OSPF STUBS
OSPF stubs allow the OSPF routers in an area to use default
routes for forwarding packets to Area Border Routers (ABRs), rather than more
specific routes. OSPF stubs reduce memory consumption and CPU processing time
on the routers inside the area because the routers in that area can have fewer
LSAs in their LSDBs.
If you configure a stub area it will block all type 5
external LSAs (E1 and E2 routes). All the prefixes you redistributed into
OSPF from another routing protocol are not welcome in the stub area. Default
routes are advertised into stub areas by the ABR. All OSPF routers in the stub area
must be configured as stubs.
In short: the OSPF stub allows the routers in an area to use the default route for forwarding packets to ABR rather than more specific routes.
OSPF has special area types called stub areas:
- Stub
- Totally Stubby
- Not-So-Stubby Area (NSSA)
- Totally Not So Stubby Area (Totally NSSA)
Stub Area Rules:
- In the OSPF Stub area, there should not be an ASBR.
- The OSPF Stub area should not be Area 0.
- No virtual link must be passing through that area.
- All routers in that area must be configured as stub routers.
A stub area is an isolated area that does not receive External
LSA. Routers in Stub areas do not receive type 4 and type 5 LSAs it is replaced
by a default route to an external autonomous system advertised by the area border
router (ABR) Stub areas can have type 1, 2, and 3 OSPF LSAs.
Totally stubby area External
LSAs are stopped (E1 and E2) and summary LSAs are stopped (OIA routes). Replaced
by a default route to other areas advertised by the ABR. Totally stubs is reducing the routing table to a minimum. This is a Cisco proprietary feature.
NSSA breaks stub area rules. NSSA area creates a special
type of link-state advertisement (LSA) known as type 7, type 7 LSAs is only
exist in the NSSA area. An NSSA autonomous system boundary router (ASBR) generates
this LSA and an NSSA area border router (ABR) translates it into a type 5 LSA, ABR
sends default routes into NSSA instead of external routes from other ASBR.
Totally NSSAs are similar to totally stubby areas, with the
exception that the routers that are internal to the totally NSSAs need to have
no knowledge of subnets outside of the area (with the exception of the routes
injected by the NSSA ASBR)
Before we start BGP configuration check out some important BGP CCIE exam topics.
Here are the lists of all OSPF labs and theories CCNA to CCIE.
Stub configuration:
GOAL:
- configure the topology and advertise the interfaces as per diagram.
- configure ospf as per the diagram and redistribute RIP routes in to OSPF on R4.
- configure area 10 to stop LSA 5 and LSA 4.
- make sure that the area 10 routers still able to reach the external routes.
R1#show ip interface brief
Interface IP-Address OK? Method Status Protocol
FastEthernet0/0 10.1.1.1 YES manual up up
Serial3/0 1.1.1.1 YES manual up up
Loopback0 11.0.0.1 YES manual up up
Loopback1 11.0.1.1 YES manual up up
Loopback2 11.0.2.1 YES manual up up
Loopback3 11.0.3.1 YES manual up up
R2#show ip interface brief
Interface IP-Address OK? Method Status Protocol
FastEthernet0/0 20.1.1.1 YES manual up up
Serial3/0 1.1.1.2 YES manual up up
Serial3/1 2.1.1.1 YES manual up up
Loopback0 12.0.0.1 YES manual up up
Loopback1 12.0.1.1 YES manual up up
Loopback2 12.0.2.1 YES manual up up
Loopback3 12.0.3.1 YES manual up up
R3#show ip interface brief
Interface IP-Address OK? Method Status Protocol
FastEthernet0/0 30.1.1.1 YES manual up up
Serial3/1 2.1.1.2 YES manual up up
Serial3/2 3.1.1.1 YES manual up up
Loopback0 13.0.0.1 YES manual up up
Loopback1 13.0.1.1 YES manual up up
Loopback2 13.0.2.1 YES manual up up
Loopback3 13.0.3.1 YES manual up up
R4#show ip interface brief
Interface IP-Address OK? Method Status Protocol
FastEthernet0/0 40.1.1.1 YES manual up up
Serial3/2 3.1.1.2 YES manual up up
Loopback0 14.0.0.1 YES manual up up
Loopback1 14.0.1.1 YES manual up up
Loopback2 14.0.2.1 YES manual up up
Loopback3 14.0.3.1 YES manual up up
R1(config)#router ospf 1
R1(config-router)# network 11.0.0.0 0.255.255.255 area 10
R1(config-router)#network 1.0.0.0 0.255.255.255 area 10
R1(config-router)#network 10.0.0.0 0.255.255.255 area 10
R2(config)#router ospf 1
R2(config-router)#network 1.0.0.0 0.255.255.255 area 10
R2(config-router)#network 2.0.0.0 0.255.255.255 area 0
R2(config-router)#network 20.0.0.0 0.255.255.255 area 0
R2(config-router)#network 12.0.0.0 0.255.255.255 area 0
R2(config-router)#end
R3(config)#router ospf 1
R3(config-router)#network 30.0.0.0 0.255.255.255 area 0
R3(config-router)#network 13.0.0.0 0.255.255.255 area 0
R3(config-router)#network 2.0.0.0 0.255.255.255 area 0
R3(config-router)#network 3.0.0.0 0.255.255.255 area 20
R4(config)#router rip
R4(config-router)#version 2
R4(config-router)#network 14.0.0.0
R4(config-router)#no auto-summary
R4(config-router)#end
R4(config)#router ospf 1
R4(config-router)#network 3.0.0.0 0.255.255.255 area 20
R4(config-router)#network 40.0.0.0 0.255.255.255 area 20
R4(config-router)#redistribute rip subnets
R4(config-router)#end
R1#show ip route ospf
Codes: L - local, 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, H - NHRP, l - LISP
+ - replicated route, % - next hop override
Gateway of last resort is not set
O IA 2.0.0.0/8 [110/128] via 1.1.1.2, 00:06:24, Serial3/0
O IA 3.0.0.0/8 [110/192] via 1.1.1.2, 00:06:24, Serial3/0
12.0.0.0/32 is subnetted, 4 subnets
O IA 12.0.0.1 [110/65] via 1.1.1.2, 00:02:08, Serial3/0
O IA 12.0.1.1 [110/65] via 1.1.1.2, 00:02:08, Serial3/0
O IA 12.0.2.1 [110/65] via 1.1.1.2, 00:02:08, Serial3/0
O IA 12.0.3.1 [110/65] via 1.1.1.2, 00:02:08, Serial3/0
13.0.0.0/32 is subnetted, 4 subnets
O IA 13.0.0.1 [110/129] via 1.1.1.2, 00:00:28, Serial3/0
O IA 13.0.1.1 [110/129] via 1.1.1.2, 00:00:28, Serial3/0
O IA 13.0.2.1 [110/129] via 1.1.1.2, 00:00:28, Serial3/0
O IA 13.0.3.1 [110/129] via 1.1.1.2, 00:00:28, Serial3/0
14.0.0.0/24 is subnetted, 4 subnets
O E2 14.0.0.0 [110/20] via 1.1.1.2, 00:04:59, Serial3/0
O E2 14.0.1.0 [110/20] via 1.1.1.2, 00:04:59, Serial3/0
O E2 14.0.2.0 [110/20] via 1.1.1.2, 00:04:59, Serial3/0
O E2 14.0.3.0 [110/20] via 1.1.1.2, 00:04:59, Serial3/0
O IA 20.0.0.0/8 [110/65] via 1.1.1.2, 00:06:24, Serial3/0
O IA 30.0.0.0/8 [110/129] via 1.1.1.2, 00:06:24, Serial3/0
R2#show ip route ospf
Codes: L - local, 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, H - NHRP, l - LISP
+ - replicated route, % - next hop override
Gateway of last resort is not set
O IA 3.0.0.0/8 [110/128] via 2.1.1.2, 00:07:04, Serial3/1
O 10.0.0.0/8 [110/65] via 1.1.1.1, 00:06:54, Serial3/0
11.0.0.0/32 is subnetted, 4 subnets
O 11.0.0.1 [110/65] via 1.1.1.1, 00:03:54, Serial3/0
O 11.0.1.1 [110/65] via 1.1.1.1, 00:03:54, Serial3/0
O 11.0.2.1 [110/65] via 1.1.1.1, 00:03:54, Serial3/0
O 11.0.3.1 [110/65] via 1.1.1.1, 00:03:54, Serial3/0
13.0.0.0/32 is subnetted, 4 subnets
O 13.0.0.1 [110/65] via 2.1.1.2, 00:01:04, Serial3/1
O 13.0.1.1 [110/65] via 2.1.1.2, 00:01:04, Serial3/1
O 13.0.2.1 [110/65] via 2.1.1.2, 00:01:04, Serial3/1
O 13.0.3.1 [110/65] via 2.1.1.2, 00:01:04, Serial3/1
14.0.0.0/24 is subnetted, 4 subnets
O E2 14.0.0.0 [110/20] via 2.1.1.2, 00:05:34, Serial3/1
O E2 14.0.1.0 [110/20] via 2.1.1.2, 00:05:34, Serial3/1
O E2 14.0.2.0 [110/20] via 2.1.1.2, 00:05:34, Serial3/1
O E2 14.0.3.0 [110/20] via 2.1.1.2, 00:05:34, Serial3/1
O 30.0.0.0/8 [110/65] via 2.1.1.2, 00:07:04, Serial3/1
R2#show ip ospf neighbor
Neighbor ID Pri State Dead Time Address Interface
13.0.3.1 0 FULL/ - 00:00:39 2.1.1.2 Serial3/1
11.0.3.1 0 FULL/ - 00:00:37 1.1.1.1 Serial3/0
R1#traceroute 14.0.0.1
Type escape sequence to abort.
Tracing the route to 14.0.0.1
VRF info: (vrf in name/id, vrf out name/id)
1 1.1.1.2 36 msec 36 msec 32 msec
2 2.1.1.2 124 msec 128 msec 144 msec
3 3.1.1.2 132 msec 92 msec 108 msec
we configure area 10 as stub, ensure that no external routes enter in to area 10.
R1(config)#router ospf 1
R1(config-router)#area 10 stub
R2(config)#router ospf 1
R2(config-router)#area 10 stub
R2#show ip ospf neighbor
Neighbor ID Pri State Dead Time Address Interface
13.0.3.1 0 FULL/ - 00:00:31 2.1.1.2 Serial3/1
11.0.3.1 0 FULL/ - 00:00:34 1.1.1.1 Serial3/0
R1#show ip route ospf
Codes: L - local, 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, H - NHRP, l - LISP
+ - replicated route, % - next hop override
Gateway of last resort is 1.1.1.2 to network 0.0.0.0
O*IA 0.0.0.0/0 [110/65] via 1.1.1.2, 00:02:22, Serial3/0
O IA 2.0.0.0/8 [110/128] via 1.1.1.2, 00:02:22, Serial3/0
O IA 3.0.0.0/8 [110/192] via 1.1.1.2, 00:02:22, Serial3/0
12.0.0.0/32 is subnetted, 4 subnets
O IA 12.0.0.1 [110/65] via 1.1.1.2, 00:02:22, Serial3/0
O IA 12.0.1.1 [110/65] via 1.1.1.2, 00:02:22, Serial3/0
O IA 12.0.2.1 [110/65] via 1.1.1.2, 00:02:22, Serial3/0
O IA 12.0.3.1 [110/65] via 1.1.1.2, 00:02:22, Serial3/0
13.0.0.0/32 is subnetted, 4 subnets
O IA 13.0.0.1 [110/129] via 1.1.1.2, 00:02:22, Serial3/0
O IA 13.0.1.1 [110/129] via 1.1.1.2, 00:02:22, Serial3/0
O IA 13.0.2.1 [110/129] via 1.1.1.2, 00:02:22, Serial3/0
O IA 13.0.3.1 [110/129] via 1.1.1.2, 00:02:22, Serial3/0
O IA 20.0.0.0/8 [110/65] via 1.1.1.2, 00:02:22, Serial3/0
O IA 30.0.0.0/8 [110/129] via 1.1.1.2, 00:02:22, Serial3/0
R1#ping 14.0.0.1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 14.0.0.1, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 80/90/100 ms
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