The OSPF implementation in Cisco IOS recognizes 5 main classifications of areas:
- Regular (non-stub) area
- Stub area
- Totally stubby area
- Not-so-stubby area (NSSA)
- Totally stub not-so-stubby area (totally NSSA)
From an OSPF design perspective, areas represent sections of the enterprise network where it is advantageous for all of the routers in an area to have detailed topology information and have the ability to make detailed routing decisions involving the shortest path to a destination (as calculated by Djikstra's algorithm). For stub areas, this isn't typically necessary because the networks outside of the stub area are reachable by 1-2 equal cost paths and are easily represented by a default route within the stub area. Stub areas automatically filter type 5 (external) LSAs and optionally filter type 3 (summary) LSAs depending on whether the totally stubby option is chosen. The only traffic flowing into a stub area is the traffic that is either locally originated or destined for that particular area.
The following topology creates a backbone area and a stub area for further consideration:
The autonomous system border router (ASBR) in the topology redistributes connected subnets (to get E2 routes and 5 LSAs into the OSPF database) for area 0. The internal routers and interfaces ( ASBR s0/0, Area0, and Area1) are configured in a similar way to the single area OSPF example. To avoid being repetitive, I will start the configuration discussion with the area border router (ABR) router. First, assign IP addresses to the interfaces,
interface Serial0/0
ip address 10.0.1.2 255.255.255.0
!
interface Serial0/1
ip address 192.168.0.1 255.255.255.0
!
Then configure OSPF as you normally would. In this case, the area stub command is necessary to have routers in area 1 agree that the area is a stub area.
router ospf 1
log-adjacency-changes
area 1 stub
network 10.0.1.0 0.0.0.255 area 0
network 192.168.0.0 0.0.0.255 area 1
!
Now, there are 3 points of interest int he network. Looking at the internal router for area 0, Area0, We see both routing table entries and database entries for all networks in the diagrams.
Area0#show ip route
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2
i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
ia - IS-IS inter area, * - candidate default, U - per-user static route
o - ODR, P - periodic downloaded static route
Gateway of last resort is not set
1.0.0.0/24 is subnetted, 1 subnets
O E2 1.1.1.0 [110/100] via 10.0.0.1, 01:22:24, Serial0/0
10.0.0.0/24 is subnetted, 2 subnets
C 10.0.0.0 is directly connected, Serial0/0
C 10.0.1.0 is directly connected, Serial0/1
O IA 192.168.0.0/24 [110/128] via 10.0.1.2, 01:19:22, Serial0/1
O IA 192.168.1.0/24 [110/129] via 10.0.1.2, 01:19:13, Serial0/1
We see that the OSPF database for area 0 has type 1 and type 2 LSAs that provide routers in area 0 with a full topology in area 0. There are type 3 (summary) LSAs originating from the ABR for area 1 that contain the 192.168.x.x networks. We also see a type 5 LSA for the E2 route advertised by the ASBR. We do not see any type 4 (ASBR Summary) LSAs because we have no ASBRs in other areas.
Area0#show ip ospf database
OSPF Router with ID (10.0.1.1) (Process ID 1)
Router Link States (Area 0)
Link ID ADV Router Age Seq# Checksum Link count
1.1.1.1 1.1.1.1 1393 0x80000006 0x008AE9 2
10.0.1.1 10.0.1.1 1264 0x80000006 0x00222B 4
192.168.0.1 192.168.0.1 1122 0x80000004 0x003A6D 2
Summary Net Link States (Area 0)
Link ID ADV Router Age Seq# Checksum
192.168.0.0 192.168.0.1 1122 0x80000003 0x0044E2
192.168.1.0 192.168.0.1 1122 0x80000003 0x0043E1
Type-5 AS External Link States
Link ID ADV Router Age Seq# Checksum Tag
1.1.1.0 1.1.1.1 1393 0x80000003 0x00C482 0
One potential point of confusion is that there are Type 1 LSAs that are being advertised by the router ID 192.168.0.1. This is only the router ID that was chosen, all of the 192.168.x.x interfaces exist in Area 1. This is evident from examining the LSA on the router that is internal to area 0:
Area0#show ip ospf database router 192.168.0.1
OSPF Router with ID (10.0.1.1) (Process ID 1)
Router Link States (Area 0)
Routing Bit Set on this LSA
LS age: 1325
Options: (No TOS-capability, DC)
LS Type: Router Links
Link State ID: 192.168.0.1
Advertising Router: 192.168.0.1
LS Seq Number: 80000004
Checksum: 0x3A6D
Length: 48
Area Border Router
Number of Links: 2
Link connected to: another Router (point-to-point)
(Link ID) Neighboring Router ID: 10.0.1.1
(Link Data) Router Interface address: 10.0.1.2
Number of TOS metrics: 0
TOS 0 Metrics: 64
Link connected to: a Stub Network
(Link ID) Network/subnet number: 10.0.1.0
(Link Data) Network Mask: 255.255.255.0
Number of TOS metrics: 0
TOS 0 Metrics: 64
Now we look at the ABR. Databases and routing tables tend to be more interesting on area border routers because they have complete topology databases for more than one area. Additionally, ABRs create and advertise default routes for other routers that are internal to stub areas, but they do not install the default route themselves. When we examine the OSPF database for the ABR, it is apparent that there are 2 complete databases (one for area 0, and one for area 1).
ABR#show ip ospf database
OSPF Router with ID (192.168.0.1) (Process ID 1)
Router Link States (Area 0)
Link ID ADV Router Age Seq# Checksum Link count
1.1.1.1 1.1.1.1 2008 0x80000006 0x008AE9 2
10.0.1.1 10.0.1.1 1880 0x80000006 0x00222B 4
192.168.0.1 192.168.0.1 1736 0x80000004 0x003A6D 2
Summary Net Link States (Area 0)
Link ID ADV Router Age Seq# Checksum
192.168.0.0 192.168.0.1 1736 0x80000003 0x0044E2
192.168.1.0 192.168.0.1 1736 0x80000003 0x0043E1
Router Link States (Area 1)
Link ID ADV Router Age Seq# Checksum Link count
192.168.0.1 192.168.0.1 1736 0x80000005 0x00E8A4 2
192.168.1.1 192.168.1.1 1887 0x80000004 0x00F819 3
Summary Net Link States (Area 1)
Link ID ADV Router Age Seq# Checksum
0.0.0.0 192.168.0.1 1736 0x80000003 0x009B36
10.0.0.0 192.168.0.1 1748 0x80000003 0x001434
10.0.1.0 192.168.0.1 1748 0x80000003 0x008601
Type-5 AS External Link States
Link ID ADV Router Age Seq# Checksum Tag
1.1.1.0 1.1.1.1 3 0x80000004 0x00C283 0
The routing table also reflects all of the routes installed from both areas (but not the default route created by the ABR and flooded to the Area1 router).
ABR#show ip route
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2
i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
ia - IS-IS inter area, * - candidate default, U - per-user static route
o - ODR, P - periodic downloaded static route
Gateway of last resort is not set
1.0.0.0/24 is subnetted, 1 subnets
O E2 1.1.1.0 [110/100] via 10.0.1.1, 01:37:34, Serial0/0
10.0.0.0/24 is subnetted, 2 subnets
O 10.0.0.0 [110/128] via 10.0.1.1, 01:37:34, Serial0/0
C 10.0.1.0 is directly connected, Serial0/0
C 192.168.0.0/24 is directly connected, Serial0/1
O 192.168.1.0/24 [110/65] via 192.168.0.2, 01:37:19, Serial0/1
Now, we look at the router that is internal to area 1, the stub area. We can see from the routing table that the default route is installed, but the E2 route advertised by ASBR has disappeared. This is because stub areas do not propagate type 5 (external) LSAs. In this configuration, type 3 LSAs are still propagated, but this can be modified using a totally stubby area.
Area1#show ip route
Gateway of last resort is 192.168.0.1 to network 0.0.0.0
10.0.0.0/24 is subnetted, 2 subnets
O IA 10.0.0.0 [110/192] via 192.168.0.1, 01:42:47, Serial0/0
O IA 10.0.1.0 [110/128] via 192.168.0.1, 01:42:47, Serial0/0
C 192.168.0.0/24 is directly connected, Serial0/0
C 192.168.1.0/24 is directly connected, Loopback0
O*IA 0.0.0.0/0 [110/65] via 192.168.0.1, 01:42:47, Serial0/0
The database also shows no type 5 (external) LSAs, but we see that the default route is propagated as a type 3 (network summary) LSA:
Area1#show ip ospf database
OSPF Router with ID (192.168.1.1) (Process ID 1)
Router Link States (Area 1)
Link ID ADV Router Age Seq# Checksum Link count
192.168.0.1 192.168.0.1 279 0x80000006 0x00E6A5 2
192.168.1.1 192.168.1.1 418 0x80000005 0x00F61A 3
Summary Net Link States (Area 1)
Link ID ADV Router Age Seq# Checksum
0.0.0.0 192.168.0.1 279 0x80000004 0x009937
10.0.0.0 192.168.0.1 279 0x80000004 0x001235
10.0.1.0 192.168.0.1 279 0x80000004 0x008402
In later posts, we will go into the details of totally stubby areas and the effects of the NSSA option.
See Also:
The Road to the CCIE
Hi, This is a good post, indeed a great job. You must have done good research for the work, i appreciate your efforts. Looking for more updates from your side. Thanks
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