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˜¡ Traffic can be directed on either the source address or both the source and destination addresses.These are
only
some of the criteria that can be used.
˜¡ Policy-based routing affects only the next hop in the path to the destination.(option C)
˜¡ Policy-based routing does not affect the destination of the packet. It affects the path used to get
to the destination.
˜¡ Policy-based routing does not allow traffic sent into another autonomous system to take a
different path from the one that would have been chosen by that autonomous system.
˜¡ It is possible to influence only how traffic will get to a neighboring autonomous system, not how
it will be routed within that autonomous system.
˜¡ As policy-based routing examines the source address, it is configured on the inbound interface.
Source: Self-Study CCNP BSCI Exam Certification Guide Third Edition P.523
Topic: Rules of Policy-Based Routing
Section 17: Explain the use of redistribution between BGP and Interior
Gateway Protocols (IGPs) (10 questions)
Q 1
When you're running an Interior Gateway Protocol (like OSPF or EIGRP); why would
you have 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.
Ans: C
Explanation: When BGP is used between autonomous systems (AS), the protocol is referred to
as External BGP (EBGP).
Note: Customer networks usually employ an Interior Gateway Protocol (IGP) such as RIP or
OSPF for the exchange of routing information within their networks
Reference: Border Gateway Protocol
http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/bgp.htm
Q2
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 CertKing 2 and it is
inserting the subnets 128.213.96.0 through 128.213.127.0. Your task is to configure the
gateway router CertKing 1 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.
CertKing 1(config)#router ospf 100
CertKing 1(config-router)#redistribute bgp 50 metric 1000 subnets
Ans: Summary-address 128.213.64.0 255.255.224.0
Explanation: Compare with Section A, question 59.
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..
Q3
What factors must you consider before redistributing BGP routes to IGP?A. IGPs are limited to 250 routesB. A full BGP routing table may contain 100,000+ routesC. Because of possible routing loops, Cisco router configuration does not allow BGP routes
to be restributed into an IGP.D. Because BGP routes are not advertised unless they are known by the IGP, Cisco
automatically redistributes routes into IGPs.
Ans: B
Explanation: The size of the BGP routing table must be taken into consideration.
Incorrect Anss
A: There is no such limit.
C: BGP routes can very well be distributed into an IGP.
D: BGP routers are not redistributed automatically.
Q4
Why is it mandatory to redistribute IGP protocols like OSPF 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.
Ans: C
Explanation: When BGP is used between autonomous systems (AS), the protocol is referred to
as External BGP (EBGP).
Note: Customer networks usually employ an Interior Gateway Protocol (IGP) such as RIP or
OSPF for the exchange of routing information within their networks
Reference: Border Gateway Protocol
http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/bgp.htm
Q5
According to Cisco's official recommendations, how would you populate 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.
Ans: 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 static routes.
Incorrect Anss
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.
Q6
You are the midst of configuring a gateway route named Router CertK . 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.
Ans:
Explanation: We want to redistribute BGP AS 75 into OSPF.
Syntax of redistributation command
router(config-router)#redistribute protocol [process-id] [metric [metric-type type-value] [routemap
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 Anss
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.
Q7
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 CertKing q and it is
inserting the subnets 142.14.96.0 through 142.14.127.0. Your task is to configure the
gateway router CertKing 1 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.
CertKing 1(config)#router ospf 200
CertKing 1(config-router)#redistribute bgp 80 metric 1100 subnets
Ans:
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 CertKing 1 router is injecting in binary to decide where to
summarize:
Decimal 1 st octet 2 nd octet 3 rd octet 4 th octet
142.14.64.0 10001110 00001110 01000000 00000000
142.14.95.0 10001110 00001110 01011111 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.
Q8
You are the administrator of an internal network configured for OSPF. Two gateway
routers ( CertKing 1 & CertKing 2) are redistributing external BGP routes. CertKing 1 is
redistributing subnets ranging from 163.150.64.0 - 163.160.95.0 and CertKing 2 is doing the
same with 163.150.96.0 - 163.150.127.0. Your task is to configure the gateway router
CertKing 1 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:
CertKing 1(config)#router ospf 100
CertKing 1(config-router)#redistribute bgp 70 metric 500 subnets
Ans:
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 1 Octet 2 Octet 3 Octet 4 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 Anss
network
The network command is not used to summarize routes.
auto-summary
The auto-summary command is used to turn autosummarization 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.
Q9
It isn't always in your best interests to redistribute dynamically learned routes from an
interior gateway protocol to a border gateway protocol. What are two mishaps of doing so?A. Routing loops can occur.B. The routers 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.
Ans: 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.
Reference: Building Scalable Cisco Networks (Ciscopress) page 408
Q0
You are the network engineer at CertKing . Your newly appointed CertKing trainee wants
to know what she should consider when she wants to redistribute BGP routes into an IGP.
What will your reply be?A. IGPs are limited to 250 routers.B. A full BGP routing table may contain more than 100,000 routers.C. Cisco router configuration does not allow BGP routes to be redistributed into an IGP
because of possible routing loops.D. Cisco automatically redistributes routes into IGPs because BGP routes are not advertised
unless they are known by the IGP.
Ans: B
Explanation:
A - Wrong IGP's are not limited to 250 routes. e.g. OSPF supports unlimited hops and size of
routing table is not limited to any routes.
C - Wrong, BGP routes can be redistributed
by commands.
D - Wrong, Routes cannot be automatically redistributed. We have to do redistribution manually
Topic 2: Implementation and Configuration
(128 questions)
Section 1: Given a set of network requirements, identify the steps to
configure an Enhanced IGRP environment and verify proper operation
(within described guidelines) of your routers (12 questions)
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