Configuring OSPF for a Stub Area

Configuring OSPF for a Stub Area
First, we’ll configure RouterB. Notice that RouterB is an ABR and that it is the only ABR in
Area 25, as recommended in our stub area design guidelines. When configuring an ABR that is
a member of a stub area, be cautious to configure only the stub area as stubby:
RouterB(config)#router ospf 10
where 10 is the Process ID.
RouterB(config-router)#network 1.0.0.0 0.255.255.255 area 0
where 1.0.0.0 0.255.255.255 is the network and wildcard mask of a network connected to
RouterB and where 0 is the area that network 1.1.1.0/24 is a member of.
RouterB(config-router)#network 10.0.0.0 0.255.255.255 area 25
where 10.0.0.0 0.255.255.255 is a summary network and wildcard mask of networks connected
to RouterB and where 25 is the area that networks 10.1.1.0/24 and 10.1.2.0/24 are members of.
RouterB(config-router)#area 25 stub
Totally Stubby Area Configuration 187
where 25 is the area that we have designated as stubby.
Notice that instead of using two network statements to represent networks 10.1.1.0/24 and
10.1.2.0/24, we used a single network statement specifying network 10.0.0.0/8, which will include
all the interfaces contained in these two networks. Using a summarized network statement like this
will by no means alter any routes that will be advertised. In other words, advertisements will still be
sent for both networks and a summary address of the two networks will not be sent.
We will also use the 10.0.0.0/8 summary when we configure RouterC and RouterD. Remember
that it’s critical that all routers that are members of a stub area be configured as stubby for that area.
Therefore, RouterC and RouterD will have identical OSPF configurations:
RouterC(config)#router ospf 1
RouterC(config-router)#network 10.0.0.0 0.255.255.255 area 25
RouterC(config-router)#area 25 stub
RouterD(config)#router ospf 100
RouterD(config-router)#network 10.0.0.0 0.255.255.255 area 25
RouterD(config-router)#area 25 stub
Let’s review some key elements of our stub area configuration example:
 The syntax to make a router stubby is area area-id stub.
 All routers that are part of Area 25 are configured as stubby.
 Area 25 has only one ABR (i.e., only one path out of the area).
 The ABR used the area area-id stub command only for Area 25, not for Area 0, which
is not stubby.