Discussion

default summary from an ABR to reach external ASs. Because a stub area has no
external routes, it cannot connect to an external area (that is, it cannot contain an
ASBR) and you cannot redistribute routes from another protocol into the stub area.
You might use stub areas when much of the topological database consists of AS
external advertisements because it reduces the size of the topological databases and
therefore the amount of memory required on the internal routers in the stub area.
Another restriction on stub areas is that you cannot create a virtual link through
them.
Not-so-stubby areas (NSSAs) are a variant of stub areas that allows a stub area to
connect to an external network. This allows external routes originated by ASBRs
within the areas to be flooded in Type 7 LSAs and then leaked into other areas. However,
external routes from other areas are not flooded into the NSSA.
For more background information about OSPF, see OSPF and IS-IS: A Comparative
Anatomy at http://www.nanog.org/mtg-0006/katz.html.
12.1 Configuring OSPF
Problem
Your want to configure OSPF on a JUNOS router.
Solution
You enable OSPF by defining the interfaces on which it will run and the area to
which the interfaces will be attached:
[edit protocols]
aviva@RouterG# set ospf area 0.0.0.0 interface fe-0/0/1.0
aviva@RouterG# set ospf area 0.0.0.0 interface fe-1/0/1.0
Discussion
The basic setup for configuring a single OSPF area is straightforward. Enable the protocol
on all router interfaces that will participate in the OSPF domain and specify which
area the interfaces are in. In this recipe, area 0 has three routers (see Figure 12-1).
In this recipe, we configure OSPF on two interfaces of a router that is in the backbone
area, which has an area identifier of 0.0.0.0. In addition, the router must have
a router ID to identify the router from which OSPF packets originate. In this recipe,
we don’t set one explicitly because we have configured a unicast IP address on the
router’s lo0 interface and this address is used as the router ID:
aviva@RouterG> show configuration interfaces lo0
unit 0 {
family inet {
address 192.168.19.1/32;
}
}
384 | Chapter 12: OSPF
This is the Title of the Book, eMatter Edition
Copyright © 2008 O’Reilly & Associates, Inc. All rights reserved.