OSPF FORWARD ADDRESS CONCEPT IN NSSA

 Forward Address selection criteria in NSSA (for LSA 7 and corresponding LSA 5):

By default, preference should be given first to the router’s internal addresses(Loopback addresses). If internal addresses are not available, preference should be given to the router’s active OSPF transit/stub network addresses. These choices avoid the possible extra hop that may happen when a transit network’s address is used. When the interface whose IP address is the LSA’s forwarding address transitions to a Down state the router must select a new forwarding address for the LSA and then re- originate it. If one is not available the LSA should be flushed. Below is the brief Forward Address election criteria in NSSA:


OSPF enabled loopback interface IP address. (Lowest Loopback IP).
OSPF enabled non-loopback interface IP address that is connected to a transit/stub network.
OSPF enabled non-loopback interface IP address that is connected to a non-transit/stub network.

Consider the diagram below: 



 Scenario 1:

In NSSA, When R3-R4 link is advertised in OSPF on ASBR/R3,  Forward-address in external LSA7 and LSA5 for 9.9.0.4/32 shows 9.9.34.4

R2#sh ip ospf database nssa-external 9.9.0.4 | i Forward
        Forward Address: 9.9.34.4

R1#sh ip ospf database external 9.9.0.4 | i Forward
        Forward Address: 9.9.34.4
 



Scenario 2:

When R3-R4 link is not advertised in OSPF on ASBR/R3, forward-address in external LSA5 for 9.9.0.4/32 shows the Internal Loopback IP (oldest Loopback IP) configured on the R3.


R2#sh ip ospf database nssa-external 9.9.0.4 | i Forward
        Forward Address: 9.9.0.3   >>>>>>>> Loopback 0

R1#sh ip ospf database external 9.9.0.4 | i Forward
        Forward Address: 9.9.0.3
 



Scenario 3:

If we shut the loopback 9.9.0.3 on  interface on ASBR/R3. The next available Loopback address will we taken as Forward address.

R2#sh ip ospf database nssa-external 9.9.0.4 | i Forward
        Forward Address: 33.33.33.33   >>>>>>>>>>>>>>>>>>>>> Loopback 1 on R3

R1#sh ip ospf database external 9.9.0.4 | i Forward
        Forward Address: 33.33.33.33 
 



Scenario 4:

If none of Loopback interfaces are up on ASBR, the internal physical interface address is taken as Forward address.

R2#sh ip ospf database nssa-external 9.9.0.4 | i Forward
        Forward Address: 9.9.23.3   >>>>>>>>>>> R3’s physical interface address connected to R2

R1#sh ip ospf database external 9.9.0.4 | i Forward
        Forward Address: 9.9.23.3
 



Scenario 5:

When we have 2 ABRs in the network. The LSA7 will be translated by the ABR with highest Router ID.

Consider the diagram below:

As we have 2 ABRs R2 and R5, in above scenario, the traffic for External prefix 9.9.0.4 will load shared between R2 and R5 as checked on R1.


R1#sh ip route 9.9.0.4
Routing entry for 9.9.0.4/32
Known via “ospf 1”, distance 110, metric 20, type extern 2, forward metric 3
Last update from 9.9.15.5 on FastEthernet0/1, 00:00:01 ago
Routing Descriptor Blocks:
9.9.15.5, from 9.9.0.5, 00:00:01 ago, via FastEthernet0/1

Route metric is 20, traffic share count is 1
* 9.9.12.2, from 9.9.0.5, 00:00:01 ago, via FastEthernet0/0
Route metric is 20, traffic share count is 1


Say in a CCIE exam, they ask that R1 should reach 9.9.0.4/32 R2 only.

The simple solution would be manipulating the OSPF metrics to prefer the R1-R2 path to reach 9.9.0.4/32 from R1. But, the examiner has also given the condition that metric change is not allowed.


The other feasible solution is as below:
————————————————————-
1st: We force  ABR R2 to do the LSA7 To LSA5 translation always, using below command:

R2(config)#router ospf 1
R2(config-router)#area 1 nssa translate type7 always
——————–
 

2nd: Also suppress Forward Address on ABR R2 with command below. This command will also change the Forward Address to 0.0.0.0.


R2(config)#router ospf 1
R2(config-router)#area 1 nssa translate type7 always suppress-fa
——————————————————

Now R1 will reach 9.9.0.4/32 only via R2. Below are outputs:
——————————————————–
R1#sh ip ospf database external 9.9.0.4
OSPF Router with ID (9.9.0.1) (Process ID 1)
Type-5 AS External Link States
LS Type: AS External Link
Link State ID: 9.9.0.4 (External Network Number )
Advertising Router: 9.9.0.2
LS Seq Number: 80000002
Metric: 20
Forward Address: 0.0.0.0
External Route Tag: 0

————————————————————-
R1#sh ip route 9.9.0.4
Routing entry for 9.9.0.4/32
Known via “ospf 1”, distance 110, metric 20, type extern 2, forward metric 1
Last update from 9.9.12.2 on FastEthernet0/0, 00:02:18 ago
Routing Descriptor Blocks:
* 9.9.12.2, from 9.9.0.2, 00:02:18 ago, via FastEthernet0/0
Route metric is 20, traffic share count is 1



 

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