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OSPF NSSA Filtering

OSPF NSSA Filtering
Di Posting Oleh : Simple Learning
Kategori : HCv1.0 Intermediate OSPF





Task 1
On SW2 create two loopbacks and configure IP addresses as shown below.
  • Loopback1: 192.168.8.8/24
  • Loopback2: 192.168.88.8/24
Configure OSPF area 28 in such a way, that R2 does not allow external prefixes but allows SW2 redistribute two new loopbacks into OSPF as external LSAs (SW2 is becoming ASBR). Ensure that routers in area 28 do have connectivity to all external prefixes already advertised.

Questions
Try to answer the following questions:
  1. What kind of solution can solve the problem?
  2. What is a potential pitfall using this solution?
Lab Solution

Solution configuration can be accessed below; solution contains task 1 and task 2 (if you want to save it, click the link, then go to File-->Download):
https://drive.google.com/file/d/0BwE5C95tpjZOd29BZ3hqZVVSM28/edit?usp=sharing

Task 1
On SW2 create two loopbacks and configure IP addresses as shown below.
  • Loopback1: 192.168.8.8/24
  • Loopback2: 192.168.88.8/24
Configure OSPF area 28 in such a way, that R2 does not allow external prefixes but allows SW2 redistribute two new loopbacks into OSPF as external LSAs (SW2 is becoming ASBR). Ensure that routers in area 28 do have connectivity to all external prefixes already advertised.

SW2 Configuration:
!
interface Loopback1
 ip address 192.168.8.8 255.255.255.0
!
interface Loopback2
 ip address 192.168.88.8 255.255.255.0
!
!
route-map CONN_TO_OSPF permit 10
 match interface loopback1 loopback2
!
router ospf 1
 no area 28 stub
 area 28 nssa
  redistribute connected subnets route-map CONN_TO_OSPF
!

In order to match the area a quick jump to R2.

R2 Config:
!
router ospf 1
 no area 28 stub
 area 28 nssa default-information-originate
!

Note!
R2 is ABR in area 28. While configuring NSSA area, ABR does NOT inject the default route by default. This way, all routers in area 28 (SW2) would not be able to reach other prefixes that area external ones redistributed in OSPF domain outside area 28. Thus, the area 28 nssa default-information-originate command on R2.

Pic. 2 - LSA Type 7 on SW2 (NSSA).


Pic. 3 - SW2 NSSA 7 Converted to LSA5 in Area 0.



Study Drill

OSPF Not-So-Stubby Area allows filtering LSA 5 but allows a presence of ASBR. The difference is that ASBR introduces external prefixes as LSA 7 (NSSA). ABR converts them into LSA 5 while introducing them into area 0.

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