OSPF Totally Stubby Area
In my previous post I talked about OSPF stub area’s and how they impact the routing table and the OSPF database. This post will be about totally stubby area’s in OSPF.
I will continue this post where the previous post stopped. The diagram that will be used in this post is also the same, see below :
As you can see in the previous post stubby-area’s remove LSA type 4 & type 5 routes and replaces the type 5 LSA with a default-route. See below the routing table when R2 & R4 are configured with the “area 2 stub” command under the OSPF proces :
R2 :
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R2#sh 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 192.168.14.4 to network 0.0.0.0
2.0.0.0/32 is subnetted, 1 subnets
C 2.2.2.2 is directly connected, Loopback0
C 192.168.14.0/24 is directly connected, FastEthernet2/0
172.16.0.0/24 is subnetted, 1 subnets
O IA 172.16.15.0 [110/4] via 192.168.14.4, 00:01:59, FastEthernet2/0
10.0.0.0/24 is subnetted, 2 subnets
O IA 10.1.35.0 [110/3] via 192.168.14.4, 00:01:59, FastEthernet2/0
O IA 10.1.34.0 [110/2] via 192.168.14.4, 00:01:59, FastEthernet2/0
O*IA 0.0.0.0/0 [110/2] via 192.168.14.4, 00:01:59, FastEthernet2/0
R2#sh ip ospf database
OSPF Router with ID (2.2.2.2) (Process ID 1)
Router Link States (Area 2) <-- LSA type 1
Link ID ADV Router Age Seq# Checksum Link count
2.2.2.2 2.2.2.2 136 0x80000005 0x009D8E 1
4.4.4.4 4.4.4.4 136 0x80000005 0x0024F4 1
Net Link States (Area 2) <-- LSA type 2
Link ID ADV Router Age Seq# Checksum
192.168.14.4 4.4.4.4 132 0x80000004 0x00B7DD
Summary Net Link States (Area 2) <-- LSA type 3
Link ID ADV Router Age Seq# Checksum
0.0.0.0 4.4.4.4 154 0x80000001 0x0039F4
10.1.34.0 4.4.4.4 154 0x80000005 0x002BD1
10.1.35.0 4.4.4.4 154 0x80000003 0x002ECE
172.16.15.0 4.4.4.4 154 0x80000003 0x001E40
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To configure an area as a totally stubby area you only need to issue the command “area 2 stub no-summary” under the OSPF process on the ABR.
So on Router 4 :
R4 :
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R4(config)#router ospf 1
R4(config-router)#area 2 stub no-summary
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Now let’s take a look at the routing-table and the OSPF database so we can compare this with the “stub area” routing-table and the OSPF database on Router R2:
R2 :
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R2#sh 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 192.168.14.4 to network 0.0.0.0
2.0.0.0/32 is subnetted, 1 subnets
C 2.2.2.2 is directly connected, Loopback0
C 192.168.14.0/24 is directly connected, FastEthernet2/0
O*IA 0.0.0.0/0 [110/2] via 192.168.14.4, 00:02:36, FastEthernet2/0
R2#sh ip ospf dat
OSPF Router with ID (2.2.2.2) (Process ID 1)
Router Link States (Area 2) <-- LSA type 1
Link ID ADV Router Age Seq# Checksum Link count
2.2.2.2 2.2.2.2 169 0x80000005 0x009D8E 1
4.4.4.4 4.4.4.4 145 0x80000005 0x0024F4 1
Net Link States (Area 2) <-- LSA type 2
Link ID ADV Router Age Seq# Checksum
192.168.14.4 4.4.4.4 145 0x80000004 0x00B7DD
Summary Net Link States (Area 2) <-- Default route replaces all LSA type 3 routes
Link ID ADV Router Age Seq# Checksum
0.0.0.0 4.4.4.4 172 0x80000001 0x0039F4
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As you can see in the routing-table and the OSPF database above not only the LSA type 4 & 5 routes are gone but also the type 3 LSA’s are no longer there. So for totally stubby area’s we can state that no type 3,4 and 5 LSA’s are allowed. A default route is injected into the area from the ABR.
The next post will be about the OSPF NSSA type.