What are areas in OSPF?

Asked By: Maddie Hough | Last Updated: 12th January, 2020
Category: technology and computing computer networking
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An OSPF network can be divided into sub-domains called areas. An area is a logical collection of OSPF networks, routers, and links that have the same area identification. A router within an area must maintain a topological database for the area to which it belongs.

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Thereof, what are the OSPF area types?

OSPF area types

  • Type 1 - Represents a router.
  • Type 2 - Represents the pseudonode (designated router) for a multiaccess link.
  • Type 3 - A network link summary (internal route)
  • Type 4 - Represents an ASBR.
  • Type 5 - A route external to the OSPF domain.
  • Type 7 - Used in stub areas in place of a type 5 LSA.

Additionally, how do I set totally stubby area in OSPF? To configure an area as a totally stubby area you'd execute the area # stub no-summary in OSPF router configuration mode on the ABR. Totally NSSA – Is an area that permits LSA's 1, 2 and 7 while blocking 3 4 and 5. This stub area receives a default route from the ABR using a type 3 LSA.

Thereof, why areas are used in OSPF?

OSPF uses areas to simplify administration and optimize traffic and resource utilization. An area is simply a logical grouping of contiguous networks and routers. All routers in the same area have the same topology table and don't know about routers in the other areas.

What is an LSA in OSPF?

The link-state advertisement (LSA) is a basic communication means of the OSPF routing protocol for the Internet Protocol (IP). It communicates the router's local routing topology to all other local routers in the same OSPF area.

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How do OSPF areas work?

Understanding OSPF Areas
It means dividing routers inside a single autonomous system running OSPF, into areas where each area consists of a group of connected routers. The idea of dividing the OSPF network into areas is to simplify administration and optimize available resources.

Does OSPF use TCP or UDP?

Open Shortest Path First (OSPF) is a link-state routing protocol. OSPF does not use a TCP/IP transport protocol (UDP, TCP), but is encapsulated directly in IP datagrams with protocol number 89. BGP uses the Transmission Control Protocol (TCP) as its transport protocol, and is assigned the reserved port 179.

Why do we need 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. When running multi area OSPF, an ABR must be used.

How many types of LSA are there in OSPF?

LSA Type 3: Summary LSA. LSA Type 4: Summary ASBR LSA. LSA Type 5: Autonomous system external LSA. LSA Type 6: Multicast OSPF LSA.

OSPF LSA Types Explained.
Link Type Description Link ID
3 Connection to stub network. IP Network
4 Virtual Link Neighbor router ID

What is DR in OSPF?

OSPF uses a DR (Designated Router) and BDR (Backup Designated Router) on each multi-access network. OSPF uses a DR (Designated Router) and BDR (Backup Designated Router) on each multi-access network.

How many routers are there in one OSPF area?

Generally, an area should have no more than 50 routers. Areas that suffer with unstable links should be smaller. Number of areas supported by any one router: A router must run the link-state algorithm for each link-state change that occurs for every area in which the router resides.

How does OSPF elect Dr?

Two rules are used to elect a DR and BDR: router with the highest OSPF priority will become a DR. By default, all routers have a priority of 1. if there is a tie, a router with the highest router ID wins the election.

What are the benefits of OSPF?

OSPF supports/provides/advantages –
  • Both IPv4 and IPv6 routed protocols.
  • Load balancing with equal cost routes for same destination.
  • VLSM and route summarization.
  • Unlimited hop counts.
  • Trigger updates for fast convergence.
  • A loop free topology using SPF algorithm.
  • Run on most routers.
  • Classless protocol.

What is Area 0 called in OSPF?

The backbone area (Area 0) is the core of an OSPF network. All other areas are connected to it and all traffic between areas must traverse it. All routing between areas is distributed through the backbone area.

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.

What is OSPF area ID?

An area ID identifies the specific group, or area, to which a network or interface belongs. Therefore, OSPF uses area IDs to organize a topology and to determine the neighbor routers to which it should send the packets that build the link-state database (LSDB).

Why use OSPF virtual link?

For this purpose, virtual-links can be used to establish connection to backbone area through non-backbone area. Virtual-links are established between area border routers. Basic OSPF configuration with virtual-link.

What is not so stubby area in OSPF?

OSPF NSSA. RFC3101 describes an OSPF area called not-so-stubby area (NSSA). The motivation behind NSSA is to allow OSPF Stub areas to carry External routes (routes learned using other routing protocols like RIP, EIGRP, BGP, etc). Stub areas are defined as areas that are not capable of importing routes external to OSPF.

What is Asbr?

Asbr stands for autonomous system boundary router when ever you perform redistribution by using commands like redistribute connected subnets (to redistribute the connected subnets to that router) then that router is called as asbr.

Which OSPF network type uses a Dr?

Point to point, point to multipoint – the broadcast and non-broadcast is the only 2 network types that require a DR/BDR.

How is OSPF metric calculated?

Open Shortest Path First (OSPF) uses "Cost" as the value of metric and uses a Reference Bandwidth of 100 Mbps for cost calculation. The formula to calculate the cost is Reference Bandwidth divided by interface bandwidth. For example, in the case of 10 Mbps Ethernet , OSPF Metric Cost value is 100 Mbps / 10 Mbps = 10.

What is stub area flag in OSPF?

Stub area flag: Two routers have to also agree on the stub area flag in the Hello packets in order to become neighbors. Stub Areas. OSPF allows certain areas to be configured as stub areas. External networks, such as those redistributed from other protocols into OSPF, are not allowed to be flooded into a stub area.