主题：CCNP认证试题[17]

Section 17: Explain the use of redistribution between BGP and Interior Gateway Protocols (IGPs) (9 questions)

QUESTION NO: 1

When you’re running an Interior Gateway Protocol (like OSPF or EIGRP); why would you want to redistribute these routes into BGP?

A. So BGP can propagate this information to other IGP neighbors.

B. So BGP can propagate this information to other IBGP neighbors.

C. So BGP can propagate this information to other EBGP neighbors.

D. So BGP can propagate this information to other OSPF neighbors.

Answer: C

Explanation:

When BGP is used between autonomous systems (AS), the protocol is referred to as External

BGP (EBGP). In order to advertise the IP subnets used within your network to other autonomous systems, these routes must be injected into BGP.

Note: Customer networks usually employ an Interior Gateway Protocol (IGP) such as RIP or

OSPF for the exchange of routing information within their networks

QUESTION NO: 2

You are the administrator of an internal network configured for OSPF. Two gateway routers are learning external routes from BGP and inserting those learned routes into your network via redistribution. One of the gateway routers is called TestKing2 and it is inserting the subnets 128.213.64.0 through 128.213.95.0. Your task is to configure the gateway router Testking1 to summarize the subnets into one range before injecting them into OSPF. The first two lines have already been configured. Select the boxes below and place the three correct boxes in the right order to complete the configuration.

TestKing1(config)#router ospf 100

TestKing1(config-router)#redistribute bgp 50 metric 1000 subnets

Answer: Summary-address 128.213.64.0 255.255.224.0

Explanation:

External route summarization, external routes that are injected into OSPF via redistribution, is one via the following router ospf subcommand: summary-address ip-address mask This command is effective only on ASBRs doing redistribution into OSPF as in this scenario.

QUESTION NO: 3

Your network is running BGP as well as EIGRP. You are considering redistributing your BGP routes into your EIGRP. What factors must you consider before redistributing BGP routes to IGP?

A. IGPs are limited to 250 routes

B. A full BGP routing table may contain 100,000+ routes

C. Because of possible routing loops, Cisco router configuration does not allow BGP routes to be redistributed into an IGP.

D. Because BGP routes are not advertised unless they are known by the IGP, Cisco automatically redistributes routes into BGP.

Answer: B

Explanation:

The size of the BGP routing table must be taken into consideration. It is generally not recommended to redistribute BGP routes into your IGP due to the large number of routes that can be obtained from BGP.

Incorrect Answers:

A: There is no such limit.

C: BGP routes can very well be distributed into an IGP.

D: BGP routers are not redistributed automatically.

QUESTION NO: 4

Why is it sometimes necessary to redistribute IGP protocols like OSPF into BGP?

A. So that BGP can propagate this information to other IGP neighbors.

B. So that BGP can propagate this information to other IBGP neighbors.

C. So that BGP can propagate this information to other EBGP neighbors.

D. So that BGP can propagate this information to other OSPF neighbors.

Answer: C

Explanation:

Customer networks usually employ an Interior Gateway Protocol (IGP) such as RIP or OSPF for the exchange of routing information within their networks. In order to advertise these networks to other network across the Internet, they must be redistributed into BGP.

QUESTION NO: 5

According to Cisco’s official recommendations, what is the preferred method for populating the BGP table on your router with IGP routes?

A. Use the network command.

B. Redistribute EBGP routes into BGP.

C. Redistribute dynamic routes into BGP.

D. Redistribute static routes into the IGP.

Answer: A

Explanation:

For BGP the network command allows BGP to advertise a network that is already in the IP table.

Note: An alternate solution, not listed here, is to redistribute the IGP routes into BGP.

Incorrect Answers:

B: We want to distribute internal routes into BGP, not external routes.

C: Redistribution from the IGP is NOT advised because there is a great reliance on the IGP table. It could cause instability.

D: We want to populate the BGP table, not the IGP table.

QUESTION NO: 6

You are the midst of configuring a gateway route named RouterTestK. Your task is to configure the router to redistribute the various subnets it learns via BGP AS75 into OSPF, all while using the default metric. Select three of the correct command phrases below and place them in the correct order in the command sequence.

Answer:

Explanation: We want to redistribute BGP AS 75 into OSPF.

Syntax of the redistribution command:

router(config-router)#redistribute protocol [process-id] [metric [metric-type type-value]

[route-map map-tag] [subnets] [tag tagvalue]

We specify the BGP protocol and the Autonomous System number 75. We also use the subnets

keyword, which is an optional OSPF parameter that specifies that subnetted routes should also be redistributed. Only routes that are not subnetted are redistributed if the subnets keyword is not specified.

Incorrect Answers:

ospf: We want to redistribute the routes learned from BGP AS 75 into OSPF so we should specify the BGP protocol. area: Is not used in the redistribute command. default-metric: Default-metric is a separate command. metric, 200: We don’t have to specify a metric since we want to use a default metric.

QUESTION NO: 7

You are the administrator of the TestKing network which is configured for OSPF. Two gateway routers are learning external routes from BGP and inserting those learned routes into your network via redistribution. One of the gateway routers is called TestKing1 and it is inserting the subnets 142.14.64.0 through 142.14.95.0. Your task is to configure router Testking1 to summarize the subnets into one range before injecting them into OSPF. The first two lines have already been configured. Select the boxes below and place the three correct boxes in the right order to complete the configuration.

TestKing1(config)#router ospf 200

TestKing1(config-router)#redistribute bgp 80 metric 1100 subnets

Answer:

Explanation:

The summary-address router configuration command is used to create aggregate addresses for OSPF.

Simplified syntax: summary-address address mask

We study the subnets that the TestKing1 router is injecting in binary to decide where to summarize:

Decimal 1st octet 2nd octet 3rd octet 4th octet

142.14.64.0 10001110 00001110 01000000 00000000

142.14.95.0 10001110 00001110 01011111 00000000

255.255.224.0 11111111 11111111 11100000 00000000

The subnet we can summarize on is marked with red. This is the 142.14.64.0/19 network. It has a

255.255.224.0 subnet mask.

QUESTION NO: 8

You are the administrator of an internal network configured for OSPF. Two gateway routers (TestKing1 & TestKing2) are redistributing external BGP routes. TestKing1 is redistributing subnets ranging from 163.150.64.0 – 163.160.95.0 and TestKing2 is doing the same with 163.150.96.0 – 163.150.127.0. Your task is to configure the gateway router Testking1 to summarize these subnets into one range before redistributing them into OSPF. The first two lines have already been configured. Select the boxes below and place the three correct boxes in the right order to complete the configuration.

Exhibit:

TestKing1(config)#router ospf 100

TestKing1(config-router)#redistribute bgp 70 metric 500 subnets

Answer:

Explanation:

External route summarization, external routes that are injected into OSPF via redistribution, is done via the following router ospf subcommand: summary-address ip-address mask

This command is effective only on ASBRs doing redistribution into OSPF as in this scenario..

We should summarize most matching leftmost bits as indicated below.

Decimal 1st Octet 2nd Octet 3rd Octet 4th Octet

163.50.64.0 10100011 00110010 01000000 00000000

163.50.95.0 10100011 00110010 01011111 00000000

163.50.64.0 10000000 00110010 01000000 00000000 IP address

255.255.224.0 11111111 11111111 11000000 00000000 Subnet mask

Incorrect Answers

Network:

The network command is not used to summarize routes.

auto-summary:

The auto-summary command is used to turn auto summarization on.

128.213.0.0:

We should use a more specific summarization.

0.0.31.256:

For summarization we should specify a network mask, not a wildcard.

QUESTION NO: 9

It isn’t always in your best interests to redistribute dynamically learned routes from an interior gateway protocol to a border gateway protocol. Which of the following are potential issues that can occur when doing so? (Select two)

A. Routing loops can occur.

B. The routes are automatically summarized.

C. External IGP learned routes might not necessarily have originated in this AS.

D. The BGP process will ignore the external IGP learned routes.

Answer: A, C

Explanation:

If redistribution is used, care must be taken that only local routes are redistributed. For example,routes learned from other autonomous systems (that were learned by redistributing BGP into the IGP) must not be sent out again from the IGP, or rooting loops could result. Incorrect Answers:

B: With redistribution into exterior protocols such as BGP, no routes are automatically summarized.

D: All routes will be included in the redistribution, including externally learned routes.

Reference: Building Scalable Cisco Networks (Cisco Press) page 408.