Chapter 4: OSPF Stubby Areas

Chapter 4: OSPF Stubby Areas


Objective

This lab focuses on implementing OSPF stubby areas to control external and inter-area route advertisements without manual route filtering configuration. By the end of this lab, participants will understand how to configure different flavors of stub areas, including totally stubby and NSSA (Not-So-Stubby Area), and their impact on OSPF routing.


Key Features Implemented

  • Configuring different flavors of OSPF Stubby Areas
  • Controlling external and inter-area route advertisements

Topology

The lab utilizes the OSPF multi-area topology as shown in the diagram from page 1 of the "Chapter 4: OSPF Stubby Areas" PDF.

OSPF Stubby Areas Topology Diagram

Configuration Tasks

  • Configure IP addresses and OSPF routing as shown in the topology:
    • Assign IP addresses to all interfaces as per the topology.
    • Configure OSPF on router interfaces and place them into the appropriate areas.
  • Redistribute external routes into OSPF:
    • On R7 and R9, redistribute their respective loopback networks into OSPF.
    • Verify that these networks appear as external routes in the routing table of all the routers and ensure connectivity.
  • Configure Area 1 as a Stub Area:
    • Modify Area 1 settings so external LSAs do not enter it, but ensure reachability to external networks is maintained.
  • Configure Area 3 as a Totally Stubby Area:
    • Modify Area 3 settings so that both external LSAs and network summary LSAs do not enter it.
    • Ensure that reachability to both external and inter-area networks is maintained.
  • Configure Area 2 as an NSSA (Not-So-Stubby Area):
    • Ensure that Area 2 has the capability to contain an ASBR (R7) and disseminate external routes into the OSPF domain using Type-7 LSAs.
    • Prevent Type-5 LSAs from entering Area 2. Ensure reachability to all the networks in the topology is maintained.
  • Configure Area 4 as a Totally Stubby NSSA:
    • Area 4 must be able to contain an ASBR (R9) and disseminate external routes into the OSPF domain using Type-7 LSAs.
    • Both Type-5 LSAs and Type-3 LSAs must be filtered within this area.
    • Reachability to all the networks in the topology must be maintained.

Base Configurations

Ready to get started? Here, you'll find the initial configurations for each device to begin this lab.
hostname R1

interface Ethernet0/0
 ip address 12.1.1.1 255.255.255.252
 ip ospf 1 area 0
interface Ethernet0/1
 ip address 13.1.1.1 255.255.255.252
 ip ospf 1 area 0
interface Ethernet0/2
 ip address 14.1.1.1 255.255.255.252
 ip ospf 1 area 0
interface Ethernet0/3
 ip address 15.1.1.1 255.255.255.252
 ip ospf 1 area 0

router ospf 1
hostname R2

interface Ethernet0/0
 ip address 12.1.1.2 255.255.255.252
 ip ospf 2 area 0
interface Ethernet0/1
 ip address 26.1.1.1 255.255.255.252
 ip ospf 2 area 1

router ospf 2
hostname R3

interface Ethernet0/0
 ip address 13.1.1.2 255.255.255.252
 ip ospf 3 area 0
interface Ethernet0/1
 ip address 37.1.1.1 255.255.255.252
 ip ospf 3 area 2

router ospf 3
hostname R4

interface Ethernet0/0
 ip address 14.1.1.2 255.255.255.252
 ip ospf 4 area 0
interface Ethernet0/1
 ip address 48.1.1.1 255.255.255.252
 ip ospf 4 area 3

router ospf 4
hostname R5

interface Ethernet0/0
 ip address 15.1.1.2 255.255.255.252
 ip ospf 5 area 0
interface Ethernet0/1
 ip address 59.1.1.1 255.255.255.252
 ip ospf 5 area 4

router ospf 5
hostname R6

interface Loopback1
 ip address 6.6.6.1 255.255.255.255
 ip ospf 6 area 1
interface Loopback2
 ip address 6.6.6.2 255.255.255.255
 ip ospf 6 area 1
interface Loopback3
 ip address 6.6.6.3 255.255.255.255
 ip ospf 6 area 1
interface Loopback4
 ip address 6.6.6.4 255.255.255.255
 ip ospf 6 area 1
interface Ethernet0/0
 ip address 26.1.1.2 255.255.255.252
 ip ospf 6 area 1

router ospf 6
hostname R7

interface Loopback1
 ip address 7.7.7.1 255.255.255.255
interface Loopback2
 ip address 7.7.7.2 255.255.255.255
interface Loopback3
 ip address 7.7.7.3 255.255.255.255
interface Loopback4
 ip address 7.7.7.4 255.255.255.255
interface Ethernet0/0
 ip address 37.1.1.2 255.255.255.252
 ip ospf 7 area 2

router ospf 7
 redistribute connected subnets
hostname R8

interface Loopback1
 ip address 8.8.8.1 255.255.255.255
 ip ospf 8 area 3
interface Loopback2
 ip address 8.8.8.2 255.255.255.255
 ip ospf 8 area 3
interface Loopback3
 ip address 8.8.8.3 255.255.255.255
 ip ospf 8 area 3
interface Loopback4
 ip address 8.8.8.4 255.255.255.255
 ip ospf 8 area 3
interface Ethernet0/0
 ip address 48.1.1.2 255.255.255.252
 ip ospf 8 area 3

router ospf 8
hostname R9

interface Loopback1
 ip address 9.9.9.1 255.255.255.255
interface Loopback2
 ip address 9.9.9.2 255.255.255.255
interface Loopback3
 ip address 9.9.9.3 255.255.255.255
interface Loopback4
 ip address 9.9.9.4 255.255.255.255
interface Ethernet0/0
 ip address 59.1.1.2 255.255.255.252
 ip ospf 9 area 4

router ospf 9
 redistribute connected subnets

Verification Commands

  • show ip ospf
    • Verify general OSPF process details, including summarization settings.
  • show ip ospf interface
    • Confirm which interfaces are participating in OSPF.
  • show ip ospf neighbor
    • Check adjacency states.
  • show ip ospf database
    • Verify LSA Type-3 and Type-5 summary LSAs.
  • show ip route ospf
    • Confirm that summarized routes are installed in the routing table.
  • show ip route
    • Verify the reachability to networks.

Expected Outcomes

  • R5 should see a single default route after Area 1 is configured as a stub, no Type-5 LSAs should be observed within Area 1.
  • Area 3 should block both external and inter-area routes while maintaining full reachability using a default route.
  • NSSA configurations should allow redistribution of external routes while maintaining stub functionality.
  • Area 4 should block both Type-5 and Type-3 LSAs while ensuring network reachability.
  • Full connectivity should be maintained.

Final Configurations

Want to take a look for yourself? Here, you will find the final configurations of each device required for this lab.
hostname R1

interface Ethernet0/0
 ip address 12.1.1.1 255.255.255.252
 ip ospf 1 area 0
interface Ethernet0/1
 ip address 13.1.1.1 255.255.255.252
 ip ospf 1 area 0
interface Ethernet0/2
 ip address 14.1.1.1 255.255.255.252
 ip ospf 1 area 0
interface Ethernet0/3
 ip address 15.1.1.1 255.255.255.252
 ip ospf 1 area 0

router ospf 1
hostname R2

interface Ethernet0/0
 ip address 12.1.1.2 255.255.255.252
 ip ospf 2 area 0
interface Ethernet0/1
 ip address 26.1.1.1 255.255.255.252
 ip ospf 2 area 1

router ospf 2
 area 1 stub
hostname R3

interface Ethernet0/0
 ip address 13.1.1.2 255.255.255.252
 ip ospf 3 area 0
interface Ethernet0/1
 ip address 37.1.1.1 255.255.255.252
 ip ospf 3 area 2

router ospf 3
 area 2 nssa default-information-originate
hostname R4

interface Ethernet0/0
 ip address 14.1.1.2 255.255.255.252
 ip ospf 4 area 0
interface Ethernet0/1
 ip address 48.1.1.1 255.255.255.252
 ip ospf 4 area 3

router ospf 4
 area 3 stub no-summary
hostname R5

interface Ethernet0/0
 ip address 15.1.1.2 255.255.255.252
 ip ospf 5 area 0
interface Ethernet0/1
 ip address 59.1.1.1 255.255.255.252
 ip ospf 5 area 4

router ospf 5
 area 4 nssa no-summary
hostname R6

interface Loopback1
 ip address 6.6.6.1 255.255.255.255
 ip ospf 6 area 1
interface Loopback2
 ip address 6.6.6.2 255.255.255.255
 ip ospf 6 area 1
interface Loopback3
 ip address 6.6.6.3 255.255.255.255
 ip ospf 6 area 1
interface Loopback4
 ip address 6.6.6.4 255.255.255.255
 ip ospf 6 area 1
interface Ethernet0/0
 ip address 26.1.1.2 255.255.255.252
 ip ospf 6 area 1

router ospf 6
 area 1 stub
hostname R7

interface Loopback1
 ip address 7.7.7.1 255.255.255.255
interface Loopback2
 ip address 7.7.7.2 255.255.255.255
interface Loopback3
 ip address 7.7.7.3 255.255.255.255
interface Loopback4
 ip address 7.7.7.4 255.255.255.255
interface Ethernet0/0
 ip address 37.1.1.2 255.255.255.252
 ip ospf 7 area 2

router ospf 7
 area 2 nssa
 redistribute connected subnets
hostname R8

interface Loopback1
 ip address 8.8.8.1 255.255.255.255
 ip ospf 8 area 3
interface Loopback2
 ip address 8.8.8.2 255.255.255.255
 ip ospf 8 area 3
interface Loopback3
 ip address 8.8.8.3 255.255.255.255
 ip ospf 8 area 3
interface Loopback4
 ip address 8.8.8.4 255.255.255.255
 ip ospf 8 area 3
interface Ethernet0/0
 ip address 48.1.1.2 255.255.255.252
 ip ospf 8 area 3

router ospf 8
 area 3 stub
hostname R9

interface Loopback1
 ip address 9.9.9.1 255.255.255.255
interface Loopback2
 ip address 9.9.9.2 255.255.255.255
interface Loopback3
 ip address 9.9.9.3 255.255.255.255
interface Loopback4
 ip address 9.9.9.4 255.255.255.255
interface Ethernet0/0
 ip address 59.1.1.2 255.255.255.252
 ip ospf 9 area 4

router ospf 9
 area 4 nssa
 redistribute connected subnets
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