Backbone and ABRs

Understand

R1 and R2 connect through area 0. R1 also attaches area 1/R3; R2 attaches area 2/R4. R1 and R2 are ABRs, maintaining separate area databases. Interfaces belong to areas; a router can participate in several. R3 and R4 are internal routers in their own areas.

Inter-area routing uses a backbone-centered hierarchy. Non-backbone areas connect to area 0 through ABRs; keep area 0 contiguous. An area number on an isolated interface is not a substitute for actual backbone connectivity. Virtual links can logically extend the backbone through a transit area, but have constraints and are a repair/design exception rather than the default solution. Stub areas cannot be virtual-link transit areas.

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

Apply the complete multi-area baseline. First verify R1–R2 in area 0, then the R1–R3 and R2–R4 links. Do not merge all interfaces into area 0 merely because a remote route is absent. Area IDs must match across each link, not across every interface on the router.

Verify and interpret

R1# show ip ospf
 Routing Process "ospf 1" with ID 10.255.0.1
 It is an area border router
 Number of areas in this router is 2
R1# show ip ospf interface brief
Interface PID Area IP Address/Mask Cost State Nbrs F/C
Gi1       1   0    192.0.2.1/30    10   P2P   1/1
Gi2       1   1    192.0.2.9/30    10   P2P   1/1

R1 participates in two areas. Omitted Loopback0 belongs to area 0; its router identity does not determine all interface areas.

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 crosses its ABR, the backbone, and the other ABR. The total is three transit costs plus R4’s loopback cost.

As a controlled fault, shut R1 Gi1. Area 1 loses its only working backbone attachment and remote area-2 reachability after convergence. Restore no shutdown and verify R3’s inter-area route and a sourced ping to R4. There is no redundant backbone path in this lab.

Technical references

OSPFv2 standard · Router configuration guide