LAB: OSPF Stub Areas with Multiple Exit Points.

- Re-enable R6’s Loopback0 interface and connection to VLAN 146.
- Modify area 2 in such a way that traffic from devices in area 2 going to area 3 uses R6 as the exit point, while traffic from area 2 going to the RIP routes learned from BB3 uses R3.
- If R3 loses its connection to area 2, traffic for External destinations should be rerouted to R6.
- If R6 loses its connection to area 2, traffic for Inter-Area destinations should be rerouted to R3.
- Do not modify the cost of any links in area 2 to accomplish this.


command sets:

R3:
router ospf 1
area 2 nssa no-summary

R6:
router ospf 1
area 2 nssa default-information-originate

SW1:
router ospf 1
area 2 nssa

SW3:
router ospf 1
area 2 nssa

stub areas can be used also for inter-area traffic engineering. in our case here there are multiple exit points out of area 2.  R3 advertises the default route as a type-3 summary LSA (not so totally stubby area) and R6 advertises all Type-3 Summary LSA, plus type-7 NSSA external default route.

for inter-area routing from devices in area 2 this means  that the longest match learned from R6 will always be used from R6.  and for default routing the type-3 summary LSA will be used from R3.

route preference:
intra-area>inter-area>external>nssa-external

verification:

Rack1SW1#show ip ospf database | begin Summary
                Summary Net Link States (Area 2)

Link ID         ADV Router      Age         Seq#       Checksum
0.0.0.0         150.1.3.3       566         0x80000003 0x00BADB
150.1.1.0       150.1.6.6       234         0x80000001 0x009D2F
150.1.2.0       150.1.6.6       234         0x80000001 0x00AE19
150.1.3.0       150.1.6.6       234         0x80000001 0x00A323
150.1.4.0       150.1.6.6       234         0x80000001 0x00982D
150.1.5.0       150.1.6.6       234         0x80000001 0x000CF8
150.1.6.0       150.1.6.6       249         0x80000001 0x00A351
150.1.6.0       155.1.67.6      391         0x80000001 0x00C9E8
150.1.8.0       150.1.6.6       235         0x80000001 0x00F40C
150.1.10.0      150.1.6.6       235         0x80000001 0x00BFA8
155.1.0.1       150.1.6.6       235         0x80000001 0x005375
155.1.0.2       150.1.6.6       235         0x80000001 0x00655E
155.1.0.3       150.1.6.6       235         0x80000001 0x005B67
155.1.0.4       150.1.6.6       235         0x80000001 0x005170
155.1.0.5       150.1.6.6       235         0x80000001 0x00C53B
155.1.5.0       150.1.6.6       235         0x80000001 0x00CA35
155.1.8.0       150.1.6.6       160         0x80000003 0x00C21D
155.1.13.0      150.1.6.6       235         0x80000001 0x007713
155.1.23.0      150.1.6.6       235         0x80000001 0x008528
155.1.45.0      150.1.6.6       236         0x80000001 0x008810
155.1.58.0      150.1.6.6       235         0x80000001 0x008149
155.1.108.0     150.1.6.6       237         0x80000001 0x003AC7
155.1.146.0     150.1.6.6       252         0x80000001 0x001126
155.1.146.0     155.1.67.6      393         0x80000001 0x0037BD
192.10.1.0      150.1.6.6       237         0x80000001 0x00E187

traffic to other areas is using R6 as a gateway:

Rack1SW1#show ip route 150.1.10.10
Routing entry for 150.1.10.0/24
  Known via "ospf 1", distance 110, metric 3482, type inter area
  Last update from 155.1.67.6 on Vlan67, 00:04:33 ago
  Routing Descriptor Blocks:
  * 155.1.67.6, from 150.1.6.6, 00:04:33 ago, via Vlan67
      Route metric is 3482, traffic share count is 1

while traffic to external-network such as 30.0.0.1 RIP routes redistributed into OSPF on R4, will use R3 as a gateway since it is advertising LSA type-3 summary default route.

Rack1SW1#show ip route 30.0.0.1
% Network not in table

Rack1SW1#show ip route | include _0.0.0.0
Gateway of last resort is 155.1.37.3 to network 0.0.0.0
O*IA 0.0.0.0/0 [110/301] via 155.1.37.3, 00:08:49, FastEthernet0/3

output below shows that sw1 has 2 default routes, one is LSA type 3 and other is type7 nssa-external.  in our case here the metric is the same. but even if the metric for type-7 nssa-external is lower than type-3, summary-type 3 will still be used.

Rack1SW1#show ip ospf database summary 0.0.0.0

            OSPF Router with ID (150.1.7.7) (Process ID 1)

                Summary Net Link States (Area 2)

  Routing Bit Set on this LSA
  LS age: 906
  Options: (No TOS-capability, DC, Upward)
  LS Type: Summary Links(Network)
  Link State ID: 0.0.0.0 (summary Network Number)
  Advertising Router: 150.1.3.3
  LS Seq Number: 80000003
  Checksum: 0xBADB
  Length: 28
  Network Mask: /0
        TOS: 0  Metric: 1

Rack1SW1#show ip ospf database nssa-external 0.0.0.0

            OSPF Router with ID (150.1.7.7) (Process ID 1)

                Type-7 AS External Link States (Area 2)

  LS age: 646
  Options: (No TOS-capability, No Type 7/5 translation, DC)
  LS Type: AS External Link
  Link State ID: 0.0.0.0 (External Network Number )
  Advertising Router: 150.1.6.6
  LS Seq Number: 80000001
  Checksum: 0x6BA2
  Length: 36
  Network Mask: /0
        Metric Type: 2 (Larger than any link state path)
        TOS: 0
        Metric: 1
        Forward Address: 0.0.0.0
        External Route Tag: 0


this will also be used to backup each other in-case R3 and R6 fails.

When R3 fails.

Rack1SW1#show ip route | include _0.0.0.0
Gateway of last resort is 155.1.67.6 to network 0.0.0.0
O*N2 0.0.0.0/0 [110/1] via 155.1.67.6, 00:00:00, Vlan67

likewise when R6 fails.

Rack1SW1#show ip route 150.1.10.10
% Subnet not in table

Rack1SW1#show ip route | include _0.0.0.0    
Gateway of last resort is 155.1.37.3 to network 0.0.0.0
O*IA 0.0.0.0/0 [110/301] via 155.1.37.3, 00:00:09, FastEthernet0/3