Wednesday, August 3, 2011

Configuring OSPF - Totally Stubby Area

In this blogtorial we are going to talk about OSPF Totally Stubby Areas. If an area does not connect to any other areas or if the area is really a spoke then Totally Stubby Areas are the way to go. It will inject a default route and that's it. 


We will be using the same topology from my previous blogtorials. So let's get started. 


Objective: To configure Area 1 as a Totally Stubby Area and see how it affects the routing table of R5 (in Area 1). 


Below is the routing table for R5 before we make Area 1 into a totally stubby area. Notice E2/IA routes. 

R5#sh ip route
<-- output shortened -->
     172.16.0.0/16 is variably subnetted, 5 subnets, 2 masks
C       172.16.5.0/24 is directly connected, Loopback0
O IA    172.16.4.1/32 [110/2] via 10.1.1.4, 00:00:03, FastEthernet1/0
O E2    172.16.6.0/24 [110/20] via 10.1.1.4, 00:00:03, FastEthernet1/0
O E2    172.16.2.0/24 [110/20] via 10.1.1.4, 00:00:03, FastEthernet1/0
O E2    172.16.3.0/24 [110/20] via 10.1.1.4, 00:00:03, FastEthernet1/0
     10.0.0.0/24 is subnetted, 3 subnets
O E2    10.2.2.0 [110/20] via 10.1.1.4, 00:00:03, FastEthernet1/0
C       10.1.1.0 is directly connected, FastEthernet1/0
O IA    10.0.0.0 [110/2] via 10.1.1.4, 00:00:03, FastEthernet1/0

Now let's configure both R4 and R5 so area 1 becomes a totally stubby area. 

R4#sh run | sec ospf
router ospf 1
 log-adjacency-changes
 area 1 stub no-summary ** This command states that area 1 is a totally stubby area. Remember Area type is one of the criteria which has to match in-order to form OSPF neighbor relationship **
 network 10.0.0.0 0.0.0.255 area 0
 network 10.1.1.0 0.0.0.255 area 1
 network 172.16.4.0 0.0.0.255 area 0
 network 192.168.1.0 0.0.0.255 area 0

R5#sh run | sec ospf
router ospf 1
 log-adjacency-changes
 area 1 stub no-summary ** This command states that area 1 is a totally stubby area. Remember Area type is one of the criteria which has to match in-order to form OSPF neighbor relationship **
 network 10.1.1.0 0.0.0.255 area 1
 network 10.3.3.0 0.0.0.255 area 2
 network 172.16.5.0 0.0.0.255 area 1
 network 192.168.1.0 0.0.0.255 area 0

Now let's look at 'sh ip route' on R5 in Area 1 which is now a 'totally stubby area'.

R5#sh ip route
<-- output shortened -->
     172.16.0.0/24 is subnetted, 1 subnets
C       172.16.5.0 is directly connected, Loopback0
     10.0.0.0/24 is subnetted, 1 subnets
C       10.1.1.0 is directly connected, FastEthernet1/0
O*IA 0.0.0.0/0 [110/2] via 10.1.1.4, 00:01:33, FastEthernet1/0 ** Nothing but a default summary route ** 

So that's how easy it is to reduce the size of the OSPF database 'area x stub no-summary'. 

Wonder if some surfer in California came up with this "Dude that is a totally stubby area" :) 

Many more articles to come so stay tuned!! If you like my posts please subscribe!!