Chapter 8
Reading Organizer
After completion of this chapter, you should be able to:
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Explain the process by which link-state routers learn about other networks.
Describe the types of packets used by Cisco IOS routers to establish and maintain an OSPF
network.
Explain how Cisco IOS routers achieve convergence in an OSPF network.
Configure an OSPF IOS router ID.
Configure single-area OSPFv2 in a small, routed network.
Explain how OSPF uses cost to determine best path.
Verify single-area OSPFv2 in a small, routed network.
Compare the characteristics and operations of OSPFv2 to OSPFv3.
Configure single-area OSPFv3 in a small, routed network.
Verify single-area OSPFv3 in a small, routed network.
8.1 Characteristics of OSPF
1. List and explain the common OSPF features.
a. Classless –
It is classless by design; therefore, it supports VLSM and CIDR.
b. Efficient –
Routing changes trigger routing updates (no periodic updates). It uses the SPF algorithm to
choose the best path.
c. Fast convergence –
It quickly propagates network changes.
d. Scalable –
It works well in small and large network sizes. Routers can be grouped into areas to support a
hierarchical system.
e. Secure –
It supports Message Digest 5 (MD5) authentication. When enabled, OSPF routers only accept
encrypted routing updates from peers with the same pre-shared password.
2. What is Administrative Distance?
Administrative distance (AD) is the trustworthiness (or preference) of the route source.
3. What is the default administrative distance for OSPF?
110
4. List and explain the three databases OSPF creates and maintains.
a. Adjacency database –
Creates the neighbor table
b. Link-state database (LSDB) –
Creates the topology table
c. Forwarding database –
Creates the routing table
5. List the five types of messages OSPF exchanges to convey routing information.
a. Hello packet
b. Database description packet
c. Link-state request packet
d. Link-state update packet
e. Link-state acknowledgment packet
6. List the generic link-state routing process OSPF routers complete to maintain routing information, in
order to reach a state of convergence.
a. Establish Neighbor Adjacencies
b. Exchange Link-State Advertisements
c. Build the Topology Table
d. Execute the SPF Algorithm
7. What is an OSPF area?
It is a group of routers that share the same link-state information in their LSDBs.
8. OSPF can be implemented in one of two ways. List and explain both.
a. Single-Area OSPF –
All routers are in one area called the backbone area (area 0).
b. Multiarea OSPF –
OSPF is implemented using multiple areas, in a hierarchal fashion. All areas must connect to the
backbone area (area 0). Routers interconnecting the areas are referred to as Area Border
Routers (ABR).
9. List and explain the advantages hierarchical-topology for multiarea OSPF.
a. Smaller routing tables –
Fewer routing table entries because network addresses can be summarized between areas.
Route summarization is not enabled by default.
b. Reduced link-state update overhead –
Minimizes processing and memory requirements.
c. Reduced frequency of SPF calculations –
Localizes the impact of a topology change within an area. For instance, it minimizes routing
update impact because LSA flooding stops at the area boundary.
10. What does OSPF use to establish and maintain neighbor adjacencies and exchange routing
updates?
Link-state packets (LSPs)
11. List the five different LSPs used by OSPF.
a. Hello packet
b. Database Description (DBD) packet
c. Link-State Request (LSR) packet
d. Link-State Update (LSU) packet
e. Link-State Acknowledgment (LSAck) packet
12. Explain what the OSPF Type 1 Hello packets are used for.
a. Discover OSPF neighbors and establish neighbor adjacencies.
b. Advertise parameters on which two routers must agree to become neighbors.
c. Elect the Designated Router (DR) and Backup Designated Router (BDR) on multiaccess
networks like Ethernet and Frame Relay. Point-to-point links do not require DR or BDR.
13. By default, how often are OSPF Hello packets transmitted on multi-access and point-to-point
networks?
Every 10 seconds
14. What is the Dead interval?
The Dead interval is the period that the router waits to receive a Hello packet before declaring
the neighbor down.
15. What does an OSPF router initially do when connected to a network?
a. Create adjacencies with neighbors
b. Exchange routing information
c. Calculate the best routes
d. Reach convergence
16. OSPF progresses through several states while attempting to reach convergence. These are:
a. Down state
b. Init state
c. Two-Way state
d. ExStart state
e. Exchange state
f. Loading state
g. Full state
17. What happens when a neighboring OSPF-enabled router receives a Hello packet with a router ID
that is not within its neighbor list?
The receiving router attempts to establish an adjacency with the initiating router.
18. When a router receives a Hello packet with its Router ID listed in the list of neighbors, the router
transitions from the Init state to the Two-Way state. The action performed in Two-Way state depends
on the type of inter-connection between the adjacent routers. Explain both.
a. If the two adjacent neighbors are interconnected over a point-to-point link, then they
immediately transition from the Two-Way state to the database synchronization phase.
b. If the routers are interconnected over a common Ethernet network, then a designated router
DR and a BDR must be elected.
19. Multiaccess networks can create two challenges for OSPF regarding the flooding of LSAs. List and
explain both.
a. Creation of multiple adjacencies –
Ethernet networks could potentially interconnect many OSPF routers over a common link.
Creating adjacencies with every router is unnecessary and undesirable. It would lead to an
excessive number of LSAs exchanged between routers on the same network.
b. Extensive flooding of LSAs –
Link-state routers flood their LSAs any time OSPF is initialized, or when there is a change in the
topology. This flooding can become excessive.
20. The solution to managing the number of adjacencies and the flooding of LSAs on a multiaccess
network is the Designated Router (DR). Explain the purpose of a DR on multiaccess networks.
OSPF elects a DR to be the collection and distribution point for LSAs sent and received.
21. What is the purpose of a Backup Designated Router (BDR)?
A BDR is also elected in case the DR fails.
22. What is a DROTHER?
A DROTHER is a router that is neither the DR nor the BDR.
23. After the topological databases are synchronized, when are updates (LSUs) sent to neighbors?
a. A change is perceived (incremental updates)
b. Every 30 minutes
8.2 Configuring Single-area OSPFv2
24. Every router requires a router ID to participate in an OSPF domain. The router ID can be defined by
an administrator or automatically assigned by the router. List and explain how the router ID is used by
an OSPF-enabled router.
a. Uniquely identify the router –
The router ID is used by other routers to uniquely identify each router within the OSPF domain
and all packets that originate from them.
b. Participate in the election of the DR –
In a multiaccess LAN environment, the election of the DR occurs during initial establishment of
the OSPF network. When OSPF links become active, the routing device configured with the
highest priority is elected the DR. Assuming there is no priority configured, or there is a tie, then
the router with the highest router ID is elected the DR. The routing device with the second
highest router ID is elected the BDR.
25. Explain the three methods that can be used to determine the router ID?
a. The router ID is explicitly configured using the OSPF router-id rid router configuration mode
command. The rid value is any 32-bit value expressed as an IPv4 address. This is the
recommended method to assign a router ID.
b. If the router ID is not explicitly configured, the router chooses the highest IPv4 address of any
of configured loopback interfaces. This is the next best alternative to assigning a router ID.
c. If no loopback interfaces are configured, then the router chooses the highest active IPv4
address of any of its physical interfaces. This is the least recommended method because it
makes it more difficult for administrators to distinguish between specific routers.
26. After a router selects a router ID, an active OSPF router does not allow the router ID to be changed
until one of two things occur. These changes are?
a. the router is reloaded
b. or the OSPF process cleared
27. What is the preferred method to reset the router ID?
Clearing the OSPF process
28. What command is used to clear the OSPF routing process?
clear ip ospf process
29. Some older versions of the Cisco IOS do not recognize the router-id command. What is the best
way to set the router ID on those routers?
Use a loopback interface.
30. Why is it a good practice to use an area ID of 0 with single-area OSPF?
This convention makes it easier if the network is later altered to support multiarea OSPF.
31. Sending out unneeded OSPF messages on a LAN affects the network in three ways. List and explain
each.
a. Inefficient Use of Bandwidth –
Available bandwidth is consumed transporting unnecessary messages. Messages are
multicasted; therefore, switches are also forwarding the messages out all ports.
b. Inefficient Use of Resources –
All devices on the LAN must process the message and eventually discard the message.
c. Increased Security Risk –
Advertising updates on a broadcast network is a security risk. OSPF messages can be
intercepted with packet sniffing software. Routing updates can be modified and sent back to
the router, corrupting the routing table with false metrics that misdirect traffic.
32. What is the passive-interface router configuration mode command used for?
To prevent the transmission of routing messages through a router interface, but still allow that
network to be advertised to other routers
33. OSPF uses cost as a metric. Is a lower cost or higher cost preferred?
A lower cost indicates a better path than a higher cost.
34. What is the cost of an OSPF route?
It is the accumulated value from one router to the destination network.
35. What must be done to assist OSPF in making the correct path determination?
The reference bandwidth must be changed to a higher value to accommodate networks with
links faster than 100 Mb/s.
36. To adjust the reference bandwidth on a router, use the auto-cost reference-bandwidth Mb/s
router configuration command. Where must this command be configured?
This command must be configured on every router in the OSPF domain.
37. What command can you use to adjust an interface’s bandwidth?
bandwidth kilobits
38. What is an alternative to setting the default interface bandwidth?
The cost can be manually configured on an interface using the ip ospf cost value
39. What is the purpose of adjusting the default interface bandwidth?
To provide an accurate value for use by OSPF in determining the best route.
40. What command can you use to verify that the router has formed an adjacency with its neighboring
routers?
show ip ospf neighbor
41. Explain the reasons two routers might not form an OSPF adjacency.
a. The subnet masks do not match, causing the routers to be on separate networks.
b. OSPF Hello or Dead Timers do not match.
c. OSPF Network Types do not match.
d. There is a missing or incorrect OSPF network command.
42. Which command is a quick way to verify vital OSPF configuration information?
show ip protocols
43. What is the quickest way to verify OSPF interface settings?
Use the show ip ospf interface
44. What command can you use to get a summary of OSPF-enabled interfaces?
show ip ospf interface brief
8.3 Configure Single-area OSPFv3
45. Which version of OSPF is required to run IPv6 addressing?
OSPFv3
46. List the similarities between OSPFv2 and OSPFv3.
a. Link-state
b. Routing algorithm
c. Metric
d. Areas
e. OSPF packet types
f. Neighbor discovery mechanism
g. DR/BDR election process
h. Router ID
47. List the differences between OSPFv2 and OSPFv3
a. Advertises
b. Source address
c. All OSPF router multicast addresses
d. DR/BDR multicast address
e. Advertise networks
f. IP unicast routing
g. Authentication
48. Link-local addresses are automatically created when an IPv6 global unicast address is assigned to
the interface.
49. Are Global unicast addresses required on an interface?
No
50. Are IPv6 link-local required on an interface?
Yes
60. A router with several interfaces can assign the same link-local address to each IPv6 interface.
Explain.
This is because the link-local address is only required for local communications.
61. What is the link-local address prefix address range?
FE80 to FEBF
62. OSPFv2 uses the network command to specify matching interface IPv4 addresses. How is this
different in OSPFv3 with IPv6 addresses?
OSPFv3 is configured directly on the interface.
63. Which command would you use verify that the router has formed an adjacency with its neighboring
routers?
show ipv6 ospf neighbor
64. The show ipv6 protocols command is a quick way to verify vital OSPFv3 configuration information,
such as:
a. the OSPF process ID
b. the router ID
c. the interfaces enabled for OSPFv3
65. What is the quickest way to verify OSPF interface settings?
Use the show ipv6 ospf interface command
66. To retrieve and view a summary of OSPFv3-enabled interfaces on a router what command would
you use?
show ipv6 ospf interface brief