Chapter 8 - OSPF

What are the 2 main versions of OSPF in networks today?

OSPFv2

OSPFv3

What are LSAs and where are they stored?

LSAs (Link State Advertisements) are packets of information that routers exchange to build and maintain a map of the network topology. They contain information about the router's directly connected links, including the neighbor routers, link costs, and other relevant details.

LSAs are stored in the Link State Database (LSDB) of each router. This database is a central repository that contains all the LSAs received by the router.

What is the function of the LSBD?

It provides the router with a complete map of the network. It uses Dijkstra’s SPF algorithm to construct a loop-free topology of shortest paths.

How would OSPF make a toplogy more manageable?

Through the use of Areas

What are OSPF Areas and where are they stored?

OSPF areas are segments of an OSPF routing domain.

Areas are stored within the LSBD

What are some of the drawbacks to OSPF areas?

• A full SPT calculation runs when a change is detected in the area

• The LSBD increases in size for a single area and makes it unmanageable

• No summarization of routes occur within a single area

What does segmenting the OSPF Domain do?

Reduces the size of the LSBD for each area. This makes SPT calc faster and decreases LSBD flooding between routers when a link flaps.

What is Area 0?

• Referred to as the backbone area

• Area 0 connects other non-backbone areas by routing advertisements into other non-backbone areas from Area 0

What are Area Border Routers (ABR)

OSPF routers connected to Area 0 and another OSPF area

Responsible for directing routes from one area into another area.

How does OSPF run (what ports, IPs etc)

• OSPF runs over protocol 89

• Uses multicast when necessary:

  • 224.0.0.5 for all routers running OSPF

  • 224.0.0.6 for all designated routers

What is the Router ID?

Router ID is a unique identifier assigned to each router in an OSPF domain. It is used to:

• Identify the router: It serves as a unique identifier for the router within the OSPF network.

• Elect the Designated Router (DR) and Backup Designated Router (BDR): The router with the highest Router ID becomes the DR, and the router with the second-highest Router ID becomes the BDR. These roles are important for efficient routing within a multicast group.

What are OSPF hello packets?

OSPF hello packets are used to establish and maintain neighbor relationships between routers in an OSPF domain. They are sent periodically by each router to its directly connected neighbors.

Key functions of OSPF hello packets:

• Neighbor discovery: Routers use hello packets to discover and establish neighbor relationships with other routers that are directly connected to them.

• Neighbor keepalive: Hello packets are also used to maintain existing neighbor relationships. If a router does not receive a hello packet from a neighbor within a specified interval, it considers the neighbor to be down.

• Router dead interval: The hello packets also help determine the router dead interval, which is the time after which a router is declared dead if no hello packets are received.

• Designated Router (DR) and Backup Designated Router (BDR) election: Hello packets are used to elect the DR and BDR for a multicast group. The router with the highest Router ID becomes the DR, and the router with the second-highest Router ID becomes the BDR.

what are OSPF neigbours?

OSPF neighbors are routers that are directly connected to each other within an OSPF domain. They exchange routing information with each other to build and maintain a map of the network topology.

What is the ASBR?

(Autonomous System Boundary Router) is a router that connects an OSPF domain to an external Autonomous System (AS).

What is an external route and what are the 2 types?

An external route is a route that originates from outside of an OSPF domain.

There are two types of external routes:

1. Type 1 external routes: These routes adds the total path metric to the ASBR. This increases the metric. They are typically used for routes that are specific to a particular destination.

2. Type 2 external routes: These are routes in which the metric is the same for every router. Default is 30. Used if you only have one exit in the network.

How does the DR help network resources?

It reduces the amount of OSPF adjacencies being flooded in the network as routers only form full adjacencies with the DR and not each other.

What is a pseudonode ?

A virtual router which manages adjacency states with all other routers on that broadcast network segment

What is the DR responsible for?

Flooding all updates to all OSPF routers on that segment as updates occur.

Explain the DR/BDR election process

The election process involves the following steps:

1. Hello Packet Exchanges: Routers exchange hello packets to discover each other and establish neighbour relationships.

2. Priority Comparison: Each router advertises its priority level in its hello packets. A higher priority value indicates a higher preference to become the DR or BDR.

3. Router ID Comparison: The router with the highest Router ID is chosen if multiple routers have the same priority level.

4. DR Election: The router with the highest priority and Router ID becomes the DR.

5. BDR Election: The router with the second-highest priority and Router ID becomes the BDR.

What failure detection does OSPF have?

Hello and dead interval timers

these timers determine how long the OSPF process waits before declaring a neighbour state to be down

What Authentication types does OSPF support ?

MD5 and Plaintext