Scalable OSPF Network Design
Question
Your OSPF network is currently operating with a single backbone area and you are concerned with the scalability of your design.
Which network design is a best practice for providing the scalability you desire?
Answers
Explanations
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A. B. C. D.B
The best answer is B. Implement areas in your network.
OSPF is a link-state routing protocol that operates by flooding link-state advertisements (LSAs) to all routers in the same OSPF domain. The OSPF domain is also known as an Autonomous System (AS) in the context of OSPF.
When all routers in the OSPF domain belong to a single area, the network is referred to as a single-area OSPF network. In this design, all routers participate in the same OSPF routing process and have a copy of the same link-state database (LSDB). As the OSPF network grows in size, the single-area design may lead to scalability issues, including excessive flooding, long convergence times, and high CPU utilization on the routers.
The solution to these scalability issues is to implement a hierarchical network design using multiple OSPF areas. In this design, the OSPF domain is divided into multiple areas, each with its own set of routers and links. A backbone area (Area 0) is defined, which connects all the other areas. The routers that connect to multiple areas are called Area Border Routers (ABRs) and they maintain separate link-state databases for each area they connect.
By dividing the OSPF domain into multiple areas, the following benefits can be achieved:
Reduced flooding: Since LSAs are not flooded beyond area boundaries, the amount of flooding is reduced, and the size of the LSDB is also reduced.
Faster convergence: Since routers within an area only have to worry about topology changes within their area, they converge faster.
Lower CPU utilization: Since routers only have to process LSAs within their area, their CPU utilization is reduced.
Better security: By confining OSPF traffic to a specific area, it is easier to apply security policies and control the flow of traffic.
Answer A, Eliminating all designated routers is not a recommended solution. Designated routers (DRs) and backup designated routers (BDRs) are used in multiaccess networks, such as Ethernet, to reduce the amount of OSPF traffic and flooding. They are elected within each network segment to represent the segment in the OSPF domain. Eliminating DRs would increase the amount of OSPF traffic and flooding.
Answer C, Reducing the database refresh timer value is not a recommended solution. The database refresh timer controls how often routers check for changes in the LSDB. Lowering the refresh timer would increase the amount of CPU utilization and flooding, which would worsen the scalability issues.
Answer D, Allowing each device to perform ASBR duties is not a recommended solution. An Autonomous System Boundary Router (ASBR) is a router that connects the OSPF domain to an external network, such as the Internet. Allowing each router to perform ASBR duties would lead to inconsistent routing and could potentially cause routing loops. It is recommended to have a centralized ASBR or a small number of ASBRs to maintain consistent routing.
