OSPF IP Prefix-List

R4 has Loopback 0 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
ip prefix-list INTO-AREA3 seq 10 permit le 32
ip prefix-list INTO-AREA3 seq 15 deny
ip prefix-list INTO-AREA3 seq 20 deny

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:

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

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

ip prefix-list INTO-AREA3 seq 15 deny
ip prefix-list INTO-AREA3 seq 20 deny
will never be checked since all routes will match sequence 10 permit le 32
jskfanAuthor Commented:
if I understand after : permit le 32
there is no other Prefix-list that will be looked at regardless of the sequence number ..Correct ?
Yes. All routes are checked until first match is found (and action permit or deny is applied from matching statement). le 32 will always be match (networks and are subsets of le32). That's why there is a rule - more specific statements should be configured before less specific.

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jskfanAuthor Commented:
Thank you
You're welcome.
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