OSPF filtering at ABR with filter-list prefix

[Barry Landon]

Topology

R1 –  [R4] = Area 0 : process ID 1
[R4] – R6 – R7 = Area 1 : process ID 2

I have a loopback interface configured on R6 and R7 which is distributed by ABR and pingable from R1.  After adding the filter list and prefix-list its still a routable path to 192.168.177.0/24 from R1. Each area has its own process ID.

Config

R6#
interface Loopback6
 ip address 192.168.166.1 255.255.255.0
 ip ospf authentication
 ip ospf authentication-key area1
 ip ospf network point-to-point
 ip ospf 2 area 1
end

R7#
interface Loopback7
 ip address 192.168.177.1 255.255.255.0
 ip ospf authentication
 ip ospf authentication-key area1
 ip ospf network point-to-point
 ip ospf 2 area 1
end

I want to filter loopback 7 from creating a LSA entering Area 0 but permit loopback 6

R4#

router ospf 1
 router-id 4.4.4.10
 redistribute ospf 2 subnets

router ospf 2
 router-id 4.4.4.21
 area 1 filter-list prefix FILTER_INTO_AREA_0 in
 redistribute ospf 1 subnets

ip prefix-list FILTER_INTO_AREA_0 seq 5 deny 192.168.177.0/24
ip prefix-list FILTER_INTO_AREA_0 seq 10 permit 0.0.0.0/0 le 32

Can somebody help me understand how I can get this to work.

[willwetherman]

Hi,

OSPF filter lists can only be applied to an ABR and used to filter type-3 LSAs between areas.

In your configuration, as you are running two separate OSPF processes on R4 and redistributing between them, R4 will not consider itself as an ABR and the filter list will not work. As R4 is redistributing OSPF process 2 into OSPF process 1, R4 is acting as an ASBR and not an ABR and will therefore be generating a type-5 LSA for 192.168.177.0/24 in OSPF process 1.

To fix this issue you will either need to configure area 0 and area 1 under the same OSPF process on R4 and use the filter list to filter the type-3 LSA, or configure a distribute list in the outbound direction on R4 under OSPF process 1. For example

router ospf 1
 router-id 4.4.4.10
 redistribute ospf 2 subnets
 distribute-list prefix FILTER_INTO_AREA_0  out

router ospf 2
 router-id 4.4.4.21
 redistribute ospf 1 subnets

ip prefix-list FILTER_INTO_AREA_0 seq 5 deny 192.168.177.0/24
ip prefix-list FILTER_INTO_AREA_0 seq 10 permit 0.0.0.0/0 le 32

[seungyoul-lee]

excellent explanations!

[paul driver]

Hello

As you performing mutual redistribution on R4 then You need to filter on the redistribution point.

R4
ip prefix-list stan_pf seq 5 permit x.x.x.x/y

route-map STAN deny 10
match ip address prefix-list stan_pf
route-map STAN permit 99

router ospf 1
redistribute ospf 2 subnets route-map STAN

Or on the router originating the subnet to suppress it from being advertised

R7
int loopback 7
ip ospf prefix-suppression

res
Paul