OSPF NSSA Area
This post will be about OSPF Not-So-Stubby-Area’s. In the previous posts we talked about stubby and not-so-stubby-area’s. For this post I will use the same topology as in the previous two posts, see below :
On Router 1 we again redistribute the interface 172.20.1.1/24 into OSPF :
R1 :
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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
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Let’s make area 2 a nssa area and see how it impacts the LSA’s and the routing-table.
We need to issue the “area 2 nssa” command on Router 2 and Router 4.
R2 :
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Router 2 :
router ospf 1
router-id 2.2.2.2
log-adjacency-changes
area 2 nssa
network 192.168.14.2 0.0.0.0 area 2
Router 4 :
router ospf 1
router-id 4.4.4.4
log-adjacency-changes
area 2 nssa
network 10.1.34.4 0.0.0.0 area 0
network 192.168.14.4 0.0.0.0 area 2
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Now let’s take a look at the routing-table and the OSPF database on Router 2 :
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, 01:29:51, FastEthernet2/0
10.0.0.0/24 is subnetted, 2 subnets
O IA 10.1.35.0 [110/3] via 192.168.14.4, 01:29:51, FastEthernet2/0
O IA 10.1.34.0 [110/2] via 192.168.14.4, 01:29:51, FastEthernet2/0
R2#sh ip ospf database
OSPF Router with ID (2.2.2.2) (Process ID 1)
Router Link States (Area 2)
Link ID ADV Router Age Seq# Checksum Link count
2.2.2.2 2.2.2.2 1412 0x80000008 0x001F02 1
4.4.4.4 4.4.4.4 1402 0x80000008 0x00AB60 1
Net Link States (Area 2)
Link ID ADV Router Age Seq# Checksum
192.168.14.4 4.4.4.4 1402 0x80000007 0x003951
Summary Net Link States (Area 2)
Link ID ADV Router Age Seq# Checksum
10.1.34.0 4.4.4.4 412 0x80000007 0x00AE44
10.1.35.0 4.4.4.4 412 0x80000005 0x00B141
172.16.15.0 4.4.4.4 412 0x80000005 0x00A1B2
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As you can see a NSSA area blocks type 4 & type 5 LSA’s and doesn’t originate a default-route by default.
Although the ABR doesn’t originate a default-route in a NSSA area by default it is possible to let the ABR originate a default route, this can be done like below on the ABR :
R4 :
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R4(config-router)#area 2 nssa default-information-originate
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Now let’s take a look on Router 2 again :
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, 01:34:36, FastEthernet2/0
10.0.0.0/24 is subnetted, 2 subnets
O IA 10.1.35.0 [110/3] via 192.168.14.4, 01:34:36, FastEthernet2/0
O IA 10.1.34.0 [110/2] via 192.168.14.4, 01:34:36, FastEthernet2/0
O*N2 0.0.0.0/0 [110/1] via 192.168.14.4, 00:01:46, FastEthernet2/0
R2#sh ip ospf database
OSPF Router with ID (2.2.2.2) (Process ID 1)
Router Link States (Area 2)
Link ID ADV Router Age Seq# Checksum Link count
2.2.2.2 2.2.2.2 1696 0x80000008 0x001F02 1
4.4.4.4 4.4.4.4 1686 0x80000008 0x00AB60 1
Net Link States (Area 2)
Link ID ADV Router Age Seq# Checksum
192.168.14.4 4.4.4.4 1686 0x80000007 0x003951
Summary Net Link States (Area 2)
Link ID ADV Router Age Seq# Checksum
10.1.34.0 4.4.4.4 695 0x80000007 0x00AE44
10.1.35.0 4.4.4.4 695 0x80000005 0x00B141
172.16.15.0 4.4.4.4 695 0x80000005 0x00A1B2
Type-7 AS External Link States (Area 2)
Link ID ADV Router Age Seq# Checksum Tag
0.0.0.0 4.4.4.4 113 0x80000001 0x00B372 0
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As you can see a default-route is now originated by the ABR. This is the type 7 (N2) LSA.
The next post will be about the OSPF totally not so stubby area type.