OSPF FORWARD ADDRESS CONCEPT IN NSSA
In this article, we will discuss OSPF Forward Address selection criteria in NSSA along with the OSPF FORWARD ADDRESS CONCEPT IN NSSA with the help of some useful lab scenarios. Let us begin with discussing the Forward Address selection criteria.
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.
Let us now understand the OSPF FORWARD ADDRESS CONCEPT IN NSSA with help of below lab topology.
OSPF FORWARD ADDRESS CONCEPT IN NSSA - 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#show ip ospf database nssa-external 9.9.0.4 | i Forward
Forward Address: 9.9.34.4
R1#show ip ospf database external 9.9.0.4 | i Forward
Forward Address: 9.9.34.4
OSPF FORWARD ADDRESS CONCEPT IN NSSA - 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#show ip ospf database nssa-external 9.9.0.4 | i Forward
Forward Address: 9.9.0.3 >>>>>>>> Loopback 0
R1#show ip ospf database external 9.9.0.4 | i Forward
Forward Address: 9.9.0.3
OSPF FORWARD ADDRESS CONCEPT IN NSSA - 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#show ip ospf database nssa-external 9.9.0.4 | i Forward
Forward Address: 33.33.33.33 >>>>>>> Loopback 1 on R3
R1#show ip ospf database external 9.9.0.4 | i Forward
Forward Address: 33.33.33.33
OSPF FORWARD ADDRESS CONCEPT IN NSSA - Scenario 4
If none of Loopback interfaces are up on ASBR, the internal physical interface address is taken as Forward address.
R2#show 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#show ip ospf database external 9.9.0.4 | i Forward
Forward Address: 9.9.23.3
OSPF FORWARD ADDRESS CONCEPT IN NSSA - 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:
R1#show 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.
First: 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
Second: 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#show 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#show 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
