Contents
Why is Area 0 the backbone area in OSPF?
Albeit OSPF is a link state protocol, the way OSPF handles inter-area traffic leaves it prone to routing loops. This is why OSPF must connect back to area 0 – to avoid routing loops.
What is the difference between backbone area and stub area in OSPF?
The backbone is considered a standard area. Stub areas can contain type 1, 2, and 3 LSAs. A default route is substituted for external routes. Totally stubby areas can only contain type 1 and 2 LSAs, and a single type 3 LSA.
Which area of OSPF network is called backbone area?
area 0
All areas in an OSPF autonomous system must be physically connected to the backbone area (area 0). In some cases where this physical connection is not possible, you can use a virtual link to connect to the backbone through a non-backbone area.
Why area is used in OSPF?
In OSPF, a single autonomous system (AS) can be divided into smaller groups called areas. This reduces the number of link-state advertisements (LSAs) and other OSPF overhead traffic sent on the network, and it reduces the size of the topology database that each router must maintain.
How do I enable OSPF?
To enable OSPF on a Cisco router and advertise interfaces, the following tasks are required:
- Use the command router ospf process ID to start OSPF.
- Use the network command to enable the interfaces.
- Identify area assignments.
- (Optional) Assign the router ID.
How does OSPF avoid loops?
Because inter-area OSPF is distance vector, it is vulnerable to routing loops. It avoids loops by mandating a loop-free inter-area topology, in which traffic from one area can only reach another area through area 0.
Why do we use area in OSPF?
What are the not so stubby areas in OSPF?
Not-so-stubby areas (NSSAs) are an extension of OSPF stub areas. Like stub areas, they prevent the flooding of AS-external link-state advertisements (LSAs) into NSSAs and instead rely on default routing to external destinations.
Why is there a need for a backbone area in OSPF?
Because OSPF is link state the LSDB must be identical within an area. This is to ensure that the SPF calculation is consistent and to prevent routing loops. If running single area OSPF any area can be used, there is no requirement to have area 0.
Can a stub area be configured as a backbone area?
Let’s start with a nice topology with 5 areas. In the middle you’ll find the backbone area and the other areas are configured as the different stub area types. If you configure an area as stub it will block all type 5 external LSAs. All the prefixes that you redistributed into OSPF from another routing protocol are not welcome in the stub area.
Can a IGRP route be propagated into an OSPF stub area?
In the network diagram, Area 1 is defined as a stub area. IGRP routes cannot be propagated into the OSPF domain because redistribution is not allowed in the stub area. However, if we define Area 1 as NSSA, we can inject IGRP routes into the OSPF NSSA domain with the creation of type 7 LSAs.