OSPF Distribute-List Filtering

OSPF supports a number of methods to filter routes but it is more restrictive compared to distance vector routing protocols like RIP or EIGRP.

As a link-state routing protocol OSPF uses LSAs to build its LSDB (Link State Database). Routers will run the SPF algorithm to find the shortest path to each destination, the topology in the LSDB has to be the same on all routers or SPF will fail.

However OSPF routers only know what the topology looks like within the area. They don’t know what the topology looks like for other areas. For inter-area routes OSPF only knows the prefix and the ABR (Area Border Router) to reach it.

You could say that OSPF acts like a distance vector routing protocol for inter-area routes. It only knows the metric (distance) and the ABR to get there (vector).

Unlike RIP or EIGRP, OSPF doesn’t advertise routes but LSAs so if we want to filter something we’ll have to filter the advertisement of LSAs.

Since the LSDB within the area has to be the same we can’t filter LSAs within the area, we can however filter routes from entering the routing table. Filtering LSAs between areas on an ABR or ASBR is no problem.

In this lesson I’ll show you how we can filter routes from entering the routing table within the area. In other lessons I will explain how to filter type 3 LSAs and type 5 LSAs.

Here’s the topology I will use:

Nothing fancy, we have three routers running OSPF in the same area. R1 has a loopback interface that is advertised in OSPF, we’ll see if we can filter this network.

Configuration

Here’s the OSPF configuration:

R1#show running-config | section ospf
router ospf 1
 network 1.1.1.0 0.0.0.255 area 0
 network 192.168.12.0 0.0.0.255 area 0

R2#show running-config | section ospf
router ospf 1
 network 192.168.12.0 0.0.0.255 area 0
 network 192.168.23.0 0.0.0.255 area 0

R3#show running-config | section ospf
router ospf 1
 network 192.168.23.0 0.0.0.255 area 0

Let’s verify if R2 and R3 have learned 1.1.1.1 /32:

R2#show ip route ospf

      1.0.0.0/32 is subnetted, 1 subnets
O        1.1.1.1 [110/2] via 192.168.12.1, 00:00:27, FastEthernet0/0

R3#show ip route ospf

      1.0.0.0/32 is subnetted, 1 subnets
O        1.1.1.1 [110/3] via 192.168.23.2, 00:00:28, FastEthernet0/0
O     192.168.12.0/24 [110/2] via 192.168.23.2, 00:00:28, FastEthernet0/0

Let’s see if we can get rid of this network on R3:

R3(config)#router ospf 1
R3(config-router)#distribute-list ?
  <1-199>      IP access list number
  <1300-2699>  IP expanded access list number
  WORD         Access-list name
  gateway      Filtering incoming updates based on gateway
  prefix       Filter prefixes in routing updates
  route-map    Filter prefixes based on the route-map

We can use a distribute-list for this, to keep it simple I’ll combine it with an access-list;

R3(config-router)#distribute-list R1_L0 in

When we want to remove something from the routing table we have to apply it inbound. The outbound distribute-list is used for LSA type 5 filtering.

Let’s create that access-list:

R3(config)#ip access-list standard R1_L0
R3(config-std-nacl)#deny host 1.1.1.1    
R3(config-std-nacl)#permit any

It will now be gone from the routing table:

R3#show ip route 1.1.1.1
% Network not in table

As you can see it’s gone…it’s still in the LSDB though:

R3#show ip ospf database router 192.168.12.1

            OSPF Router with ID (192.168.23.3) (Process ID 1)

		Router Link States (Area 0)

  LS age: 664
  Options: (No TOS-capability, DC)
  LS Type: Router Links
  Link State ID: 192.168.12.1
  Advertising Router: 192.168.12.1
  LS Seq Number: 80000003
  Checksum: 0xF14F
  Length: 48
  Number of Links: 2

    Link connected to: a Stub Network
     (Link ID) Network/subnet number: 1.1.1.1
     (Link Data) Network Mask: 255.255.255.255
      Number of MTID metrics: 0
       TOS 0 Metrics: 1

    Link connected to: a Transit Network
     (Link ID) Designated Router address: 192.168.12.2
     (Link Data) Router Interface address: 192.168.12.1
      Number of MTID metrics: 0
       TOS 0 Metrics: 1

You have to be very careful if you use this command. If you are not careful you can end up in a scenario where you blackhole some traffic. For example, let’s see what happens when I filter this network on R2 instead of R3.