subject: OSPF NSSA For Beginners (ABR As Default Gateway for External and Internal Addresses 3.2) [print this page] 1. What NSSA? 1. What NSSA?
Answer: This is an area stub that can import external destinations in OSPF ASBR.
2. How Does NSSA?
Answer: Let's describe a scenario as follows:
- ASBR learns a new external address, creates a type 7 LSA and floods in NSSA.
- Routers internal receive this LSA and create a new entry for this address in their routing tables.
- ABR receives a Type 7 LSA, converts an external LSA and floods in other areas. ABR LSA indicates that it is the external origin. It is, behaves like an ABR ASBR for routers in other areas.
- Routers in other areas think the ASBR ABR is for this external address specified in LSA type 5.
3. How do you compare Stub and NSSA area?
Answer: Commons: They do not support type 4, 5 LSA. internal use roads as defult gateway ABR.
Differences: There are three differences:
1) Stub zone can not import external destinations.
2) Stub zone ABR routers use to access the outside world. NSSA access addresses for external routers in two ways: a) For addresses announced by NSSA ASBR, ASBR use as next hop. b) For other external addresses using ABR as the next hop.
3) Type 7 NSSA LSA uses
4) In NSSA, ABR plays a dual role. It is an ABR and ASBR. It creates Type 5 LSA for the external address announced by ASBR and flooding in other areas.
4. What do ABR routers in other areas think it's a ASBR?
Answer: The short answer is just E. , We consider an ABR connects Zone 0, 1. The area is an NSSA.
1) When configuring a router NSSA ABR in Area 1, Area 0 ABR changes router LSA by setting its E bit to 1. This means that ABR is an ASBR for zone 0. Then re-ABR's router LSA flooding in Zone 0.
2). When Zone 1 of ASBR learns a new external address, it floods the LSA type 7 in Zone 1.
3) When ABR receives this LSA, it converts outside LSA and floods the LSA in Zone 0.
4) When area 0 external routers LSA receive ABR, they see that ABR ASBR from the LSA.
5. How does a home in a non-NSSA send packets to destinations outside discovered by NSSA ASBR's?
Answer. It takes 3 steps:
1) host uses non-NSSA router as default gateway and sends packets to it.
2) The router forwards packets to non-NSSA ABR zone next hop. This external address is given in an LSA type 5, which was issued by SLA. ESAF As these routers (in non-NSSA) CONCERE, ABR ASBR for these is their external addresses.
3) ABR forwards packets to the NSSA ASBR.
4) ASBR forwards packets to the next hop in foreign> AS
This article is a simulation FAQ OSPF: Not-So-Stub-Zone. To play this interactive simulation, see external links below....
OSPF NSSA For Beginners (ABR As Default Gateway for External and Internal Addresses 3.2)
