In this series of posts lets configure OSPF Totally Stubby Area, but before proceeding further let’s summarise the below topology
Two OSPF Areas Area 0 and Area 5
R1, R2 and R4 are part of Area 0 and OSPF is configured on the directly connected links on each router ( R1 – R2 link , R1-R4 link)
R4 has four loop back interfaces loopback 1 (10.0.1.1) , loopback 2 (10.0.2.2) ,loopback 3 (10.0.3.3) and loopback 4 (10.0.4.4) ,these loopback interfaces networks are redistributed into OSPF
R2-R3 are part of Area 5, R2 happened to be a ABR
OSPF Area 5 is configured on the interfaces connected between R2-R3.
Currently Area 5 is a normal area and its not been configured as a totally stubby area, R2 installs the R4 loopback interface networks as Type 5 LSA and forward the same to Area 5
We can see from the below snap shot R2 received R4 loopback networks as Type 5 LSAs and the routes are installed as Type 2 External OSPF routes, also we could see the interface connecting R1-R4 are also advertised as Type 3 LSA
R3 sees R4 loopback interfaces network as Type 5 LSA and R1-R4 , R1-R2 links network 192.168.14.0/29 , 192.168.12.0/29 as Type 3 LSAs
In next post, let’s see by what impact Area 5 will have especially after configuring it as OSPF Totally Stubby Area.
We all know OSPF as routing protocol is one of the most widely used IGP protocol, OSPF happens to be the most scalable IGP protocol. OSPF also happens to be one of the complex protocols as it deals with various concepts and terminologies. One such a topic where people get confused is OSPF Area types.
I will try to simplify them and present them in an easy language, I am not going to reinvent the wheel, as one can find plenty of resources for OSPF.
What is a OSPF Stub Area ?
OSPF Stub Area basically filters out information of an OSPF database purely based on the LSA types, Basically, an ABR in a Stub Area prevents LSA type 5 to be flooded into a Stub Area, it removes Type 5 LSA and replaces them with a default route which is a Type 3 LSA. To simplify ABR creates a default route using LSA 3, listing a 0.0.0.0 with a subnet mask 0.0.0.0 and flood the same into the stub area. By using Stub Area feature one can reduce the CPU utilization of a Router.
From the above scenario, we can see Type 3 LSA is exchanged between Area 0 and 5, however, when a Type 5 LSA reaches R2 which is an ABR, it will strip External LSAs (Type 5 LSAs) and replace them with a default route towards the Router R3.
OSPF Stub Area is configured in Cisco Routers using an IOS command
router ospf 1
area 5 stub
In the upcoming post, let’s see how to configure OSPF stub area in Cisco Routers, we will build a sample topology using Cisco VIRL.