Inter-area routing

Understand

Within an area, Type 1 and Type 2 LSAs describe topology. Across an area boundary, the ABR originates Type 3 network summaries from eligible reachability. A Type 3 may describe a single prefix; “summary” does not imply aggregation. The receiving router adds its path cost to the advertising ABR.

For the same prefix, an intra-area path is preferred over an inter-area path before cost comparison. This is distinct from longest-prefix forwarding and from competition between routing protocols. A lower-cost inter-area route does not override a valid intra-area route to the identical prefix. ABRs use backbone reachability for the inter-area hierarchy; adding arbitrary non-backbone shortcuts does not replace that design.

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, R4: area 2.

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
interface GigabitEthernet2
 ip address 192.0.2.9 255.255.255.252
 ip ospf network point-to-point
 ip ospf cost 10
 ip ospf 1 area 1
 no shutdown
 exit
end

configure terminal
hostname 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 2
 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 192.0.2.10 255.255.255.252
 ip ospf network point-to-point
 ip ospf cost 10
 ip ospf 1 area 1
 no shutdown
 exit
end

configure terminal
hostname R4
router ospf 1
 router-id 10.255.0.4
 passive-interface Loopback0
 exit
interface Loopback0
 ip address 10.255.0.4 255.255.255.255
 ip ospf 1 area 2
 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 2
 no shutdown
 exit
end

Apply

Use the baseline, with no summarization or redistribution. Trace R4’s loopback from area 2 to R2, across area 0 to R1, and into area 1 at R3. Compare advertising router and metric at each stage.

Verify and interpret

R1# show ip route ospf | include 10.255.0.4
O IA 10.255.0.4/32 [110/21] via 192.0.2.2, GigabitEthernet1
R1# show ip ospf database summary 10.255.0.4
  LS Type: Summary Links(Network)
  Advertising Router: 10.255.0.2
  Network Mask: /32
  Metric: 11
R3# show ip ospf database summary 10.255.0.4
  LS Type: Summary Links(Network)
  Advertising Router: 10.255.0.1
  Network Mask: /32
  Metric: 21

R2 advertises cost 11 into area 0. R1 calculates 21, then originates its own area-1 summary with that metric.

R3# show ip route ospf | include 10.255.0.4
O IA 10.255.0.4/32 [110/31] via 192.0.2.9, GigabitEthernet1
R3# ping 10.255.0.4 source Loopback0
!!!!!

R3 adds its cost 10. The ping also depends on area-1 reachability being advertised in the reverse direction.

If the LSA exists but the route is missing, check reachability to the advertising ABR, route-type competition and local route filters. If the LSA itself is absent, work backward across the ABRs and the source interface.

Technical references

OSPFv2 standard · Router configuration guide