Overview: Stub area.
Goal: Routers in a stub area don;t keep external addresses. They delgate to ABR. This simulation shows the process of deleting external address from routing table and add 0.0.0.0 as default route.
Topology: Two AS. OSPF AS has two areas, R2 is ABR. RIP AS contains R3, R4. R3 is ASBR.
Steps: 1) Run OSPF at R1, R2, R3. Run RIP at R3, R4.
2) Configure R1, R2 as stub area 1 and make R2 (ABR) a default gateway to access external addresses.
3) Configure R2 as Total stub area 1 and make it a gateway to access addresses in other areas.
Init: Create LSAs
Run RIP on R3, R4. Run OSPF at R1, R2, R3. Configure R3 and impirt address from RIP (redistribution).
- R1, R2, R3 created theor Router LSA (type 1).
- R2 is ABR. It also generates 2 additonal Network Summary LSA (type 3).
- R3 is ASBR. It generates 2 External LSA (type 5).
（There is no Network LSA since routers are connected by serial links.)
Synch LSA within area
In Area 1, routers exchange Update to sycnh their LSA. R1 receives 3 LSA from R2. R2 receives 1 LSA from R1.
In area 0, R3 receives 2 LSA from R2. R2 receives 3 LSA from R3. R2 also creates 2 LSA:
A Network Summary LSA (R1's stub network 172.16.1.0 / 24)
An ASBR Summary LSA (type 4).
R2 sync LSA across areas
R2 is ABR. Aftre updating LSDB from receiving R1 and R3's LSAs, it sends Update again:
- 4 LSAs to Area 0: Router LSA (R1, R2) ； Network Summary LSA (subnet 172.16.255.4/ 30；
ASBR Summary LSA (R3)； External LSA (RIP AS的192.168.1.0, 10.0.0.0)。
- 3 LSAs to area 1: Router LSA (R2, R3)； Network Summary LSA (172.16.255.0 / 30)；
External LSA (192.168.1.0, 10.0.0.0)。
Configure area 1 as a stub area
To make area 1 a stub area, on each area 1 router (R1, R2), type "area 1 stub"
It triggers following actions:
- R1 deletes type 4, 5 LSA (ASBR Summary LSA, External LSA) .
- R2 (ABR) deletes type 4 LSA and creates a Network Summary LSA 0.0.0.0
R1's default gateway is R2
R2 floods an Update to Area 1. This Update contains a type 3 LSA 0.0.0.0. It intends to add a default route to area 1 routers.
When R1 receives this LSA, it learns that R2 (ABR) is a default gateway to reach external networks..
Without any type 4, 5 LSA, R1's routing table does not contains any external address.
Now R1's routing table contains OSPF subnets and a default route to reach R2.
Configure Totally Stub Area
To make R1's routing table even smaller, configure ARea 1 as Totally Stub. At R2 (ABR), type command:
area 1 stub no-summary.
Summary Network LSA (Type = 3) is deleted (except for Link State ID = 0.0.0.0）.
Now R2 is an area gateway for area 1 routers. R2 sends an Update to R1 to notify the change.
R2 deletes type 3 LSA (area 1)
R2 floods an Update to Area 1. The Update contains all Network Summary LSA with age 360.
When R1 receives Update, R1 deletes obsolete LSAs from its LSDB (age=3600 means it reaches maximum age).
Now R1 contains only type 1 LSA (Router).
R2 sends default route 0.0.0.0
Then R2 sends another Update which contains a Newtrok Summary LSA with Link State ID = 0.0.0.0.
Upon receiving this LSA, R1 adds it to LSDB.
Now R1 does not have type 3, 4, 5 LSA. In Total Stub Area, internal routers use ABR to forward all packets outside its area.
R2: default gateway
R1 receives type 3 default address 0.0.0.0 from R2.
Now R1 only knows addresses in area 1. R1's routing table becomes much smaller.
To reach addresses outside area 1 (in other areas or outside OSPF AS), R1 forwards packets to ABR and let it figure out how to forward.
1. What is Stub Area?
Answer: Internal routers don't keep external destinations. They use ABR as default gateway to reach external destinations outside OSPF AS. That is, routers don't have type 4, 5 LSA.
2. What is Totally Stub?
Answer: Routers don't even carry addresses from other areas. They use ABR as default gateway to reach destinations in other areas as well as external destinations. They don't keep type 3, 4, 5 LSA (except a default address 0.0.0.0 which is type 3)
3. Why Stub Area?
Answer: An OSPF internal router needs External LSA (type 5) and ASBR Summary LSA (type 4) to reach destinations in the outside world (non-OSPF addresses). However, when the number of external addresses grow to thousands, it's getting difficult to maintain. To solve this problem, we can make the area stub: Internal routers don't keep external addresses any more. They use ABR as the default gateway when want to access the outside world. This is very much the same way as hosts. They focus on running applications and rely on default gateway to access Internal.
4. Only ABR is configured as Totally Stub Area, not internal routers. Why is that?
Answer: Internal routers now don't need to track OSPF network addresses from other areas. They rely on ABR as the default gateway. Therefore, they don't need to keep Type 3 LSA anymore. They could delete Type 3 LSA when configured in an aream that is totally stub. But it takes several steps of manual operations. It is simpler to let the originator of Type 3 LSA - ABR - to cancel these LSA it announced earlier. ABR simply re-floods Type 3 LSA with age 3600.
5. When configure ABR as a Stub Area router, it deletes type 4 in Stub Area, not type 5 LSA. Why?
Answer: Type 4 is ASBR Summary LSA. It tells internal routers in how to reach ASBR when accessing external addresses. ABR creates and floods this LSA to internal routers. In a Stub Area, internal routers use ABR as default gateway to reach external addresses. They don't need to know how to reach ASBR any more. Therefore, ABR does not need to keep Type 4 LSA in Stub Area.
But External LSA is different. If deleted, ABR won't be able to forward packets to external destinations for internal routers.
6. When a host in a Stub Area ping an external destination, how is the ping packet being forwarded?
Answer: Here is the analysis of end-to-end forwarding path between a host and an external destination:
- The host forwards ping to its default gateway, which is an internal router.
- The router's routing table does not have the external address, it forwards ping to its default gateway, ABR.
- ABR uses the external address to lookup its routing table, finds a match, and forwards it to ASBR.
- ASBR forwards ping to next hop in an alien AS.
- The next hop forwards ping to next hops until ping is delivered to the external address.
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