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.

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 9.9.34.4R2#show ip ospf database nssa-external 9.9.0.4 | i Forward Forward Address: 9.9.34.4R1#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 0R1#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 R3R1#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 R2R1#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:

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#show ip route 9.9.0.4Routing entry for 9.9.0.4/32Known via “ospf 1”, distance 110, metric 20, type extern 2, forward metric 3Last update from 9.9.15.5 on FastEthernet0/1, 00:00:01 agoRouting Descriptor Blocks:9.9.15.5, from 9.9.0.5, 00:00:01 ago, via FastEthernet0/1Route metric is 20, traffic share count is 1* 9.9.12.2, from 9.9.0.5, 00:00:01 ago, via FastEthernet0/0Route metric is 20, traffic share count is 1Say 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 1R2(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 1R2(config-router)#area 1 nssa translate type7 always suppress-faNow R1 will reach 9.9.0.4/32 only via R2. Below are outputs:R1#show ip ospf database external 9.9.0.4OSPF Router with ID (9.9.0.1) (Process ID 1)Type-5 AS External Link StatesLS Type: AS External LinkLink State ID: 9.9.0.4 (External Network Number )Advertising Router: 9.9.0.2LS Seq Number: 80000002Metric: 20

Forward Address: 0.0.0.0

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