ENSA 1-2 study guide

By default, what is the OSPF cost for any link with a bandwidth of 100 Mb/s or greater?

1

What is the default router priority value for all Cisco OSPF routers?

1

Refer to the exhibit. If no router ID was manually configured, what would router Branch1 use as its OSPF router ID?

192.168.1.100

Refer to the exhibit. What is the OSPF cost to reach the router A LAN 172.16.1.0/24 from B?

65

Single-Area OSPF

All routers are in one area. Best practice is to use area 0.

What is the recommended Cisco best practice for configuring an OSPF-enabled router so that each router can be easily identified when troubleshooting routing issues?

Configure a value using the router-id command.

After modifying the router ID on an OSPF router, what is the preferred method to make the new router ID effective?

HQ# clear ip ospf process

Init State

Hello packets are received from the neighbor, They contain the Router ID of the sending router, Transition to Two-Way state.

Refer to the exhibit. Suppose that routers B, C, and D have a default priority, and router A has a priority 0. Which conclusion can be drawn from the DR/BDR election process?

If the DR fails, the new DR will be router B.

Two-Way State

In this state, communication between the two routers is bidirectional, On multiaccess links, the routers elect a DR and a BDR, Transition to ExStart state.

Which three OSPF states are involved when two routers are forming an adjacency?

Init, two-way, down

Refer to the exhibit. A network administrator has configured OSPFv2 on the two Cisco routers but PC1 is unable to connect to PC2. What is the most likely problem?

Interface Fa0/0 has not been activated for OSPFv2 on router R2

In an OSPFv2 configuration, what is the effect of entering the command network 192.168.1.1 0.0.0.0 area 0 ?

It tells the router which interface to turn on for the OSPF routing process.

Loading State

LSRs and LSUs are used to gain additional route information, Routes are processed using the SPF algorithm, Transition to the Full state.

Down State

No Hello packets received = Down, Router sends Hello packets, Transition to Init state.

Multiarea OSPF

OSPF is implemented using multiple areas, in a hierarchical fashion. All areas must connect to the backbone area (area 0). Routers interconnecting the areas are referred to as Area Border Routers (ABRs).

ExStart State

On point-to-point networks, the two routers decide which router will initiate the DBD packet exchange and decide upon the initial DBD packet sequence number.

Refer to the exhibit. On which router or routers would a default route be statically configured in a corporate environment that uses single area OSPF as the routing protocol?

R0-A

Refer to the exhibit. Which command did an administrator issue to produce this output?

R1# show ip ospf interface serial0/0/1

Refer to the exhibit. Which three statements describe the results of the OSPF election process of the topology that is shown in the exhibit?

R3 will be elected BDR. The R4 router ID is 172.16.1.1. R2 will be elected DR.

Link-State Request (LSR) packet

Receiving routers can then request more information about any entry in the DBD by sending an LSR.

Refer to the exhibit. If the switch reboots and all routers have to re-establish OSPF adjacencies, which routers will become the new DR and BDR?

Router R4 will become the DR and router R1 will become the BDR.

Exchange State

Routers exchange DBD packets, If additional router information is required then transition to Loading; otherwise, transition to the Full state.

A network engineer has manually configured the hello interval to 15 seconds on an interface of a router that is running OSPFv2. By default, how will the dead interval on the interface be affected?

The Dead interval will now be 60 seconds.

Refer to the exhibit. A network administrator has configured the OSPF timers to the values that are shown in the graphic. What is the result of having those manually configured timers?

The R1 dead timer expires between hello packets from R2.

Shortest path first

The SPF algorithm is based on the cumulative cost to reach a destination. The SPF algorithm creates an SPF tree by placing each router at the root of the tree and calculating the shortest path to each node. The SPF tree is then used to calculate the best routes. OSPF places the best routes into the forwarding database, which is used to make the routing table.

Full State

The link-state database of the router is fully synchronized.

What indicates to a link-state router that a neighbor is unreachable?

The router no longer receives Hello packets.

Database Description (DBD) packet

This contains an abbreviated list of the LSDB of the sending router and is used by receiving routers to check against the local LSDB. The LSDB must be identical on all link-state routers within an area to construct an accurate SPF tree.

Adjacency database

This creates the neighbor table. List of all neighbor routers to which a router has established bidirectional communication, This table is unique for each router, Can be viewed using the show ip ospf neighbor command.

Link-state Database

This creates the topology table. Lists information about all other routers in the network, This database represents the network topology, All routers within an area have identical LSDB, Can be viewed using the show ip ospf database command.

Hello packet

This is used to establish and maintain adjacency with other OSPF routers.

Link-State Update (LSU) packet

This is used to reply to LSRs and to announce new information. LSUs contain several different types of LSAs.

what is a benefit of multiarea OSPF routing?

Topology changes in one area do not cause SPF recalculations in other areas

Link-State Acknowledgment (LSAck) packet

When an LSU is received, the router sends an LSAck to confirm receipt of the LSU. The LSAck data field is empty.

What is the format of the router ID on an OSPF-enabled router?

a 32-bit number formatted like an IPv4 address

What will an OSPF router prefer to use first as a router ID?

any IP address that is configured using the router-id command

Which step in the link-state routing process is described by a router building a link-state database based on received LSAs?

building the topology table

Which step in the link-state routing process is described by a router inserting best paths into the routing table?

choosing the best route

Which type of OSPFv2 packet contains an abbreviated list of the LSDB of a sending router and is used by receiving routers to check against the local LSDB?

database description

To establish a neighbor adjacency two OSPF routers will exchange hello packets. Which two values in the hello packets must match on both routers?

dead interval, hello interval

Which step in the link-state routing process is described by a router sending Hello packets out all of the OSPF-enabled interfaces?

establishing neighbor adjacencies

When an OSPF network is converged and no network topology change has been detected by a router, how often will LSU packets be sent to neighboring routers?

every 30 minutes

Which step does an OSPF-enabled router take immediately after establishing an adjacency with another router?

exchanges link-state advertisements

Which step in the link-state routing process is described by a router flooding link-state and cost information about each directly connected link?

exchanging link-state advertisements

Which step in the link-state routing process is described by a router running an algorithm to determine the best path to each destination?

executing the SPF algorithm

Which OSPF data structure is identical on all OSPF routers that share the same area?

link-state database

An OSPF router has three directly connected networks; 172.16.0.0/16, 172.16.1.0/16, and 172.16.2.0/16. Which OSPF network command would advertise only the 172.16.1.0 network to neighbors?

router(config-router)# network 172.16.1.0 255.255.255.0 area 0

Which command will a network engineer issue to verify the configured hello and dead timer intervals on a point-to-point WAN link between two routers that are running OSPFv2?

show ip ospf interface serial 0/0/0

What command would be used to determine if a routing protocol-initiated relationship had been made with an adjacent router?

show ip ospf neighbor

Which command is used to verify that OSPF is enabled and also provides a list of the networks that are being advertised by the network?

show ip protocols

A network technician issues the following commands when configuring a router: R1(config)# router ospf 11 R1(config-router)# network 10.10.10.0 0.0.0.255 area 0 What does the number 11 represent?

the OSPF process ID on R1

what is used to facilitate hierarchical routing in OSPF

the use of multiple areas

What is the reason for a network engineer to alter the default reference bandwidth parameter when configuring OSPF?

to more accurately reflect the cost of links greater than 100 Mb/s

What are the two purposes of an OSPF router ID?

to uniquely identify the router within the OSPF domain, to facilitate router participation in the election of the designated router

In an OSPF network when are DR and BDR elections required?

when the routers are interconnected over a common Ethernet network