15 deny 192.168.14.0/24
ip prefix-list INTO-AREA3 seq 20 deny 192.168.24.0/24
will never be checked since all routes will match sequence 10 permit 0.0.0.0/0 le 32
jskfan

Author

Commented:
if I understand after : permit 0.0.0.0/0 le 32
there is no other Prefix-list that will be looked at regardless of the sequence number ..Correct ?
Distinguished Expert 2018
Commented:
Yes. All routes are checked until first match is found (and action permit or deny is applied from matching statement). 0.0.0.0/0 le 32 will always be match (networks 192.168.14.0/24 and 192.168.24.0/24 are subsets of 0.0.0.0/0 le32). That's why there is a rule - more specific statements should be configured before less specific.
jskfan

Author

Commented:
Thank you
JustInCase
Distinguished Expert 2018

Commented:
You're welcome.

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