Multiarea OSPF 4

CCNP ROUTE Practice Exam:

Multiarea OSPF 4

Chris Bryant, “The Computer Certification Bulldog”

Here’s the 4th practice exam in this series. Dig in!

Question 1:

You’re configuring an OSPF ASBR, and have determined that all the routes injected into OSPF via redistribution can be summarized with the address 23.0.0.0 /12.

What is the full OSPF command to advertise this summary into the OSPF domain in place of the specific routes?

Question 2:

You’re performing two-way route redistribution between an EIGRP AS and an OSPF domain. On the OSPF routers, what is the default routing table code for the routes learned from EIGRP?

Extra credit: What code will appear on the EIGRP routers next to any routes that are injected into the EIGRP AS from OSPF?

Consider the following OSPF configuration and answer the following questions.

R4(config)#router ospf 1

R4(config-router)#network 172.12.0.0 0.0.0.255 area 0

R4(config-router)#network 1.1.1.1 0.0.0.0 area 1

R4(config-router)#router-id 20.1.1.1

< router reloaded at this point >

R4(config)#interface loopback0

R4(config-if)#ip address 50.1.1.1 255.255.255.255

Question 3:

Is that a legal configuration? If not, why not?

Question 4:

What is the OSPF process number?

Question 5:

What is the RID of this particular router?

Question 6:

Is this router an ABR, ASBR, both, or neither?

Explain why this router is or isn’t an ABR.

Explain why this router is or isn’t an ASBR.

This ends the questions about that router configuration.

Question 7:

An OSPF total stub router can contain what route types?

A. Default route only

B. E2 routes and a default route

C. E1 routes and a default route

D. Intra-area routes and a default route

E. Inter-area routes and a default route

Answers:

1. The full command is:

summary-address 23.0.0.0 255.240.0.0

2. These routes will appear as OSPF External Type 2 routes and will have a router table code of “O E2”.

The routes that are redistributed from OSPF into EIGRP will appear in the EIGRP routing tables with a code of “D EX”.

3. Yep, it’s legal.

4. The OSPF process number is 1, as indicated by the router ospf 1 command.

5. The OSPF RID was hardcoded as 20.1.1.1, and it took effect after the router reload I mentioned.

6. This router is an ABR since it borders Area 1 and Area 0.

There is no route redistribution shown on this router, so it’s not an ASBR.

7. “D”. The routing table of a total stub router can contain intra-area routes and a default inter-area route using the ABR as the next hop.

Thanks for taking this CCNP practice exam – here are the other OSPF multiarea exams!

OSPF Questions 2

OSPF Questions 2

Question 1

Into which two types of areas would an area border router (ABR) inject a default route? (Choose two)

A. the autonomous system of a different interior gateway protocol (IGP)
B. area 0
C. totally stubby
D. NSSA
E. stub
F. the autonomous system of an exterior gateway protocol (EGP)

 

Answer: C E

Explanation

Both stub area & totally stubby area allow an ABR to inject a default route. The main difference between these 2 types of areas is:

+ Stub area replaces LSA Type 5 (External LSA – created by an ASBR to advertise network from another autonomous system) with a default route
+ Totally stubby area replaces both LSA Type 5 and LSA Type 3 (Summary LSA – created by an ABR to advertise network from other areas, but still within the AS, sometimes called interarea routes) with a default route.

Below summarizes the LSA Types allowed and not allowed in area types:

Area Type Type 1 & 2 (within area) Type 3 (from other areas) Type 4 Type 5 Type 7
Standard & backbone Yes Yes Yes Yes No
Stub Yes Yes No No No
Totally stubby Yes No No No No
NSSA Yes Yes No No Yes
Totally stubby NSSA Yes No No No Yes

Question 2

Which three restrictions apply to OSPF stub areas? (Choose three)

A. No virtual links are allowed.
B. The area cannot be a backbone area.
C. Redistribution is not allowed unless the packet is changed to a type 7 packet.
D. The area has no more than 10 routers.
E. No autonomous system border routers are allowed.
F. Interarea routes are suppressed.

 

Answer: A B E

Question 3

Refer to the partial configurations in the exhibit. What address is utilized for DR and BDR identification on Router1?

Router1#show run**** output omitted ******

interface serial1/1
ipv6 address 2001:410:FFFE:1::64/64
ipv6 ospf 100 area 0
!
interface serial2/0
ipv6 address 3FFF:B00:FFFF:1::2/64
ipv6 ospf 100 area 0
!
ipv6 router ospf
router-id 10.1.1.3

A. the serial 1/1 address
B. the serial 2/0 address
C. a randomly generated internal address
D. the configured router-id address

 

Answer: D

Explanation

In OSPFv3 and OSPF version 2, the router uses the 32-bit IPv4 address to select the router ID for an OSPF process. The router ID selection process for OSPFv3 is described below (same as OSPF version 2):

1. The router ID is used if explicitly configured with the router-id command.
2. Otherwise, the highest IPv4 loopback address is used.
3. Otherwise, the highest active IPv4 address.
4. Otherwise, the router ID must be explicitly configured.

In this case the router ID 10.1.1.3 is explicitly configured -> D is correct.

Question 4

By default, which statement is correct regarding the redistribution of routes from other routing protocols into OSPF?

A. They will appear in the OSPF routing table as type E1 routes.
B. They will appear in the OSPF routing table as type E2 routes.
C. Summarized routes are not accepted.
D. All imported routes will be automatically summarized when possible.
E. Only routes with lower administrative distances will be imported.

 

Answer: B

Explanation

Type E1 external routes calculate the cost by adding the external cost to the internal cost of each link that the packet crosses while the external cost of E2 packet routes is always the external cost only. E2 is useful if you do not want internal routing to determine the path. E1 is useful when internal routing should be included in path selection. E2 is the default external metric when redistributing routes from other routing protocols into OSPF -> B is correct.

Question 5

Which statement is true about OSPF Network LSAs?

A. They are originated by every router in the OSPF network. They include all routers on the link, interfaces, the cost of the link, and any known neighbor on the link.
B. They are originated by the DR on every multi-access network. They include all attached routers including the DR itself.
C. They are originated by Area Border Routers and are sent into a single area to advertise destinations outside that area.
D. They are originated by Area Border Router and are sent into a single area to advertise an Autonomous System Border Router.

 

Answer: B

Explanation

Popular LSA Types are listed below:

LSA Type Description Details
1 Router LSA Generated by all routers in an area to describe their directly attached links
2 Network LSA Advertised by the DR of the broadcast network (does not cross ABR)
3 Summary LSA Advertised by the ABR of originating area
4 Summary LSA Generated by the ABR of the originating area to advertise an ASBR to all other areas in the autonomous system
5 AS external LSA Used by the ASBR to advertise networks from other autonomous systems
7 Defined for NSSAs Generated by an ASBR inside a Not-so-stubby area (NSSA) to describe routes redistributed into the NSSA

Question 6

Refer to the exhibit. OSPF is configured on all routers in the network. On the basis of the show ip ospf neighbor output, what prevents R1 from establishing a full adjacency with R2?

show_ip_ospf_neighbor.jpg

A. Router R1 will only establish full adjacency with the DR and BDR on broadcast multiaccess networks.
B. Router R2 has been elected as a DR for the broadcast multiaccess network in OSPF area
C. Routers R1 and R2 are configured as stub routers for OSPF area 1 and OSPF area 2.
D. Router R1 and R2 are configured for a virtual link between OSPF area 1 and OSPF area 2.
E. The Hello parameters on routers R1 and R2 do not match.

 

Answer: A

Explanation

From the output, we learn that R4 is the DR and R3 is the BDR so other routers will only establish full adjacency with these routers. All other routers have the two-way adjacency established -> A is correct.

Question 7

Refer to the exhibit. On the basis of the configuration provided, how are the Hello packets sent by R2 handled by R5 in OSPF area 5?

OSPF_Hello_packets.jpg

A. The Hello packets will be exchanged and adjacency will be established between routers R2 and R5.
B. The Hello packets will be exchanged but the routers R2 and R5 will become neighbors only.
C. The Hello packets will be dropped and no adjacency will be established between routers R2 and R5.
D. The Hello packets will be dropped but the routers R2 and R5 will become neighbors.

 

Answer: C

Explanation

Recall that in OSPF, two routers will become neighbors when they agree on the following: Area-id, Authentication, Hello and Dead Intervals, Stub area flag.

We must specify Area 5 as a stub area on the ABR (R2) and all the routers in that area (R5 in this case). But from the output, we learn that only R2 has been configured as a stub for Area 5. This will drop down the neighbor relationship between R2 and R5 because the stub flag is not matched in the Hello packets of these routers.

Question 8

When an OSPF design is planned, which implementation can help a router not have memory resource issues?

A. Have a backbone area (area 0) with 40 routers and use default routes to reach external destinations.
B. Have a backbone area (area 0) with 4 routers and 30,000 external routes injected into OSPF.
C. Have less OSPF areas to reduce the need for interarea route summarizations.
D. Have multiple OSPF processes on each OSPF router. Example, router ospf 1, router ospf 2

 

Answer: A

Question 9

When verifying the OSPF link state database, which type of LSAs should you expect to see within the different OSPF area types? (Choose three)

A. All OSPF routers in stubby areas can have type 3 LSAs in their database.
B. All OSPF routers in stubby areas can have type 7 LSAs in their database.
C. All OSPF routers in totally stubby areas can have type 3 LSAs in their database.
D. All OSPF routers in totally stubby areas can have type 7 LSAs in their database.
E. All OSPF routers in NSSA areas can have type 3 LSAs in their database.
F. All OSPF routers in NSSA areas can have type 7 LSAs in their database.

 

Answer: A E F

Explanation

Below summarizes the LSA Types allowed and not allowed in area types:

Area Type Type 1 & 2 (within area) Type 3 (from other areas) Type 4 Type 5 Type 7
Standard & backbone Yes Yes Yes Yes No
Stub Yes Yes No No No
Totally stubby Yes No No No No
NSSA Yes Yes No No Yes
Totally stubby NSSA Yes No No No Yes

Popular LSA Types are listed below:

LSA Type Description Details
1 Router LSA Generated by all routers in an area to describe their directly attached links
2 Network LSA Advertised by the DR of the broadcast network (does not cross ABR)
3 Summary LSA Advertised by the ABR of originating area
4 Summary LSA Generated by the ABR of the originating area to advertise an ASBR to all other areas in the autonomous system
5 AS external LSA Used by the ASBR to advertise networks from other autonomous systems
7 Defined for NSSAs Generated by an ASBR inside a Not-so-stubby area (NSSA) to describe routes redistributed into the NSSA

Question 10

You are troubleshooting an OSPF problem where external routes are not showing up in the OSPF database. Which two options are valid checks that should be performed first to verify proper OSPF operation? (Choose two)

A. Are the ASBRs trying to redistribute the external routes into a totally stubby area?
B. Are the ABRs configured with stubby areas?
C. Is the subnets keyword being used with the redistribution command?
D. Is backbone area (area 0) contiguous?
E. Is the CPU utilization of the routers high?

 

Answer: A C

Explanation

A totally stubby stubby area cannot have an ASBR so it will discard this type of LSA (LSA Type 5) -> A is a valid check.

Each stubby area needs an ABR to communicate with other areas so it is normal -> B is not a valid check.

When pulling routes into OSPF, we need to use the keyword “subnets” so that subnets will be redistributed too. For example, if we redistribute these EIGRP routes into OSPF:

+ 10.0.0.0/8
+ 10.10.0.0/16
+ 10.10.1.0/24

without the keyword “subnets”

router ospf 1
redistribute eigrp 1

Then only 10.0.0.0/8 network will be redistributed because other routes are not classful routes, they are subnets. To redistribute subnets we must use the keyword “subnets”

router ospf 1
redistribute eigrp 1 subnets

-> C is a valid check.

We don’t need to care if area 0 is contiguous or not -> D is not a valid check.

CPU utilization cannot be the cause for this problem -> E is not a valid check.

For this question, please read our tutorial about OSPF LSA Types for more detail.

CCNP ROUTE Practice Exam Questions

CCNP ROUTE Practice Exam Questions:

source: http://www.thebryantadvantage.com/CCNPExamBSCIPracticeExamMultiAreaOSPF3.htm 

 

Multi-Area OSPF (Set #3)

Links to the other OSPF multiarea practice exams can be found at the end of the answers to this one.

As always with my practice exams, all multiple choice questions are “select all that apply”. Let’s get started!

Question 1:

The cost of an E2 route indicates what?

A. The cost of the path from the local router to the ASBR.

B. The cost of the path from the ASBR to the destination network.

C. The full cost of the path to the destination network from the local router.

D. The cost of the path from the ABR to the destination network.

Question 2:

What LSA type (numeric value) is generated by an ABR and describes inter-area links?

Question 3:

The cost of an E1 route indicates which of the following?

A. The cost of the path from the local router to the ASBR.

B. The cost of the path from the ASBR to the destination network.

C. The cost of the path from the local router to the destination network.

D. The cost of the path from the ASBR to the destination network.

Question 4:

By default, how does an OSPF routing table indicate routes that were injected into the OSPF domain via redistribution?

A. O EX

B. O *IA

C. O E2

D. O E1

E. O N2

Question 5:

Which of the following commands will show whether a router’s neighbors are or are not an ABR or ASBR?

A. show ip ospf database

B. show ip route ospf

C. show ip ospf neighbors

D. show ip ospf border-routers

Question 6:

You see a route marked “O IA” in your Cisco routing table. You know what the “O” stands for, or you wouldn’t be taking this exam. 🙂

However, what does the “IA” indicate regarding the destination network?

A. It’s in the same area as the local router.

B. It’s in another area.

C. It will be reached via a default route.

D. The route was learned via route redistribution.

Question 7:

What’s the number of the LSA type that is generated by every router for every area it belongs to, and is flooded to a single area?

Answers:

1. “B”. The default setting for a route learned by OSPF via redistribution is E2, and the metric of an E2 route is only the cost of the path from the ASBR to the destination network.

2. Type 3 LSAs are generated by ABRs and describe inter-area routes.

3. “C”. OSPF E1 routes reflect the entire cost of the path from the local router to the destination network.

4. “C”. As mentioned in the answer to Question 1. 🙂

5. “D”. It’s not one of the most common OSPF commands, but you can get some important information from the show ip ospf border-routers command.

6. “B”. The “IA” indicates a network that is in another OSPF area.

7. Type 1 LSAs are generated by every router for every area that it belongs to, and these LSAs are never flooded outside that particular area.

Thanks for taking this CCNP practice exam – here are the other OSPF multiarea exams on the site!

EIGRP Questions 3

EIGRP Questions 3

Here you will find answers to EIGRP Questions – Part 3

Note: If you are not sure about EIGRP, please read my EIGRP tutorial.

Question 1

Refer to the exhibit. ROUTE.com has just implemented this EIGRP network. A network administrator came to you for advice while trying to implement load balancing across part of their EIGRP network.
If the variance value is configured as 2 on all routers and all other metric and K values are configured to their default values, traffic from the Internet to the data center will be load balanced across how many paths?

EIGRP_load_balancing.jpg

A. 1
B. 2
C. 3
D. 4

 

Answer: C

Explanation

First we should list all the paths from the Internet to the data center:

+ A-B-C-H with a metric of 70 (40 + 15 + 15)
+ A-B-E-H with a metric of 60 (40+10+10)
+ A-D-E-H with a metric of 30 (10+10+10)
+ A-D-E-B-C-H with a metric of 60 (10+10+10+15+15)
+ A-D-E-F-G-H with a metric of 70 (10+10+10+20+20)
+ A-F-G-H with a metric of 60 (20+20+20)
+ A-F-E-H with a metric of 40 (20+10+10)

So the path A-D-E-H will be chosen because it has the best metric. But EIGRP can support unequal cost path load balancing. By configuring the variance value of 2, the minimum metric is increased to 60 (30 * 2) and all the routes that have a metric of less than or equal to 60 and satisfy the feasibility condition will be used to send traffic.

Besides the main path A-D-E-H we have 4 more paths that have the metric of less than or equal to 60 (we also include the Advertised Distances of these routes for later comparison):

+ A-B-E-H with an AD of 20
+ A-D-E-B-C-H with an AD of 50
+ A-F-G-H with an AD of 40
+ A-F-E-H with an AD of 20

Now the last thing we need to consider is the feasible condition. The feasible condition states:

“To qualify as a feasible successor, a router must have an AD less than the FD of the current successor route”

The FD of the current successor route here is 30 (notice that the variance number is not calculated here). Therefore there are only 2 paths that can satisfy this conditions: the path A-B-E-H & A-F-E-H.

In conclusion, traffic from the Internet to the data center will be load balanced across 3 paths, including the main path (successor path) -> C is correct.

Question 2

Which condition must be satisfied before an EIGRP neighbor can be considered a feasible successor?

A. The neighbor’s advertised distance must be less than or equal to the feasible distance of the current successor.
B. The neighbor’s advertised distance must be less than the feasible distance of the current successor.
C. The neighbor’s advertised distance must be greater than the feasible distance of the current successor.
D. The neighbor’s advertised distance must be equal to the feasible distance of the current successor.
E. The neighbor’s advertised distance must be greater than or equal to the feasible distance of the current successor.

 

Answer: B

Explanation

As explained in question 1, this is called the feasible condition.

Question 3

Which statement about a non-zero value for the load metric (k2) for EIGRP is true?

A. A change in the load on an interface will cause EIGRP to recalculate the routing metrics and send a corresponding update out to each of its neighbors.
B. EIGRP calculates interface load as a 5-minute exponentially weighted average that is updated every 5 minutes.
C. EIGRP considers the load of an interface only when sending an update for some other reason.
D. A change in the load on an interface will cause EIGRP to recalculate and update the administrative distance for all routes learned on that interface.

 

Answer: C

Explanation

The load metric (k2) represents the worst load on a link between source and destination.

EIGRP routing updates are triggered only by a change in network topology (like links, interfaces go up/down, router added/removed), and not by change in interface load or reliability -> A & D are not correct.

The load is a five minute exponentially weighted average that is updated every five seconds (not five minutes) -> B is not correct.

EIGRP considers the load of an interface only when sending an update for some other reason (like a link failure, topology change). Updates are not sent out each time the load changes -> C is correct.

Note: To learn how to calculate EIGRP metric, please read my EIGRP tutorial – Part 3.

Question 4

Your network consists of a large hub-and-spoke Frame Relay network with a CIR of 56 kb/s for each spoke.
Which statement about the selection of a dynamic protocol is true?

A. EIGRP would be appropriate if LMI type ANSI is NOT used.
B. EIGRP would be appropriate, because the Frame Relay spokes could be segmented into their own areas.
C. EIGRP would be appropriate, because by default, queries are not propagated across the slow speed Frame Relay links.
D. EIGRP would be appropriate, because you can manage how much bandwidth is consumed over the Frame Relay interface.

 

Answer: D

Explanation

By default, EIGRP will limit itself to using no more than 50% of the interface bandwidth. The primary benefit of controlling EIGRP’s bandwidth usage is to avoid losing EIGRP packets, which could occur when EIGRP generates data faster than the interface line can absorb it. This is of particular benefit on Frame Relay networks, where the access interface bandwidth and the PVC capacity may be very different.

For example, in our Frame Relay topology a Hub is connected with 4 Spoke routers. The main Frame Relay interface on Hub router is 512Kpbs which is not enough to use for 6 links of 128 Kbps ( = 768 Kbps).

EIGRP_Frame_Relay_CIR.jpg

The solution here is we can use 512 / 6 = 85 Kbps on each subinterface of Hub by using “bandwidth 85” command. For example:

Hub(config)#interface Serial0/0.1 point-to-point
Hub(config-subif)#bandwidth 85

Also on Spoke routers we need to set this value. For example on Spoke1:

Spoke1(config)#interface Serial0/1.0 point-to-point
Spoke1(config-subif)#bandwidth 85

Notice that by default, EIGRP limits itself to use no more than 50% of the configured interface bandwidth. In this case EIGRP will not use more than 42.5 Kbps (50% of 85 Kbps).

(For more information about implementing EIGRP over Frame Relay, please read http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094063.shtml)

Question 5

When an EIGRP topology change is detected, what is the correct order of events when there is a FS?

A.
The neighbor adjacency is deleted.
The feasible route is used.
DUAL is notified.
Remove all topology entries learned from that neighbor.

B.
DUAL is notified.
Remove all topology entries learned from that neighbor.
The neighbor adjacency is deleted.
Routes enter the Active state and the feasible route is used.

C.
The neighbor adjacency is deleted.
Routes enter the Active state and the feasible route is used.
DUAL is notified.
Remove all topology entries learned from that neighbor.

D.
DUAL is notified.
The neighbor adjacency is deleted.
Remove all topology entries learned from that neighbor.
The feasible route is used.

 

Answer: D

Question 6

Refer to the exhibit. You want to use all the routes in the EIGRP topology for IP load balancing.

EIGRP_show_ip_eigrp_topology.jpg

Which two EIGRP subcommands would you use to accomplish this goal? (Choose two)

A. traffic-share balanced
B. distance
C. maximum-paths
D. default-network
E. variance

 

Answer: C E

Explanation

Notice that the “maximum-paths” command is used to share traffic to equal cost path while the “variance” command can share traffic to unequal cost path.

In the output above we learn that EIGRP is using 2 successors to send traffic. By using the “variance 2” command we can share traffic to other feasible successor routes. But by default, EIGRP only shares traffic to 4 paths. So we need to use the “maximum-paths 6” to make sure all of these routes are used.

Question 7

Refer to the exhibit. R1 accesses the Internet using E0/0. You have been asked to configure R1 so that a default route is generated to its downstream devices (191.0.0.1 and 192.0.0.1). Which commands would create this configuration?

EIGRP_default_route.jpg

A.
router eigrp 190
redistribute static
!
ip route 0.0.0.0 0.0.0.0 Null0

B. ip default-network 20.0.0.0

C.
router eigrp 190
redistribute static
!
ip route 0.0.0.0 255.255.255.255 Null0

D. ip default-network 20.20.20.0

 

Answer: A

Question 8

Which command will display EIGRP packets sent and received, as well as statistics on hello packets, updates, queries, replies, and acknowledgments?

A. debug eigrp packets
B. show ip eigrp traffic
C. debug ip eigrp
D. show ip eigrp interfaces

 

Answer: B

Explanation

Below is the output of the “show ip eigrp traffic” command:

EIGRP_show_ip_eigrp_traffic.jpg

Question 9

Which three statements are true about EIGRP operation? (Choose three)

A. When summarization is configured, the router will also create a route to null 0.
B. The summary route remains in the route table, even if there are no more specific routes to the network.
C. Summarization is configured on a per-interface level.
D. The maximum metric for the specific routes is used as the metric for the summary route.
E. Automatic summarization across major network boundaries is enabled by default.

 

Answer: A C E

Question 10

Which two statements about the EIGRP DUAL process are correct? (Choose two)

A. An EIGRP route will go active if there are no successors or feasible successors in the EIGRP topology table.
B. An EIGRP route will go passive if there are no successors in the EIGRP topology table.
C. DUAL will trigger an EIGRP query process while placing the flapping routes in the holddown state.
D. A feasible successor in the EIGRP topology table can become the successor only after all the query requests have been replied to.
E. The stuck in active state is caused when the wait for the query replies have timed out.
F. EIGRP queries are sent during the loading state in the EIGRP neighbor establishment process.

 

Answer: A E

Question 11

What are three key concepts that apply when configuring the EIGRP stub routing feature in a hub and spoke network? (Choose three)

A. A hub router prevents routes from being advertised to the remote router.
B. Only remote routers are configured as stubs.
C. Stub routers are not queried for routes.
D. Spoke routers connected to hub routers answer the route queries for the stub router.
E. A stub router should have only EIGRP hub routers as neighbors.
F. EIGRP stub routing should be used on hub routers only.

 

Answer: B C E

Redistribute Questions 3

Redistribute Questions 3

Here you will find answers to Redistribute Questions – Part 3

Question 1

Given the accompanying output, which additional command is needed to redistribute IGRP into EIGRP?

Router eigrp 123
Network 10.10.10.0
No auto-summary
!
Router igrp 123
Network 172.16.0.0
Network 172.17.0.0

A. Under the router igrp mode add redistribute eigrp 123
B. Under the router eigrp mode add redistribute igrp 123
C. Under the router eigrp mode add redistribute igrp 123 subnets
D. None, EIGRP and IGRP are automatically redistributed in this instance.

 

Answer: D

Explanation

If IGRP and EIGRP use the same Autonomous System (AS) then redistribution occurs automatically. In this case both IGRP & EIGRP use the same AS 123 so they are automatically redistributed.

If IGRP and EIGRP use different AS numbers then redistribution must be done manually.

Question 2

Study the exhibit carefully. Router R1 is connected to networks 172.16.1.0/26 and 172.16.1.64/27. Based on the partial output in the exhibit, which description is correct?

prefix-list.jpg

A. Router R1 should be reconfigured with an ACL instead of an ip prefix-list command.
B. Router R1 will advertise both routes.
C. Router R1 will deny the 172.16.1.0/27 route while permitting the 172.16.1.0/26 route to be advertised.
D. Router R1 will deny the 172.16.1.0/26 route while permitting the 172.16.1.64/27 route to be advertised.

 

Answer: C

Explanation

Prefix lists are configured with permit or deny keywords to either permit or deny the prefix based on the matching condition. A prefix list consists of an IP address and a bit mask. The IP address can be a classful network, a subnet, or a single host route. The bit mask is entered as a number from 1 to 32.

Prefix lists are configured to match an exact prefix length or a prefix range. The ge and le keywords are used to specify a range of the prefix lengths to match, providing more flexible configuration than can be configured with just the network/length argument. The prefix list is processed using an exact match when neither ge nor le keyword is entered.

Therefore in this case the exact 172.16.1.0/26 network is permitted while other networks are denied.

(Reference: http://www.cisco.com/en/US/docs/ios/12_3t/ip_route/command/reference/ip2_i2gt.html)

Question 3

Refer to the exhibit. The partial configuration for an OSPF ASBR and an Area 0 ABR is shown. Assume the OSPF configurations throughout the network are operable. Which statement about these configurations is true?

Redistribute_OSPF_ASBR_ABR.jpg

A. The ASBR route-maps are basically useless, because there are no deny prefix-lists.
B. LSA Type 5s will not be received by the ABR from the ASBR.
C. The OSPF backbone will not learn any RFC 1918 addresses.
D. The matched prefix-list addresses will be given a metric of 255, which is essentially unreachable.

 

Answer: C

Explanation

The ASBR accepts RFC 1918 addresses and set these networks to “tag 255” but when advertising into Area 0, the ABR Area 0 filters out these networks because they match “tag 255” so the OSPF backbone will not learn any RFC 1918 addresses.

Note that if you use an ACL in a route-map deny clause, routes that are permitted by the ACL are not redistributed.

All the networks with “tag 255” are blocked by the clause 10 while all other networks are permitted by the clause 20 of the route-map (if a match command is not present, all routes match the clause).

Note:

RFC 1918 addresses include:

+ Class A: 10.0.0.0 – 10.255.255.255 (10/8 prefix)
+ Class B: 172.16.0.0 – 172.31.255.255 (172.16/12 prefix)
+ Class C: 192.168.0.0 – 192.168.255.255 (192.168/16 prefix)

Question 4

A network administrator is troubleshooting a redistribution of RIP routes into OSPF. Given the exhibited configuration commands, which statement is true?

rooter rip
network 10.0.0.0
!
router ospf 5
network 172.10.0.0 0.0.255.255 area 0
redistribute rip

A. Redistributed routes will be tagged as external type 1 (E1) with a metric of 30.
B. Redistributed routes will be tagged as external type 2 (E2) with a metric of 20.
C. Redistributed routes will maintain their original RIP routing metric.
D. Redistributed routes will have a default metric of 0 and will be treated as unreachable and not advertised.
E. Redistributed routes will have a default metric of 0 but will not be treated as reachable and will be advertised.

 

Answer: B

Explanation

By default, all routes redistributed into OSPF will be tagged as external type 2 (E2) with a metric of 20, except for BGP routes (with a metric of 1).

Note: The cost of a type 2 route is always the external cost, irrespective of the interior cost to reach that route. A type 1 cost is the addition of the external cost and the internal cost used to reach that route.

Question 5

Refer to the exhibit. On the basis of the partial configuration, which two statements are correct? (Choose two)


!
router rip
distribute-list 2 out ethernet 0
distribute-list 1 out
!
access-list 1 permit 10.0.0.0 0.255.255.255
access-list 2 permit 10.0.1.0 0.0.0.255
!

A. Only routes matching 10.0.1.0/24 will be advertised out Ethernet 0.
B. Only routes 10.0.1.0/24 will be sent out all interfaces.
C. Only routes 10.0.1.0/24 will be allowed in the routing table.
D. Only routes matching 10.0.0.0/8 will be advertised out Ethernet 0.
E. Only routes matching 10.0.0.0/8 will be advertised out interfaces other than Ethernet 0.
F. All routes will be advertised out interfaces other than Ethernet 0.

 

Answer: A E

Explanation

In this case, the following algorithm is used when multiple distribute-lists are used:

1. First check which interface is being sent out. If it is Ethernet 0, distribute-list 2 is applied first. If the network is denied then no further checking is done for this network. But if distribute-list 2 permits that network then distribute-list 1 is also checked. If both distribute-lists allow that network then it will be sent out.

2. If the interface is not Ethernet 0 then only distribute-list 1 is applied.

Now let’s take some examples.
+ If the advertised network is 10.0.1.0/24, it will be sent out all interfaces, including Ethernet 0.
+ If the advertised network is 10.0.2.0/24, it will be sent out all interfaces, excepting Ethernet 0.
+ If the advertised network is 11.0.0.0/8, it will be dropped.

Note: It is possible to define one interface-specific distribute-list per interface and one protocol-specific distribute-list for each process/autonomous-system.

(For more information, please read: http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080208748.shtml)

Question 6

Refer to the exhibit. Examine the partial configuration and the routing table excerpt. Which routes would be redistributed into OSPF area 1?

redistribute_ospf_subnets.jpg

A. 10.10.10.16/28 only
B. 10.10.10.16/28 and 10.10.10.64/26
C. 10.10.10.16/28, 10.10.10.64/26, and 172.16.10.0/24
D. 10.10.10.64/26 only

 

Answer: B (but in the exam you should choose D)

Explanation

The network 172.16.10.0/24 belongs to OSPF (we know from the “network 172.16.0.0 0.0.255.255 area 1” command) so it will not be redistributed.

When using the “subnets” keyword, all the connected networks will be redistributed so 10.10.10.16/28 & 10.10.10.64/26 will be redistributed, too. You can read my GNS3 lab about this topic here: http://www.digitaltut.com/redistribute-eigrp-and-ospf-gns3-lab.

Therefore the correct answer should be B but in the exam you should choose D. Maybe it is a mistake of Cisco.

Question 7

Refer to the exhibit. A partial routing configuration is shown. Complete the configuration so that only the default-network is redistributed from EIGRP 190 into EIGRP 212. Which ACL statement completes the configuration correctly?

router eigrp 190
redistribute eigrp 212
network 192.0.0.0 0.0.0.3
!
router eigrp 212
redistribute eigrp 190 route-map default_route
network 212.50.185.96 0.0.0.31
!
route-map defau1t_route permit 10
match ip address 100

A. access-list 100 permit ip 0.0.0.0 0.0.0.0 0.0.0.0 0.0.0.0
B. access-list 100 permit ip host 0.0.0.0 any
C. access-list 100 permit ip any host 0.0.0.0
D. A default-network cannot be redistributed between routing processes.

 

Answer: C

Explanation

The command “access-list 100 permit ip any host 0.0.0.0” means permit any source address with the destination of 0.0.0.0/0, which is the default route

Note:

any equals 0.0.0.0 255.255.255.255

host 0.0.0.0 equals 0.0.0.0 0.0.0.0

Question 8

Refer to the exhibit. Router B and router C are performing mutual redistribution between OSPF and EIGRP, and their default metrics are configured the same. Router D has equal cost paths to networks where both paths are not really equal cost. For example, network 172.16.54.0 shows equal cost through both router B and router C, though in reality the cost is greater using router C. Other routers, though not shown, are connected to the 172.16.54.0 and 172.16.55.0 networks, and the same issues exist to those routers and the networks connected to them.
What can be done so that data will be routed along the most optimal path in the network?

mutual_redistribute_EIGRP_OSPF.jpg

A. Redistribute connected interfaces on router B and router C.
B. Set the maximum number of equal cost paths to 1 in all routers.
C. When redistributing EIGRP into OSPF, set the external metric type to type E1.
D. Adjust the default metrics in router B and router C so that the values are different in each router.
E. None of these solutions will fix the problem. Migrate to a single dynamic routing protocol.

 

Answer: E

Explanation

Let’s discuss about answers C & D first.

From the output, we learn that all the External OSPF routes have metrics of 100 (the second parameters in [110/100]). This is not the default metric of OSPF Type 2 External route (the default value is 20) so the metrics of redistributed routes have been modified. Maybe when redistributing into OSPF, the “metric” in the “redistribute” command or the “default-metric” command was used on router B & C to assign the metric of these routes. Something like this:

router ospf 1
redistribute eigrp 1 metric 100 subnets

or

router ospf 1
…..
default-metric 100

Therefore even if we use the metric type E1 the problem still exists because the link B-D & C-D seems to have the same metric -> the total metrics remains the same -> C is not correct.

We can use route-map and set different metrics for each networks but some unshown networks will have the same issues -> D is not a good choice

So the best answer should be E.

Question 9

Refer to the exhibit. A new TAC engineer comes to you for advice. The engineer wants to configure RIPv2-OSPF two-way redistribution while avoiding routing loops. Which two additions to the router B1 configuration should the engineer make? (Choose two)

RIPv2_OSPF_Redistribute.jpg

A. access-list 40 deny 172.16.1.0 0.0.0.255
access-list 40 permit any
router rip
redistribute ospf 100 metric 5
distribute-list 40 out ospf 100

B. ip prefix-list rip_routes permit 172.16.1.16/25 ge 26 le 28
route-map redis-ospf deny 10
match ip address prefix-list rip_routes
router rip
redistribute ospf 10 route-map redis-ospf subnets

C. ip prefix-list rip-to-ospf permit 10.1.1.8/25 ge 26 le 28
route-map redis-rip deny 20
match ip address prefix-list rip-to-ospf
router ospf 100
redistribute rip route-map redis-rip subnets

D. access-list 15 deny 10.1.1.0 0.0.0.63
access-list 15 permit any
route-map redis-rip deny 10
match ip address 15
route-map redis-rip permit 20
router ospf 100
redistribute rip route-map redis-rip subnets

 

Answer: A D

Explanation

B1 is not the only router that redistributes between RIP & OSPF. The “small” router below B1 can be configured for this task too so B1 can try to redistribute networks advertised by that “small” router again. Therefore it is necessary to filter out networks that have been advertised by the “small” router. For example, we need to prevent network 172.16.1.0/24 from advertised back into RIPv2 or network 10.1.1.0/26 from advertised back into OSPF. Notice that all networks in OSPF domain (including 10.1.1.8/30, 10.1.1.12/30, 10.1.1.48/28, 10.1.1.32/28) can be summarized as 10.1.1.0/26 and all networks in RIP domain (including 172.16.1.24/30, 172.16.1.20/30, 172.16.1.32/28, 172.16.1.48/28) can be summarized as 172.16.1.0/24 -> answers A & D are correct.

In answer B, the command “ip prefix-list rip_routes permit 172.16.1.16/25 ge 26 le 28” means:

+ First check the first 25 bits of the address -> this will allow addresses from 172.16.1.0 to 172.16.1.127

ip_prefix_list.jpg

+ If those match then check the subnet mask, which in this case can be GREATER THAN or EQUAL to 26 bits & LESS THAN or EQUAL to 28 bits -> meaning that /26, /27, /28 subnet masks would match.

For example, networks 172.16.1.0/26; 172.16.1.16/28 would match (but notice networks 172.16.1.0/25; 172.16.1.128/26 wouldn’t).

In the “ip prefix-list rip_routes permit 172.16.1.16/25 ge 26 le 28”, the prefix-list “rip_routes” only covers networks 172.16.1.32/28 & 172.16.1.48/28 but can’t cover networks 172.16.1.24/30 & 172.16.1.20/30. Also, the OSPF process in the “redistribute” command should be 100, not 10 -> B is not correct.

Same problem as answer B, the prefix-list in answer C can’t cover networks 10.1.1.8/30 & 10.1.1.12/30 -> C is not correct.

Question 10

Refer to the exhibit. The network administrator is trying to configure mutual redistribution between EIGRP and OSPF. Autosummarization in EIGRP 100 AS is disabled. After adding OSPF configuration to router E31, the network administrator checked the routing table of router B2, but none of the EIGRP routes appeared there.
To redistribute the EIGRP AS 100 routes into OSPF, which command should be added, or edited, on router B1 under router ospf 10?

Redistribute_OSPF_EIGRP_subnets.jpg

A. redistribute eigrp 100 metric-type 1
B. redistribute eigrp 100 subnets
C. no auto-summary 10.0.0.0 255.0.0.0
D. area 0 range 10.10.0.0 255.255.0.0

 

Answer: B

Explanation

When redistributing into OSPF without keyword “subnets”, only classful networks will be redistributed. Classful networks here mean networks with the default major subnet masks (for example 10.0.0.0/8; 180.1.0.0/16; 200.200.200.0/24…).

In fact, the routing table on the exhibit above is not totally correct. The network 192.168.110.0/24 will be redistributed and shown in the routing table of B2 even if the keyword “subnets” is not used because it belongs to class C with the default subnet mask of class C.

To make all the networks, including subnets appear in the routing table of B2 we must use keyword “subnets” when redistributing into OSPF. This is also an important thing to remember when redistributing into OSPF.

Please read my Redistribute EIGRP and OSPF – GNS3 Lab if you are still not sure about this.

Question 11

Refer to the exhibit. Routers R1 and R2 are running EIGRP and have converged. On the basis of the information that is presented, which statement is true?

Redistribute_distribute-list-passive-interface.jpg

A. All outgoing routing updates from router R1 to router R2 will be suppressed, but the inbound updates will continue to be received.
B. All incoming routing updates from R2 will be suppressed, but the outgoing updates will continue to be sent.
C. Both outgoing and incoming routing updates on R1 will be stopped because of the passive-interface Serial0/0 configuration statement.
D. Both outgoing and incoming routing updates on R1 will be permitted because the distribute-list 20 out Serial0/0 command cannot be used with association with the outgoing interface.

 

Answer: C

Explanation

In EIGRP (and OSPF) the passive-interface command stops sending outgoing hello packets, hence the router can not form any neighbor relationship via the passive interface. This behavior stops both outgoing and incoming routing updates -> the distribute-list has no use here.