Ch 10 - Route Redistribution Flashcards
Which of the following answers is the least likely reason for an engineer to choose to use route redistribution?
a. To exchange routes between merged companies
b. To give separate control over routing to different parts of one company
c. To support multiple router vendors
d. To knit together an OSPF area if the area becomes discontiguous
D. The three incorrect answers list typical reasons for using route redistribution. The correct answer—the least likely reason among the answers for using route redistribution—lists a problem for which an OSPF virtual link is often used. Route redistribution could be attempted to solve a problem with a discontiguous OSPF area, but the redistribution completely changes the LSAs that would have otherwise been known and could have negative impacts on route summaries and cause routing loops, and have other problems as well.
For a router to successfully redistribute routes between OSPF and EIGRP, which of the following are true? (Choose two.)
a. The router must have one routing protocol configured, but configuration for both routing protocols is not necessary.
b. The router must have at least one working link connected to each routing domain.
c. The redistribute command must be configured under EIGRP to send the routes to OSPF.
d. The redistribute command should be configured under OSPF to take routes from EIGRP into OSPF.
B and D. For a router to redistribute routes between two routing protocols, the router must have both routing protocols configured, have a working link into each routing domain, and configure redistribute commands under each routing process. The redistribute command, issued in routing protocol configuration mode, pulls routes into that routing process from another routing process as referenced on the redistribute command.
Process EIGRP 1 is redistributing routes from process OSPF 2. Which of the following methods can be used to set the metrics of the redistributed routes? (Choose two.)
a. Let the metrics default.
b. Set the metric components using the redistribute command’s metric keyword.
c. Set the metric components using the default-metric subcommand under router configuration mode.
d. Set the integer (composite) metric using the redistribute command’s metric keyword.
B and C. Because the metrics come from a different routing protocol than EIGRP, the metric must be set. The metric must be set with five components; EIGRP will then use those components as it would for an internal route. The metric components can be set as listed in the two correct answers, plus using a route map as referenced by the redistribute command.
Examine the following excerpt from the show ip eigrp topology 10.2.2.0/24 command on Router R1. Which answer can be verified as definitely true based on this output?
External data:
Originating router is 10.1.1.1
AS number of route is 1
External protocol is OSPF, external metric is 64
Administrator tag is 0 (0x00000000)
a. R1 is the router that redistributed the route.
b. R1’s metric to reach subnet 10.2.2.0/24 is 64.
c. The route was redistributed on a router that has a router ospf 1 command configured.
d. R1 is redistributing a route to prefix 10.2.2.0/24 into OSPF.
C. This output is the external data section of a detailed view of an EIGRP topology table entry for an external route. This output confirms that this route was redistributed into EIGRP. If R1 were the redistributing router, the output would include the phrase “(this system)”; this example does not include that notation. The output means that on the router that did the redistribution, the route was redistributed from OSPF process 1, and the OSPF metric was 64. R1’s metric is not based on the OSPF metric of the route.
Router R1 has a connected route for 10.1.1.0/24 off interface Fa0/0. Interface Fa0/0 has been enabled for OSPF because of a router ospf 1 and network 10.1.1.0 0.0.0.255 area 0 command. R1 also has EIGRP configured, with the redistribute ospf 1 metric 1000 100 10 1 1500 command configured under EIGRP. Which of the following is true?
a. R1 will not redistribute 10.1.1.0/24 into EIGRP, because R1 knows it as a connected route and not as an OSPF route.
b. For any OSPF routes redistributed into EIGRP, the metric components include a value equivalent to 1 Mbps of bandwidth.
c. For any OSPF routes redistributed into EIGRP, the metric components include a value equivalent to 100 microseconds of delay.
d. No subnets of network 10.1.1.0 will be redistributed because of the omission of the subnets parameter.
B. The redistribute ospf command will attempt to redistribute OSPF routes and connected routes from interfaces on which OSPF is enabled. The metric components include 1000 kbps (or 1 Mbps), 100 tens-of-microseconds (or 1000 microseconds), 10 for the loading, 1 for the reliability, and 1500 for MTU. The EIGRP version of the redistribute command does not include a subnets option.
Process OSPF 1 is redistributing routes from process OSPF 2. Which of the following methods can be used to set the metrics of the redistributed routes? (Choose two.)
a. Let the metrics default.
b. Use each redistributed route’s OSPF metric using the redistribute command’s
metric transparent keywords.
c. Set the metric using the default-metric subcommand under router configuration
mode.
d. Redistribution is not allowed between two OSPF processes.
A and C. Because the routes come from OSPF and feed into OSPF, the metrics can be set with the usual tools or the metric can default. When taking routes from OSPF into another OSPF process, the default metric is taken from the source route’s OSPF cost. Alternatively, the metric can be set for all routes, regardless of the route source, using the default-metric OSPF subcommand. The metric transparent keywords cannot be used for an OSPF redistribute command.
Examine the following excerpt from the show ip ospf database asbr-summary command on Router R1 (RID 1.1.1.1). Which answer can be verified as definitely true based on this output?
LS Type: Summary Links (AS Boundary Router)
Link State ID: 9.9.9.9 (AS Boundary Router address)
Advertising Router: 3.3.3.3
LS Seq Number: 8000000D
Checksum: 0xE43A
Length: 28
Network Mask: /0
TOS: 0 Metric: 100
a. The output describes the contents of a Type 5 LSA.
b. 3.3.3.3 identifies a router as being the router performing redistribution.
c. R1’s metric for its best route to reach the router with RID 9.9.9.9 is 100.
d. The router with RID 3.3.3.3’s metric for its best route to reach the router with RID 9.9.9.9 is 100.
D. This command lists the output of Type 4 Summary ASBR LSAs. The LSID identifies the redistributing ASBR (9.9.9.9). The advertising router is the ABR that created and flooded the LSA (3.3.3.3), and the metric is the ABR’s best metric route to reach the ASBR.
Router R1 sits inside OSPF area 1. Router R2 redistributes an E1 route into OSPF for prefix 2.2.2.0/24, with external metric 20. Router R22 redistributes an E2 route for the same prefix/length, external metric 10. Under what conditions will R1 choose as its best route the route through R22?
a. R1 will always choose the route through R22.
b. As long as R1’s best internal OSPF cost to reach R22 is less than 10.
c. As long as R1’s best internal OSPF cost to reach R22 is less than 20.
d. R1 will never choose the route through R22 if the E1 route through R2 is available.
D. Routers add internal and external costs for E1 routes and use only external costs for E2 routes, so the cost for the route through R22 will always be lower. However, for a given prefix/length, OSPF always prefers intra-area routes first, then interarea, then E1, and finally, E2, all regardless of metric.
Router R1 has been configured with the redistribute ospf 1 route-map fred command under router eigrp 1. The route map named fred needs to be configured to match routes to determine which routes are redistributed into EIGRP. Which of the following answers lists an item that cannot be matched by route map fred?
a. Subnet number
b. Next-hop router IP address of the route
c. Whether the route is an E1 or E2 route
d. The route’s tag
e. The number of router hops between the router and the subnet
E. Because OSPF does not use hop count as a metric, the information about the number of hops is not available in OSPF routes in the IP routing table. The other answers list items that can be matched with the route map match subcommand.
Router R1 refers to route map fred when redistributing from EIGRP into OSPF. The entire route map is listed next. Which of the following answers must be true based on the configuration as shown?
route-map fred deny 10
match ip address one
route-map fred deny 20
match ip address two
route-map fred permit 100
a. The third route map clause will allow any routes not already filtered by the first two clauses.
b. Routes permitted by ACL “two” will be redistributed.
c. Routes denied by ACL “one” will be redistributed.
d. All routes will be filtered.
A. The deny clauses in the route map mean that the route map will filter routes matched by that clause. The permit or deny action of the referenced ACLs just defines whether the route is matched. So, routes permitted by ACL “two” will be matched and then filtered because of the route map clause deny action. Routes denied by ACL “one” simply do not match the route map clause numbered 10;
such routes might or might not be redistributed depending on the next two clauses. Clause number 100 does not have a match command, meaning that it matches all routes not otherwise matched, with a permit action, allowing these routes to be redistributed.
On Router R1, process EIGRP 1 is redistributing routes from process OSPF 2, calling route map fred with the redistribute ospf 2 route-map fred command. R1 has learned intra-area routes for 10.1.1.0/24 and 10.1.2.0/24 in part because of the Type 2 LSAs known for each subnet. The route map filters route 10.1.1.0/24 and allows 10.1.2.0/24 through. Which of the following commands on Router R1 list subnet 10.1.1.0/24? (Choose two.)
a. show ip route
b. show ip eigrp topology
c. show ip ospf database
d. show ip eigrp topology 10.1.1.0/24
A and C. The problem states that R1 has learned OSPF intra-area routes for 10.1.1.0/24, so show ip route will display that subnet. As an intra-area route based on a Type 2 LSA, the show ip ospf database command lists the summary of the LSAs, including the 10.1.1.0 subnet number for that Type 2 LSA. However, because the redistribution filtering discards subnet 10.1.1.0/24, this value will not be included in the EIGRP topology table.
Router R1 is redistributing between two OSPF processes. Given the configuration shown, which includes all commands in the route map named fred, which of the following answers is true regarding the redistribution into OSPF process 1?
router ospf 1
redistribute ospf 2 match external 2 route-map fred
!
route-map fred permit 10
match ip address 1
set metric-type type-1
a. No routes are redistributed because a route cannot be both E1 and E2.
b. Only OSPF E2 routes in the OSPF 2 domain will be considered for
redistribution.
c. Inside the OSPF 2 domain, any formerly E2 routes will become E1 routes.
d. Routes permitted by ACL 1 will be redistributed, regardless of whether the routes are E1 or E2 routes.
B. The external 2 parameters on the redistribute command act as matching logic. Only routes from the source routing protocol (in this case OSPF 2) that match this extra logic will be considered for redistribution by this redistribute command. The set metric-type type-1 route map subcommand sets the route type as it is injected into the destination routing protocol (in this case, OSPF 1); this logic is not used for matching the source routes. The routes permitted by ACL 1 will be redistributed, but only those that are also E2 routes from the (source) OSPF 2 domain. The redistribute function will not change the attributes of routes inside a single routing domain, but only in the destination routing domain (OSPF 1), so the configuration has no effect on the OSPF 2 routes that remain in OSPF 2.
Which of the following is not true regarding Cisco IOS default settings for administrative distance?
a. EIGRP internal: 90
b. OSPF external: 110
c. EIGRP external: 90
d. RIP: 120
e. OSPF internal: 110
C. EIGRP, by default, sets a different AD for internal (90) and external (170) routes. The rest of the answers are accurate regarding default settings.
A network includes a RIPv2 domain, an EIGRP domain, and an OSPF domain. Each pair of routing domains has multiple routers redistributing routes between the pair of domains. The design requires that the redistribution configuration avoid matching based on prefix/length because of the trouble in maintaining such configurations. Which of the following tools can be used in all three routing domains to attempt to prevent domain loops? (This book uses the term domain loop to refer to the long routes that might be chosen for routes when redistribution exists—for example, a route might forward packets from the EIGRP domain, to the OSPF domain, back to EIGRP, and then to subnet X in the RIP domain.)
a. Setting route tags
b. Setting the default administrative distance differently for internal and external
routes
c. Setting administrative distance differently per route
d. Setting metrics much higher for all external routes than for all internal routes
A. All the answers list reasonable options in some cases, but the only feature listed that is useful with all three routing protocols is the route tag feature. RIPv2 does not support the concept of differentiating between internal and external routes, so the two answers that suggest setting administrative distance (AD) based on the route type (internal or external) could not be used in all three routing domains, as required by the question. All three routing protocols support setting route tags and setting the AD per route. However, because RIPv2 cannot match based on the route type (internal/external), the option to set the route tags is the only option that applies to all three routing domains.
A coworker is developing an implementation plan for a design that uses OSPF 2 and RIPv2 routing domains, with two routers redistributing between the two domains. The coworker asks your help in choosing how to prevent domain loops by setting administrative distance. Assuming that all other related settings use defaults, which of the following would solve the domain loop problem?
a. The distance ospf intra-area 80 inter-area 80 OSPF subcommand
b. The distance ospf external 80 OSPF subcommand
c. The distance ospf intra-area 180 inter-area 180 OSPF subcommand
d. The distance ospf external 180 OSPF subcommand
D. AD can be used to prevent the domain loop problem with two routing domains by making each routing protocol’s AD for internal routes be better (lower) than the other routing protocol’s AD for external routes. RIP uses AD 120 for all routes, with no distinction of internal or external. As such, OSPF’s internal default AD settings of 110 meet the requirement that OSPF’s internal AD (110) is better than RIP’s external (120). However, RIP’s default of 120 is not better than OSPF’s default for externals (110), so the distance ospf external 180 command changes that setting to meet both requirements. The three wrong answers, while syntactically valid, do not help meet the requirements.
Router R1 sets a route tag for subnet 10.1.1.0/24 when redistributing from OSPF into EIGRP. Which of the following units is assigned to the route tag?
a. Kilobits/second
b. Tens-of-microseconds
c. Cost
d. Hop count
e. No units assigned
E. Route tags are unitless integers that can be given to a route and even passed between different routing protocols by routers that perform redistribution.
What do these commands do:
router ospf 2
redistribute eigrp 1
Tells router to take EIGRP routes and put them into the OSPF topo table.
What 3 things are required of at least one router for route redistribution to happen?
- have at least one working physical link in each routing domain
- A working routing protocol config for each domain.
- Have the ‘redistribute’ command configured.
What is the metric of routes redistributed with this command?
router eigrp 1
redistribute ospf 2
Redistributed routes are treated as external routes. Routes redistributed into EIGRP will have a metric of 170.
What does OSPF do when a route is redistributed into a normal area, i.e. not a stub area?
OSPF will create a Type 5 External LSA for each redistributed subnet. It will also assign an integer metric to the LSA.
What does OSPF do when a route is redistributed into a NSSA?
OSPF will create a Type 7 External LSA for each redistributed subnet. It will also assign an integer metric to the LSA.
Which of these will be redistributed?
a. routes learned from a routing protocol
b. connected routes from interfaces not enabled with routing protocol
c. connected routes from interfaces enabled with routing protocol.
d. connected routes from passive interfaces enabled with routing protocol
e. connected routes from passive interfaces not enabled with routing protocol
a, c, d
What do other routers use to match a tag with?
route-map
Which of the following routing protocols can be redistributed?
a. bgp
b. connected routes,
c. eigrp
d. isis
e. ospf
f. static routes
g. rip
all.
T/F: When redistributing from OSPF the match parameter can be used to match route types like internal, external by type E1 or E2, or NSSA external.
True.
T/F: EIGRP requires component metrics to be set when redistributing from OSPF.
True. There are 3 ways to do this.
- set default for all redistribute commands in EIGRP
- set metrics with each redistribute command
- set different metrics on different routes from a single source by using a route map, matching and setting the component metrics with the ‘set-metric’ command.
Note: EIGRP to EIGRP redistribution just takes the metrics from the original route. They don’t need to be set.
In the following command, what are the component metric values and for the delay, how many microseconds is the delay?
router eigrp 1
default-metric 1000 33 255 1 1500
end
bw, delay, reliability, load and mtu
330 microseconds is the delay, it listed in 10s of microseconds. (Thanks Cisco!)
Big Dogs Really Like Me – the order of K values in EIGRP: bandwidth, delay, reliability, load, MTU
What does ‘via redistributed’ mean when attached to a route in the routing table?
It means you have traveled to a parallel dimension. ‘via redistributed’ show up in the topo table on redistributed routes!
In the following command, what does the ‘metric-type 2’ mean?
router ospf 2
redistribute eigrp 1 metric-type 2
The parameter ‘metric-type’ defines external metric types for routes redistributed by this command 1=E1 routes, 2=E2 routes.
For redistributed routes, by default OSPF creates Type E2 routes.
