3.2.b Configure simple OSPFv2/v3 environments, including multiple normal areas, summarization, and filtering (neighbor adjacency, point-to-point, and broadcast network types, and passive-interface)

Open Shortest Path First (OSPF) is the link-state protocol the exam uses to test areas, summarization, and filtering. This article builds OSPFv2 and OSPFv3 networks, then proves them with show output.

What this objective tests

The objective says "configure". The exam wants a working multi-area OSPFv2 network and a working OSPFv3 network. You must also prove both with show output.

Open Shortest Path First (OSPF) works in four steps:

1. Routers become neighbors on a link.
2. Neighbors exchange link-state information.
3. Each router builds a link-state database.
4. Each router runs Shortest Path First (SPF) and installs the best routes.

Troubleshooting means finding which step failed.

OSPFv2 multi-area configuration

Topology:

LAN A 10.1.1.0/24 --+
                    |
LAN B 10.1.2.0/24 --+-- R1 -- area 1 -- R2 -- area 0 -- R3 -- Lo0 3.3.3.3/32
                    ABR = R2
R1-R2 link: 10.0.12.0/30
R2-R3 link: 10.0.23.0/30

R1:

interface Loopback0
 ip address 1.1.1.1 255.255.255.255
!
interface GigabitEthernet0/0
 description To R2
 ip address 10.0.12.1 255.255.255.252
 ip ospf network point-to-point
!
interface GigabitEthernet0/1
 description LAN A
 ip address 10.1.1.1 255.255.255.0
!
interface GigabitEthernet0/2
 description LAN B
 ip address 10.1.2.1 255.255.255.0
!
router ospf 10
 router-id 1.1.1.1
 passive-interface default
 no passive-interface GigabitEthernet0/0
 network 10.0.12.0 0.0.0.3 area 1
 network 10.1.1.0 0.0.0.255 area 1
 network 10.1.2.0 0.0.0.255 area 1
 network 1.1.1.1 0.0.0.0 area 1

R2 (Area Border Router, summarizes area 1):

interface Loopback0
 ip address 2.2.2.2 255.255.255.255
!
interface GigabitEthernet0/0
 description To R1
 ip address 10.0.12.2 255.255.255.252
 ip ospf network point-to-point
!
interface GigabitEthernet0/1
 description To R3
 ip address 10.0.23.1 255.255.255.252
 ip ospf network point-to-point
!
router ospf 10
 router-id 2.2.2.2
 passive-interface default
 no passive-interface GigabitEthernet0/0
 no passive-interface GigabitEthernet0/1
 network 10.0.12.0 0.0.0.3 area 1
 network 10.0.23.0 0.0.0.3 area 0
 network 2.2.2.2 0.0.0.0 area 0
 area 1 range 10.1.0.0 255.255.0.0

R3:

interface Loopback0
 ip address 3.3.3.3 255.255.255.255
!
interface GigabitEthernet0/0
 description To R2
 ip address 10.0.23.2 255.255.255.252
 ip ospf network point-to-point
!
router ospf 10
 router-id 3.3.3.3
 passive-interface default
 no passive-interface GigabitEthernet0/0
 network 10.0.23.0 0.0.0.3 area 0
 network 3.3.3.3 0.0.0.0 area 0

The config uses three exam features:

• ip ospf network point-to-point skips the DR/BDR election on a two-router link.
• passive-interface default keeps hellos off LAN ports. The LAN subnets still get advertised.
• area 1 range on the ABR advertises one summary for all area 1 networks.

Expected verification output

Neighbors on R2. Point-to-point links show state FULL/ - because there is no DR:

R2# show ip ospf neighbor

Neighbor ID     Pri   State           Dead Time   Address         Interface
1.1.1.1           0   FULL/  -        00:00:37    10.0.12.1       GigabitEthernet0/0
3.3.3.3           0   FULL/  -        00:00:31    10.0.23.2       GigabitEthernet0/1

Routes on R3. The summary appears as one O IA route instead of two /24 routes:

R3# show ip route ospf
...
      1.0.0.0/32 is subnetted, 1 subnets
O IA     1.1.1.1 [110/3] via 10.0.23.1, 00:21:14, GigabitEthernet0/0
      2.0.0.0/32 is subnetted, 1 subnets
O        2.2.2.2 [110/2] via 10.0.23.1, 00:21:14, GigabitEthernet0/0
      10.0.0.0/8 is variably subnetted, 3 subnets, 2 masks
O IA     10.0.12.0/30 [110/2] via 10.0.23.1, 00:21:14, GigabitEthernet0/0
O IA     10.1.0.0/16 [110/3] via 10.0.23.1, 00:21:14, GigabitEthernet0/0

Read this output:

• O means intra-area. O IA means inter-area.
• 10.1.0.0/16 is the summary from area 1 range. R3 never sees 10.1.1.0/24 or 10.1.2.0/24.

Neighbor states and adjacency faults

Note: 2-WAY/DROTHER on a broadcast segment is normal. Do not troubleshoot a state that is correct.

OSPFv3 with address families

OSPFv3 uses the router ospfv3 process. Each address family sits inside the process. Interfaces join an area per address family.

R1:

ipv6 unicast-routing
!
interface Loopback0
 ipv6 address 2001:db8:1::1/128
!
interface GigabitEthernet0/0
 description To R2
 ipv6 address 2001:db8:12::1/64
 ospfv3 10 ipv6 area 0
 ospfv3 network point-to-point
!
router ospfv3 10
 router-id 1.1.1.1
 !
 address-family ipv6 unicast
  passive-interface default
  no passive-interface GigabitEthernet0/0
 exit-address-family

R2:

ipv6 unicast-routing
!
interface GigabitEthernet0/0
 description To R1
 ipv6 address 2001:db8:12::2/64
 ospfv3 10 ipv6 area 0
 ospfv3 network point-to-point
!
router ospfv3 10
 router-id 2.2.2.2
 !
 address-family ipv6 unicast
 exit-address-family

Note: OSPFv3 still needs a 32-bit router ID. Set it manually with router-id under the process.

Verify:

R1# show ipv6 ospf neighbor

            OSPFv3 Router with ID (1.1.1.1) (Process ID 10)

Neighbor ID     Pri   State           Dead Time   Interface ID    Interface
2.2.2.2           0   FULL/  -        00:00:35    4               GigabitEthernet0/0

OSPFv3 neighbors use link-local addresses. The output shows an interface ID instead of a neighbor address. The state must still be FULL.

Lab: build multi-area OSPFv2 and add OSPFv3

Topology: the R1-R2-R3 diagram from the OSPFv2 example above.

Do these steps:

1. Cable and address the links per the example. Give each router its loopback.
2. Configure OSPFv2 on R1, R2, and R3 with the configs above.
3. Verify neighbors on R2 with show ip ospf neighbor. Both neighbors must be FULL/ -.
4. Check routes on R3 with show ip route ospf. Confirm O IA 10.1.0.0/16 exists and the /24 LAN routes do not.
5. Check passive interfaces with show ip ospf interface brief on R1. Confirm Gi0/1 and Gi0/2 are passive.
6. Remove area 1 range on R2. Watch R3 learn the two /24 routes. Restore the summary.
7. Add the OSPFv3 address-family config on R1 and R2. Enable IPv6 routing first.
8. Verify with show ipv6 ospf neighbor and show ipv6 route ospf. The neighbor must be FULL.
9. Break the adjacency: set ospfv3 10 ipv6 area 1 on one side. Observe the neighbor fail. Fix the area.

Exam traps

• A passive interface advertises the network but forms no neighbors.
• O is intra-area. O IA is inter-area. The exam tests this difference.
• Summarization with area range happens on the ABR, not on internal routers.
• Stuck in EXSTART or EXCHANGE usually means an MTU mismatch.
• OSPFv3 needs ipv6 unicast-routing and a manual router ID.
• OSPFv3 neighbors use link-local addresses. Do not look for the global address in the neighbor table.

Pass check

You are ready when you can do these things:

• Build a two-area OSPFv2 network with summarization, passive interfaces, and point-to-point links.
• Prove the summary works by reading show ip route ospf on a router in another area.
• Configure OSPFv3 with address families and prove the adjacency.
• Name the usual cause for each stuck neighbor state.