Redistribution

Understand

Redistribution imports reachability from another routing source. The importing router becomes an ASBR and originates external LSAs in a normal area. Native OSPF interface activation is preferable for internal connected networks; do not redistribute everything connected just to enable OSPF.

Define the ownership boundary first: permitted prefixes, source protocols, metric type, seed metric and loop-prevention policy. Use an allowlist with implicit deny and retain subnets in this command syntax. Multiple mutual redistribution points can feed routes back into their source domain. Tags help only when an explicit policy matches and rejects them.

A source route must be eligible in the local routing table. Redistributing a static Null0 route advertises reachability while discarding packets at the ASBR; it is a deliberate control-plane test here, not a real service exit.

Lab setup

Use a fresh lab for this lesson; changes from other lessons are not cumulative. All outputs are illustrative excerpts, not recorded router tests. Transit cost is explicitly 10 and Loopback0 cost is 1. Interface names may vary by image.

Loopback0 is 10.255.0.<router number>/32; area assignments: R1: area 0, R2: area 0, R3: area 1.

configure terminal
hostname R1
router ospf 1
 router-id 10.255.0.1
 passive-interface Loopback0
 exit
interface Loopback0
 ip address 10.255.0.1 255.255.255.255
 ip ospf 1 area 0
 ip ospf cost 1
 exit
interface GigabitEthernet1
 ip address 192.0.2.1 255.255.255.252
 ip ospf network point-to-point
 ip ospf cost 10
 ip ospf 1 area 0
 no shutdown
 exit
end

configure terminal
hostname R2
router ospf 1
 router-id 10.255.0.2
 passive-interface Loopback0
 exit
interface Loopback0
 ip address 10.255.0.2 255.255.255.255
 ip ospf 1 area 0
 ip ospf cost 1
 exit
interface GigabitEthernet1
 ip address 192.0.2.2 255.255.255.252
 ip ospf network point-to-point
 ip ospf cost 10
 ip ospf 1 area 0
 no shutdown
 exit
interface GigabitEthernet2
 ip address 192.0.2.5 255.255.255.252
 ip ospf network point-to-point
 ip ospf cost 10
 ip ospf 1 area 1
 no shutdown
 exit
end

configure terminal
hostname R3
router ospf 1
 router-id 10.255.0.3
 passive-interface Loopback0
 exit
interface Loopback0
 ip address 10.255.0.3 255.255.255.255
 ip ospf 1 area 1
 ip ospf cost 1
 exit
interface GigabitEthernet1
 ip address 192.0.2.6 255.255.255.252
 ip ospf network point-to-point
 ip ospf cost 10
 ip ospf 1 area 1
 no shutdown
 exit
end

Apply

Use the border baseline with all areas normal. R1 owns the import. Add two discard statics but allow only 198.51.100.0/24; 203.0.113.0/24 is the negative test.

R1

configure terminal
ip route 198.51.100.0 255.255.255.0 Null0
ip route 203.0.113.0 255.255.255.0 Null0
ip prefix-list OSPF-IMPORT seq 5 permit 198.51.100.0/24
route-map STATIC-IN permit 10
 match ip address prefix-list OSPF-IMPORT
 set tag 100
 exit
router ospf 1
 redistribute static subnets metric 20 metric-type 1 route-map STATIC-IN
end

Verify and interpret

R3# show ip route ospf | include 198.51.
O E1 198.51.100.0/24 [110/40] via 192.0.2.5, GigabitEthernet1
R3# show ip route 203.0.113.0
% Network not in table
R3# show ip ospf database external 198.51.100.0
  Advertising Router: 10.255.0.1
  Network Mask: /24
  Metric Type: 1
  Metric: 20
  Forward Address: 0.0.0.0
  External Route Tag: 100

The permitted E1 metric is 20 internal + 20 external = 40. The second static is deliberately excluded. With forwarding address zero, the path resolves toward the ASBR.

Remove the permitted source static, then confirm the LSA and route withdraw after convergence. Restore it and verify the permitted route returns. Do not widen the allowlist to fix an unrelated adjacency failure.

R1 · withdraw then restore in separate steps

configure terminal
no ip route 198.51.100.0 255.255.255.0 Null0
end

R1 · restore

configure terminal
ip route 198.51.100.0 255.255.255.0 Null0
end

Technical references

OSPFv2 standard · Router configuration guide · Redistribution guidance