OSPF Stub Area
December 3rd, 2009 in
CCIE,
OSPF | tags:
OSPF,
ospf stub,
stub,
stub area In this post I will explain OSPF stub area types and the impact this will have on the routing table / OSPF database. Below is the diagram I will use in this post :
Below the configuration for OSPF on the routers with which we will start.
OSPF Configurations :
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Router 1 :
router ospf 1
router-id 1.1.1.1
log-adjacency-changes
redistribute connected subnets route-map CONNECTED
network 172.16.15.1 0.0.0.0 area 51
!
route-map CONNECTED permit 10
match interface FastEthernet3/0
!
interface FastEthernet3/0
ip address 172.20.1.1 255.255.255.0
duplex auto
speed auto
end
Router 5 :
router ospf 1
router-id 5.5.5.5
log-adjacency-changes
network 10.1.35.5 0.0.0.0 area 0
network 172.16.15.5 0.0.0.0 area 51
Router 3 :
router ospf 1
router-id 3.3.3.3
log-adjacency-changes
network 10.1.34.3 0.0.0.0 area 0
network 10.1.35.3 0.0.0.0 area 0
Router 4 :
router ospf 1
router-id 4.4.4.4
log-adjacency-changes
network 10.1.34.4 0.0.0.0 area 0
network 192.168.14.4 0.0.0.0 area 2
Router 2 :
router ospf 1
router-id 2.2.2.2
log-adjacency-changes
network 192.168.14.2 0.0.0.0 area 2
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Let’s take a look at the routing table and the OSPF database of Router R2 since this will be the area we will make stub (and stub no-summary in the next post).
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 not set
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, 00:52:10, FastEthernet2/0
172.20.0.0/24 is subnetted, 1 subnets
O E2 172.20.1.0 [110/20] via 192.168.14.4, 00:52:00, FastEthernet2/0
10.0.0.0/24 is subnetted, 2 subnets
O IA 10.1.35.0 [110/3] via 192.168.14.4, 00:52:10, FastEthernet2/0
O IA 10.1.34.0 [110/2] via 192.168.14.4, 00:52:10, 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 1252 0x80000003 0x0083A8 1
4.4.4.4 4.4.4.4 1243 0x80000003 0x000A0F 1
Net Link States (Area 2) <-- LSA type 2
Link ID ADV Router Age Seq# Checksum
192.168.14.4 4.4.4.4 1243 0x80000002 0x009DF7
Summary Net Link States (Area 2) <-- LSA type 3
Link ID ADV Router Age Seq# Checksum
10.1.34.0 4.4.4.4 1243 0x80000004 0x000FEC
10.1.35.0 4.4.4.4 1243 0x80000002 0x0012E9
172.16.15.0 4.4.4.4 1243 0x80000002 0x00025B
Summary ASB Link States (Area 2) <-- LSA type 4
Link ID ADV Router Age Seq# Checksum
1.1.1.1 4.4.4.4 1243 0x80000002 0x00F033
Type-5 AS External Link States <-- LSA type 5
Link ID ADV Router Age Seq# Checksum Tag
172.20.1.0 1.1.1.1 1238 0x80000002 0x0007D2 0
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As you can see there are Inter-Area routes (IA) and External E2 routes (External E2 - these are the redistributed routes into OSPF on router R1) in the routing table.
In the OSPF database you can see LSA types 1,2,3,4 and 5. Now let’s change Area 2 into a Stub area and see what it will do to the routing table and the OSPF database.
R2 & R4 :
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R4(config)#router ospf 1
R4(config-router)#area 2 stub
R2(config)#router ospf 1
R2(config-router)#area 2 stub
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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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In the routing table you can see that the External E2 route is gone, in the OSPF database you can also see the type 4 and 5 LSA’s are gone. So looking at the routing table and the OSPF database we can now state that a stub area blocks type 5 LSA’s and therefore make the type 4 LSA’s unnecessary, the type 5 LSA’s are replaced with a default route. It does allow type 3 lsa’s.
In the next post I will write about stub no-summary area’s and show the impact on the routing-table and OSPF database.