Saturday 21 February 2015
In Case of OSPF LSA type 5, there is a field in the update "forwarding-address", this field, you will generally see having a value 0.0.0.0 but in some cases, it will be a non-zero IP address. We will discuss this field in this post.
When LSA 5 is generated by ASBR in an area and forwarding address is set to 0.0.0.0 then it means that the other routers will be sending traffic for that particular subnet towards the ASBR which is the originator of this update.
But, if the following conditions are matched the forwarding address will be set to the "next-hop address for the subnet" on ASBR.
- The next-hop interface on ASBR is enabled with the OSPF process and
- The next-hop interface is non-passive and
- The next-hop interface is not running point-to-point OSPF and
- The next-hop interface is not running point-to-multipoint OSPF and
- The next-hop interface address of the ASBR falls in the same subnet that is defined under router OSPF
Any other condition will set the forwarding address to 0.0.0.0
To understand the significance of this architecture we will use following network topology
In this topology only ROUTER-3 is performing bi-directional redistribution between OSPF and RIP, so initially from R1 to R5 traffic is following the path R1-R2-R3-R5 and also the forwarding address for the network 5.5.5.1 on R1 can be seen as 0.0.0.0 (show ip ospf database external 5.5.5.1)
Now let us make the conditions favorable for a non-zero forwarding address for LSA-5 (5.5.5.1) by completing the following steps.
- Enable OSPF on R3's RIP side interface(10.0.0.3).
- This will be an OSPF broadcast interface as it is an Ethernet interface.
- The next-hop for subnet 5.5.5.1 lies in the same subnet which is defined under router ospf
If we check on R1 now, the forwarding address for 5.5.5.1 is 10.0.0.5 and R1 will do a recursive lookup and look for the best forward-metric for subnet 10.0.0.5
The traffic will still travel through path R1-R2-R3-R5 because 10.0.0.0/24 subnet is visible to R1 via R3 only.
But if we observe the topology closely, we see that R1-R4-R5 is the best path to make communication between R1 and R5
Let us advertise the 10.0.0.0/24 subnet into OSPF on R4 as well, which will give R1 two options to reach 10.0.0.0/24. It can be seen clearly from topology that R1-R4 is the best path to reach 10.0.0.0/24 subnet and hence R1 gets a lower forward-metric.
Now, 5.5.5.1 network is redistributed by ASBR R1 into the OSPF but the forward metric to the next hop of this subnet (10.0.0.5) is lower from the ASBR R4 so, R1-R4-R5 will be the preferred path for R1-R5 communication.
