1: Which command in OSPF shows the network LSA information?

** The command show ip ospf [process-id area-id] database network displays the network link-state
information.
2: What command would you use to create a totally stubby area?
** The command area area-id stub no-summary will create a totally stubby area. This is a subcommand to
the router ospf process-id command. It is necessary only on the ABR, but all the other routers in the area
must be configured as stub routers.
3: What is a virtual link, and what command would you use to create it?
** A virtual link is a link that creates a tunnel through an area to the backbone (Area 0). This allows an area
that cannot connect directly to the backbone to do so virtually. The command to create the link is area
area-id virtual-link router-id. Note that the area-id that is supplied is that of the transit area, and the router-
id is that of the router at the other end of the link. The command needs to be configured at both ends of the
tunnel.
4: Where would you issue the command to summarize IP subnets? State the command that is
used.
** Summarization is done at area boundaries. The command to start summarization is the area range
command, with the syntax area area-id range address mask. To summarize external routes, use the
summary-address command on the ASBRs.
5: How would you summarize external routes before injecting them into the OSPF domain?
** The command summary-address address mask is the command that you would use.
6: When is a virtual link used?
** A virtual link is used when an area is not directly attached to the backbone area (Area 0). This may be
due to poor design and a lack of understanding about the operation of OSPF, or it may be due to a link
failure. The most common cause of an area separating from the backbone is link failure, which can also
cause the backbone to be segmented. The virtual link is used in these instances to join the two backbone
areas together. Segmented backbone areas might also be the result of two companies merging.
7: Give the command for defining the cost of a default route propagated into an area.
** The command to define the cost of a default route propagated into another area is area area-id default-
cost cost.
8: Give an example of when it would be appropriate to define a default cost.
** It is appropriate to define a default cost for the default route when a stub area has more than one ABR.
This command allows the ABR or exit point for the area to be determined by the network administrator. If
this link or the ABR fails, the other ABR will become the exit point for the area.
9: On which router is the area default cost defined?
** The default cost for the default route is defined on the ABR. The ABR will then automatically generate
and advertise the route cost along with the default route.
10: Give the command to configure a stub area and state on which router it is configured.
** The command syntax to configure a stub area is area area-id stub. This command is configured on the
ABR connecting to the area and on all the routers within the area. Once the configuration is completed, the
Hellos are generated with the E bit set to 0. All routers in the area will only form adjacencies with other
routers that have the E bit set.
11: What is the purpose of the area range command, and why is it configured on the ABR?
** The area range command is configured on an ABR because it dictates the networks that will be
advertised out of the area. It is used to consolidate and summarize the routes at an area boundary.
12: Give the commands to configure a router to place subnets 144.111.248.0 through to
144.111.255.0 in Area 1 and to put all other interfaces into Area 0.
** The commands are as follows:
network 144.111.248.0 0.0.7.255 area 1
network 0.0.0.0 255.255.255.255 area 0
13: Give the syntax to summarize the subnets 144.111.248.0 to 144.111.254.255 into another
autonomous system.
** The syntax is as follows:
summary-address 144.111.248.0 255.255.248.0
14: Explain briefly the difference between the area range command and the summary-address
command.
** The area range command is used to summarize networks between areas and is configured on the ABR.
The summary-address command is used to summarize networks between autonomous systems and is
configured on the ASBR.
15: Explain the following syntax and what it will achieve: area 1 stub no-summary.
** The command area 1 stub no-summary creates a totally stubby area. The number after the word area
indicates the area that is being defined as a totally stubby area. This is necessary because the router might
be an ABR with connections to many areas. Once this command is issued, it prevents summarized and
external routes from being propagated by the ABR into the area. To reach the networks and hosts outside
the area, routers must use the default route advertised by the ABR into the area.

16: Why would you configure the routing process to log adjacency changes as opposed to
turning on debug for the same trigger?
** The reason to configure the router process to log adjacency changes to syslog as opposed to running
debug is an issue of resources. It takes fewer router and administrator resources to report on a change of
state as it happens than to have the debugger running constantly. The debug process has the highest
priority and thus everything waits for it.
17: Give some of the common reasons that neighbors fail to form an adjacency.
** Many OSPF problems stem from adjacency problems that propagate throughout the network. Many
problems are often traced back to neighbor discrepancies.
If a router configured for OSPF routing is not seeing an OSPF neighbor on an attached network, do the
following:
- Make sure that both routers are configured with the same IP mask, MTU, Interface Hello timer, OSPF Hello
interval, and OSPF dead interval.
- Make sure that both neighbors are part of the same area and area type.
- Use the debug and show commands to trace the problem.
18: When configuring a virtual link, which routers are configured?
** The configuration is between the ABRs, where one of the ABRs resides in Area 0 and the other in the area
that is disconnected from the backbone. Both of the ABRs are also members of the transit area. Having
created the virtual link, both ABRs are now members of Area 0, the disconnected area, and the transit area.
19: What does the command area 1 default-cost 15 achieve?
** The command area 1 default-cost 15 will assign a cost of 15 to the default route that is to be propagated
into the stub area. This command is configured on the ABR attached to the stub area.
20: Explain what is placed in the parameters area-id and router-id for the command area area-
id virtual-link router-id.
** The parameter area-id is the area ID of the transit area. So if the ABR in Area 0 is creating a virtual link
with the ABR in Area 3 through Area 2, the area ID stated in the command is Area 2. The router ID is the
router ID of the router with whom the link is to be formed and a neighbor relationship and adjacency
established.

Q. Why are loopbacks advertised as /32 host routes in OSPF?

A. Loopbacks are considered host routes in OSPF, and they are advertised as /32. For more information, refer to section
9.1 of RFC 2328 . In Cisco IOS Software Releases 11.3T and 12.0, if the ip ospf network point−to−point command is
configured under loopbacks, OSPF advertises the loopback subnet as the actual subnet configured on loopbacks. ISDN
dialer interface advertises /32 subnet instead of its configured subnet mask. This is an expected behavior if ip ospf network
point−to−multipoint is configured.
Q. How do I change the reference bandwidth in OSPF?
A. You can change the reference bandwidth in Cisco IOS Software Release 11.2 and laterusing the ospf auto−cost
reference−bandwidth command under router ospf. By default,reference bandwidth is 100 Mbps.
Q. How does OSPF calculate its metric or cost?
A. OSPF uses a reference bandwidth of 100 Mbps for cost calculation. The formula tocalculate the cost is reference
bandwidth divided by interface bandwidth. For example, in thecase of Ethernet, it is 100 Mbps / 10 Mbps = 10.
Note: If ip ospf cost cost is used on the interface, it overrides this formulated cost.
Q. Are OSPF routing protocol exchanges authenticated?
A. Yes, OSPF can authenticate all packets exchanged between neighbors. Authentication maybe through simple
passwords or through MD5 cryptographic checksums. To configure simple password authentication for an area, use the
command ip ospf authentication−key to assign a password of up to eight octets to each interface attached to the area.
Then, issue the area x authentication command to the OSPF router configuration to enable authentication. (In the
command, x is the area number.) Cisco IOS Software Release 12.x also supports the enabling of authentication on a
per−interface basis. If you want to enable authentication on some interfaces only, or if you want different authentication
methods on different interfaces that belong to the same area, use the ip ospf authentication interface mode command.
Q. What is the link−state retransmit interval, and what is the command to set it?
A. OSPF must send acknowledgment of each newly received link−state advertisement (LSA).It does this by sending LSA
packets. LSAs are retransmitted until they are acknowledged.The link−state retransmit interval defines the time between
retransmissions. You can use thecommand ip ospf retransmit−interval to set the retransmit interval. The default value is 5
seconds.
Q. What is the purpose of the variable IP−OSPF−Transmit−Delay?
A. This variable adds a specified time to the age field of an update. If the delay is not added before transmission over a
link, the time in which the link−state advertisement (LSA) propagates over the link is not considered. The default value is 1
second. This parameter has more significance on very low−speed links.

Q. Is it true that only the static option of the virtual link in OSPF allows discontiguous networks, regardless of the
mask propagation properties?
A. No, virtual links in OSPF maintain connectivity to the backbone from nonbackbone areas,but they are unnecessary for
discontiguous addressing. OSPF provides support for discontiguous networks because every area has a collection of
networks, and OSPF attaches a mask to each advertisement.
Q. Are the multicast IP addresses mapped to MAC−level multicast addresses?
A. OSPF sends all advertisements using multicast addressing. Except for Token Ring, the multicast IP addresses are
mapped to MAC−level multicast addresses. Cisco maps Token Ring to MAC−level broadcast addresses.
Q. Does the Cisco OSPF implementation support IP TOS−based routing?
A. Cisco OSPF only supports TOS 0. This means that routers route all packets on the TOS 0 path, eliminating the need to
calculate nonzero TOS paths.
Q. Does the offset−list subcommand work for OSPF?
A. The offset−list command does not work for OSPF. It is used for distance vector protocols such as Interior Gateway
Routing Protocol (IGRP), Routing Information Protocol (RIP), and RIP version 2.
Q. Can an OSPF default be originated into the system based on external information on a router that does not
itself have a default?
A. OSPF generates a default only if it is configured using the command default−information originate and if there is a
default network in the box from a different process. The default route in OSPF is 0.0.0.0. If you want an OSPF−enabled
router to generate a default route even if it does not have a default route itself, use the command default−information
originate always.
Q. Can I use the distribute−list in/out command with OSPF to filter routes?
A. The distribute−list commands are supported in OSPF but work differently than
distance−vector routing protocols such as Routing Information Protocol (RIP) and Enhanced
Interior Gateway Routing Protocol (EIGRP). OSPF routes cannot be filtered from entering
the OSPF database. The distribute−list in command only filters routes from entering the
routing table; it does not prevent link−state packets from being propagated. Therefore, this
command does not help conserve router memory, and it does not prohibit a router from
propagating filtered routes to other routers.
Caution: Use of the distribute−list in command in OSPF may lead to routing loops in
the network if not implemented carefully.
The command distribute−list out works only on the routes being redistributed by the
Autonomous System Boundary Routers (ASBRs) into OSPF. It can be applied to external
type 2 and external type 1 routes, but not to intra−area and interarea routes.
Q. How can I give preference to OSPF interarea routes over intra−area
routes?
A. According to section 11 of RFC 2328 , the order of preference for OSPF routes is:
¨ intra−area routes, O
¨ interarea routes, O IA
¨ external routes type 1, O E1
¨ external routes type 2, O E2
This rule of preference cannot be changed. However, it applies only within a single OSPF
process. If a router is running more than one OSPF process, route comparison occurs. With
route comparison, the metrics and administrative distances (if they have been changed) of the
OSPF processes are compared. Route types are disregarded when routes supplied by two
different OSPF processes are compared.
Q. Do I need to manually set up adjacencies for routers on the Switched
Multimegabit Data Service (SMDS) cloud with the OSPF neighbor
subcommand?
A. In Cisco IOS Software releases earlier than Cisco IOS Software Release 10.0, the
neighbor command was required to establish adjacencies over nonbroadcast multiaccess
(NBMA) networks (such as Frame Relay, X.25, and SMDS). With Cisco IOS Software
Release 10.0 and later, you can use the ip ospf network broadcast command to define the
network as a broadcast network, eliminating the need for the neighbor command. If you are not using a fully meshed
SMDS cloud, you must use the ip ospf network
point−to−multipoint command.
Q. When routes are redistributed between OSPF processes, are all
shortest path first algorithm (SPF) metrics preserved, or is the default
metric value used?
A. The SPF metrics are preserved. The redistribution between them is like redistribution
between any two IP routing processes.
Q. How does Cisco accommodate OSPF routing on partial−mesh Frame
Relay networks?
A. You can configure OSPF to understand whether it should attempt to use multicast facilities
on a multi−access interface. Also, if multicast is available, OSPF uses it for its normal
multicasts.
Cisco IOS Software Release 10.0 includes a feature called subinterfaces. You can use
subinterfaces with Frame Relay to tie together a set of virtual circuits (VCs) to form a virtual
interface, which acts as a single IP subnet. All systems within the subnet should be fully
meshed. With Cisco IOS Software Releases 10.3, 11.0 and later, the ip ospf
point−to−multipoint command is also available.
Q. Which address−wild−mask pair should I use for assigning an
unnumbered interface to an area?
A. When an unnumbered interface is configured, it references another interface on the router.
When enabling OSPF on the unnumbered interface, use the address−wild−mask pair of
interfaces to which the unnumbered interface is pointing.
Q. Can I have one numbered side and leave the other side unnumbered
in OSPF?
A. No, OSPF does not work if you have one side numbered and the other side unnumbered.
This creates a discrepancy in the OSPF database that prevents routes from being installed in
the routing table.
Q. Why do I receive the "cannot allocate router id" error message when I
configure Router OSPF One?
A. OSPF picks up the highest IP address as a router ID. If there are no interfaces in up/up
mode with an IP address, it returns this error message. To correct the problem, configure a
loopback interface.
Q. Why do I receive the "unknown routing protocol" error message when
I configure Router OSPF One?
A. Your software may not support OSPF. This error message occurs most frequently with the
Cisco 1600 series routers. If you are using a 1600 router, you need a Plus image to run OSPF.
Q. What do the states DR, BDR, and DROTHER mean in show ip ospf
interface command output?
A. DR means designated router. BDR means backup designated router. DROTHER indicates a
router that is neither the DR or the BDR. The DR generates a Network Link−State
Advertisement, which lists all the routers on that network.
Q. When I issue the show ip ospf neighbor command, why do I only see
FULL/DR and FULL/BDR, with all other neighbors showing
2−WAY/DROTHER?
A. To reduce the amount of flooding on broadcast media, such as Ethernet, FDDI, and Token
Ring, the router becomes full with only designated router (DR) and backup designated router
(BDR), and it shows 2−WAY for all other routers.
Q. Why do I not see OSPF neighbors as FULL/DR or FULL/BDR on my
serial link?
A. This is normal. On point−to−point and point−to−multipoint networks, there are no
designated routers (DRs) or backup designated routers (BDRs).
Q. Do I need any special commands to run OSPF over BRI/PRI links?
A. In addition to the normal OSPF configuration commands, you should use the dialer map
command. When using the dialer map command, use the broadcast keyword to indicate that
broadcasts should be forwarded to the protocol address.
Q. Do I need any special commands to run OSPF over asynchronous
links?
A. In addition to the normal OSPF configuration commands, you should use the async
default routing command on the asynchronous interface. This command enables the router to
pass routing updates to other routers over the asynchronous interface. Also, when using the
dialer map command, use the broadcast keyword to indicate that broadcasts should be
forwarded to the protocol address.
Q. Which Cisco IOS Software release began support for per−interface
authentication type in OSPF?
A. Per−interface authentication type, as described in RFC 2178 , was added in Cisco IOS
Software Release 12.0(8).
Q. Can I control the P−bit when importing external routes into a
not−so−stubby area (NSSA)?
A. When external routing information is imported into an NSSA in a type 7 link−state
advertisement (LSA), the type 7 LSA has only area flooding scope. To further distribute the
external information, type 7 LSAs are translated into type 5 LSAs at the NSSA border. The
P−bit in the type 7 LSA Options field indicates whether the type 7 LSA should be translated.
Only those LSAs with the P−bit set are translated. When you redistribute information into the NSSA, the P−bit is
automatically set. A possible workaround applies when the Autonomous
System Boundary Router (ASBR) is also an Area Border Router (ABR). The NSSA ASBR
can then summarize with the not−advertise keyword, which results in not advertising the
translated type 7 LSAs.
Q. Why are OSPF show commands responding so slowly?
A. You may experience a slow response when issuing OSPF show commands, but not with
other commands. The most common reason for this delay is that you have the ip ospf
name−lookup configuration command configured on the router. This command causes the
router to look up the device Domain Name System (DNS) names for all OSPF show
commands, making it easier to identify devices, but resulting in a slowed response time for
the commands. If you are experiencing slow response on commands other than just OSPF
show commands, you may want to start looking at other possible causes, such as the CPU
utilization.
Q. What does the clear ip ospf redistribution command do?
A. The clear ip ospf redistribution command flushes all the type 5 and type 7 link−state
advertisements (LSAs) and scans the routing table for the redistributed routes. This causes a
partial shortest path first algorithm (SPF) in all the routers on the network that receive the
flushed/renewed LSAs. When the expected redistributed route is not in OSPF, this command
may help to renew the LSA and get the route into OSPF.
Q. Does OSPF form adjacencies with neighbors that are not on the same
subnet?
A. The only time that OSPF forms adjacencies between neighbors that are not on the same
subnet is when the neighbors are connected through point−to−point links. This may be
desired when using the ip unnumbered command, but in all other cases, the neighbors must
be on the same subnet.
Q. How often does OSPF send out link−state advertisements (LSAs)?
A. OSPF sends out its self−originated LSAs when the LSA age reaches the link−state refresh
time, which is 1800 seconds.
Q. How do I stop individual interfaces from developing adjacencies in an
OSPF network?
A. To stop routers from becoming OSPF neighbors on a particular interface, issue the
passive−interface command at the interface.
In Internet service provider (ISP) and large enterprise networks, many of the distribution
routers have more than 200 interfaces. Configuring passive−interface on each of the 200
interfaces can be difficult. The solution in such situations is to configure all the interfaces as
passive by default using a single passive−interface default command. Then, configure
individual interfaces where adjacencies are desired using the no passive−interface command.
For more information, refer to Default Passive Interface Feature.
There are some known problems with the passive−interface default command. Workarounds
are listed in Cisco bug ID CSCdr09263 ( registered customers only) .

Q. When I have two type 5 link−state advertisements (LSAs) for the same
external network in the OSPF database, which path should be installed
in the IP routing table?
A. When you have two type 5 LSAs for the same external network in the OSPF database,
prefer the external LSA that has the shortest path to the Autonomous System Boundary
Router (ASBR) and install that into the IP routing table. Use the show ip ospf
border−routers command to check the cost to the ASBR.
Q. Why is it that my Cisco 1600 router does not recognize the OSPF
protocol?
A. Cisco 1600 routers require the Plus feature set image of Cisco IOS Software to run OSPF.
Refer to Table 3: Cisco 1600 Series Routers Feature Sets in the Release Notes for Cisco IOS
Release 11.2(11) Software Feature Packs for Cisco 1600 Series Routers for more information.
Q. Why is it that my Cisco 800 router does not run OSPF?
A. Cisco 800 routers do not support OSPF. However, they do support Routing Information
Protocol (RIP) and Enhanced Interior Gateway Routing Protocol (EIGRP). You can use the
Software Advisor ( registered customers only) tool for more information on feature support.
Q. Should I use the same process number while configuring OSPF on
multiple routers within the same network?
A. OSPF, unlike Border Gateway Protocol (BGP) or Enhanced Interior Gateway Routing
Protocol (EIGRP), does not check the process number (or autonomous system number) when
adjacencies are formed between neighboring routers and routing information is exchanged.
The only case in which the OSPF process number is taken into account is when OSPF is used
as the routing protocol on a Provider Edge to Customer Edge (PE−CE) link in a Multiprotocol
Label Switching (MPLS) VPN. PE routers mark OSPF routes with the domain attribute
derived from the OSPF process number to indicate whether the route originated within the
same OSPF domain or from outside it. If the OSPF process numbering is inconsistent on PE
routers in the MPLS VPN, the domain−id OSPF mode command should be used to mark that
the OSPF processes with different numbers belong to the same OSPF domain.
This means that, in many practical cases, you can use different autonomous system numbers
for the same OSPF domain in your network. However, it is best to use consistent
OSPF−process numbering as much as possible. This consistency simplifies network
maintenance and complies with the network designer intention to keep routers in the same
OSPF domain.
Q. I have a router that runs Cisco Express Forwarding (CEF) and OSPF,
who does load−balancing when there are multiple links to a destination?
A. CEF works by performing the switching of the packet based on the routing table which is
populated by the routing protocols such as OSPF. CEF does the load−balancing once the
routing protocol table has been calculated. For more details on load balancing, refer to How
does load−balancing work?

Q. How does OSPF use two Multilink paths to transfer packets?

A. OSPF uses the metric aCost, which is related to the bandwidth. If there are equal cost paths
(the same bandwidth on both multilinks), OSPF installs both routes in the routing table. The
routing table tries to use both links equally, regardless of the interface utilization. If one of the
links in the first multilink fails, OSPF does not send all the traffic down the second multilink.
If the first multilink peaks 100%, OSPF does not send any traffic down the second multilink
because OSPF tries to use both links equally, regardless of the interface utilization. The
second is used fully only when the first multilink goes down.
Q. How can you detect the topological changes rapidly?
A. In order to have a rapid fault detection of topology changes, the hello timer value needs to
be set to 1 second. The hold timer value, which is is four times that of the hello timer, also
needs to be configured. There is a possibility of more routing traffic if the hello and hold
timer values are reduced from their default values.
Q. Does the 3825 Series Router support the OSPF Stub feature?
A. Yes, the 3800 Series Router that runs Advanced IPServices image supports the OSPF Stub
feature.
Q. What does the error message %OSPF−4−FLOOD_WAR: Process
process−id re−originates LSA ID ip address type−2 adv−rtr ip address in
area area id means?
A. The error message is due to the some router that is flushing the network LSA because the
network LSA received by the router whose LSA ID conflicts with the IP address of one of the
router's interfaces and flushes the LSA out of the network. For OSPF to function correctly the
IP addresses of transit networks must be unique. If it is not unique the conflicting routers
reports this error message. In the error message the router with the OSPF router ID reported
as adv−rtr reports this message.
Q. Can we have OSPF run over a GRE tunnel?
A. Yes, refer to Configuring a GRE Tunnel over IPSec with OSPF.