7b - OSPF Concepts

Question 1

What is Link State?

Answer 1

A routing protocol that uses an SPF algorithm with an LSDB to compute routes.

Question 2

What is the OSPF metric?

Answer 2

OSPF totals the cost associated with each interface in the end-to-end route, with the cost based on link bandwidth.

Question 3

When is a 2-way state reached?

Answer 3

• Once routers have agreed to be neighbors* Both routers reach a 2-way state with each other, it means that both routers meet all OSPF configuration requirements to become neighbors. Effectively, at that point, they are neighbors, and ready to exchange their LSDB with each other.

Question 4

When is a full state reached?

Answer 4

After the routers have completed the exchange of LSUs and the LSDB exchange has been completed.

Question 5

What is an ABR

Answer 5

Area Border Router - a router with interfaces connected to the backbone area and at least one other area.

Question 6

What is a Backbone Router

Answer 6

A router connected to the backbone area including ABRs.

Question 7

What is a Internal Router

Answer 7

A router with all interfaces within a single area but not the backbone area.

Question 8

What is an Area

Answer 8

A set of routers and links that share the same LSDB information, but not with routers in other areas for efficiency.

Question 9

What is the Backbone Area

Answer 9

Area 0. A special area to which all other areas must connect.

Question 10

What is an Intra-area Route?

Answer 10

A route to a subnet inside the same area as the router.

Question 11

What is an Inter-area Route?

Answer 11

A route to a subnet in an area of which the router is not a part.

Question 12

What is a DR?

Answer 12

Designated Router - a router on a shared link like Ethernet to which all the other routers - DROther - exchange database information. The DR router makes sure the other routers on the shared link get a copy of each LSA.

Question 13

What is a BDR?

Answer 13

Backup Designated Router. Takes over the DR duties if the DR fails.

Question 14

What is a DROther?

Answer 14

Routers on a shared link that are NOT DR or BDR. They sync with the DR but not with each other. DROther routers never go beyond 2-way state with each other.

Question 15

What is Fully Adjacent?

Answer 15

In OSPF, a characterization of the state of a neighbor in which the two neighbors have reached the full state.

Question 16

What is a Hello Interval

Answer 16

an interface timer that dictates how often the router should send out Hello messages.

Question 17

What is the Dead Interval?

Answer 17

By default, 4 times the Hello interval. When a neighbor is silent for the Dead Interval the router assumes the neighbor has gone down.

Question 18

What is an LSU?

Answer 18

Link State Update. A packet containing LSA data structures.

Question 19

What is a neighbor?

Answer 19

another router with which a router decides to exchange routing information.

Question 20

What is the Router_ID?

Answer 20

a 32-bit number, written in dotted decimal that identifies a router.

Question 21

What is a Topology Database?

Answer 21

The structured data that describes the network topology to a routing protocol and from which routers build their routes.

Question 22

What are the requirements for OSPF routers to become neighbors?

Answer 22

• Same primary subnet and same mask* Same OSPF area * Same area type (regular, stub, not-so-stubby area [NSSA]) * Must not have duplicate RIDs * OSPF Hello and Dead timers must be equal

Question 23

How do you determine the cost of an out-going interface?

Answer 23

show ip ospf interface fa0/1