OSPF Routing Fundamentals

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Questions and Answers

What type of routers are F, G, and H?

Non-backbone internal routers (correct)

Backbone internal routers

Autonomous system boundary routers

Link-state routers

What route does the ABR router advertise to the non-backbone internal router?

A static route

A routing table

A dynamic route

A default route (correct)

What type of area can be configured in OSPF?

Stub area

Totally stubby area

Not-so-stubby area

All of the above (correct)

What is an ASBR?

Autonomous system boundary router Signup and view all the answers

What type of routing protocol maintains a complex database of the network topology?

Link-state protocol Signup and view all the answers

What is the purpose of link-state packets (LSPs)?

To construct a topological database using all received LSPs Signup and view all the answers

What triggers the LSP exchange in a network?

An event in the network Signup and view all the answers

What is the purpose of an SPF algorithm in link-state routing protocols?

To compute reachability to networked destinations Signup and view all the answers

What happens when a link-state router receives an LSA?

It updates its link-state database and forwards the LSA to all neighboring devices Signup and view all the answers

What can the process of link-state routing protocols discover?

Changes in the network topology caused by component failure or network growth Signup and view all the answers

What is the purpose of the link-state database?

To calculate the best paths through the network Signup and view all the answers

What is a link in the context of link-state routing?

An interface on a router Signup and view all the answers

Why are link-state protocols preferred in larger networks?

Because they support classless addressing and can be segmented into area hierarchies Signup and view all the answers

What algorithm is used by link-state routers to find the best paths to a destination?

Dijkstra's SPF algorithm Signup and view all the answers

What happens when a topology changes in a link-state routing protocol?

Routers always send updates when a topology changes Signup and view all the answers

What is a result of segmenting networks into area hierarchies?

Limited scope of route changes Signup and view all the answers

What is an advantage of link-state routing protocols over distance vector protocols?

More infrequent periodic refresh updates Signup and view all the answers

What is the purpose of the SPF tree?

To select the best paths for the routing table Signup and view all the answers

What is the advantage of using link-state routing protocol?

It allows the networked routers to converge faster Signup and view all the answers

What is the concern in Figure 4 when using link-state routing protocol?

There is no concern Signup and view all the answers

What is the purpose of metrics in link-state routing protocol?

To discriminate between different paths to the same destination Signup and view all the answers

Which router is the next hop for destination 185.134.0.0 from router A?

B Signup and view all the answers

What is the cost of the route from router A to destination 192.168.33.0?

1 Signup and view all the answers

Which router is the next hop for destination 10.0.0.0 from router B?

A Signup and view all the answers

What is the cost of the route from router C to destination 185.134.0.0?

2 Signup and view all the answers

Which router is the next hop for destination 192.168.157.0 from router C?

D Signup and view all the answers

What is the cost of the route from router D to destination 10.0.0.0?

2 Signup and view all the answers

What is the advantage of using a link-state routing protocol in a network?

It allows for quicker convergence after a topological change Signup and view all the answers

What is the main difference between link-state routing protocols and distance vector protocols?

Link-state routing protocols use events to drive updates Signup and view all the answers

What is the benefit of using a link-state routing protocol in terms of bandwidth availability?

It makes more bandwidth available for routing traffic Signup and view all the answers

What type of networks is link-state routing best suited for?

Larger and more complex networks Signup and view all the answers

What is the metric used by the OSPF protocol?

Cost, which is equal to 100Mbps / BW in Mbps Signup and view all the answers

How are OSPF update messages sent?

As multicast messages Signup and view all the answers

How often does OSPF send LSA refresh update messages?

Every 30 minutes Signup and view all the answers

What is the Administrative Distance of the OSPF protocol?

110 Signup and view all the answers

What feature does OSPF support?

Manual summarization Signup and view all the answers

What do link-state protocols use to choose paths through the network?

Cost metrics Signup and view all the answers

What happens when a link-state routing protocol detects a change in the network topology?

It begins forwarding packets through an alternate path Signup and view all the answers

What is a benefit of link-state protocols over traditional distance vector algorithms?

They can immediately report changes in the network topology Signup and view all the answers

Why is it difficult for routing loops to occur in link-state protocols?

Because each router has a complete and synchronized picture of the network Signup and view all the answers

What is a benefit of sequencing and aging LSPs?

Routers can base their routing decisions on the latest set of information Signup and view all the answers

What is a characteristic of link-state routing protocols during initial topology discovery?

They flood the network with LSPs Signup and view all the answers

What is a result of careful network design in link-state protocols?

Smaller link-state database sizes Signup and view all the answers

What is the main advantage of using triggered, flooded updates in link-state protocols?

Immediate reporting of changes in the network topology Signup and view all the answers

What is the main application of link-state routing protocols?

Networks of any size Signup and view all the answers

Study Notes

Link-State Routing Protocol

Each link-state router takes a copy of the LSA, updates its link-state database, and forwards the LSA to all neighboring devices.
LSAs cause every router within the area to recalculate routes, so it's recommended to limit the number of link-state routers in an area.
A link is similar to an interface on a router, and the state of the link is a description of that interface and its relationship to its neighboring routers.

Why Link-State Protocol is Preferred in Larger Networks

Link-state protocols always send updates when a topology changes.
Periodic refresh updates are more infrequent than for distance vector protocols.
Networks running link-state routing protocols can be segmented into area hierarchies, limiting the scope of route changes.
Networks running link-state routing protocols support classless addressing.

Non-Backbone Internal Routers

Non-backbone internal routers are aware of the topology within their respective areas and maintain identical link-state databases about the areas.
The ABR router will advertise a default route to the non-backbone internal router.
The internal router will use the default route to forward all inter-area or interdomain traffic to the ABR router.

Link-State Routing Protocol Algorithms

Link-state routing algorithms, known as SPF protocols, maintain a complex database of the network topology.
Each router that has exchanged LSPs constructs a topological database using all received LSPs.
An SPF algorithm is then used to compute reachability to networked destinations.
This information is employed to update the routing table.

Benefits of Link-State Routing Protocols

Link-state protocols use cost metrics to choose paths through the network.
By using triggered, flooded updates, link-state protocols can immediately report changes in the network topology to all routers in the network.
Because each router has a complete and synchronized picture of the network, it is difficult for routing loops to occur.
Because LSPs are sequenced and aged, routers always base their routing decisions on the latest set of information.
With careful network design, the link-state database sizes can be minimized, leading to smaller SPF calculations and faster convergence.

Features of OSPF Protocol

OSPF is an open standard protocol supported by many vendors.
OSPF can run many instances in parallel.
The metric used by OSPF is called Cost, which is equal to 100Mbps / BW in Mbps.
OSPF update messages are triggered and incremental updates are sent as multicast messages using the addresses 224.0.0.5 & 224.0.0.6.
OSPF periodically sends LSA refresh update messages every 30 minutes.
OSPF update messages are called as LSU - Link State Update, which may contain many LSAs - Link State Advertisement.
There are as many as 11 types of LSUs in OSPF.
The Administrative Distance of OSPF protocol is 110.
OSPF protocol supports manual summarization and VLSM.
Route announcements carry subnet mask.

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Description

Learn about OSPF routing and its router types. Identify the type of routers F, G, and H, and understand how ABR routers advertise routes to non-backbone internal routers.