CCNP Job Interview Questions & Answers Part 2
CCNP Job Interview Questions & Answere Part 2
Q101: What is a recursive table lookup?
Ans: A recursive routing table lookup occurs when a router cannot acquire all the information it needs to forward a packet with a single routing table lookup. For example, the router may perform one lookup to find the route to a destination and then perform another lookup to find a route to the next hop router of the first route.
Q102: What is a routing protocol?
Ans: A routing protocol is a "language" that routers speak to each other to share information about
network destinations.
Q103: What basic procedures should a routing algorithm perform?
Ans: At a minimum, a routing protocol should define procedures for
Passing reachability information about networks to other routers
Receiving reachability information from other routers
Determining optimal routes based on the reachability information it has and for recording
this information in a route table
Reacting to, compensating for, and advertising topology changes in an internetwork
Q104: Why do routing protocols use metrics?
Ans: A route metric, also called a route cost or a route distance, is used to determine the best path to a destination. Best is defined by the type of metric used.
Q105: What is convergence time?
Ans: Convergence time is the time a group of routers take to complete the exchange of routing
information.
Q106: What is load balancing? Name four different types of load balancing.
Ans: Load balancing is the process of sending packets over multiple paths to the same destination. Four types of load balancing are
Equal cost, per packet
Equal cost, per destination
Unequal cost, per packet
Unequal cost, per destination
Q107: What is a distance vector routing protocol?
Ans: A distance vector protocol is a routing protocol in which each router calculates routes based on the routes of its neighbors and then passes its routes to other neighbors.
Q108: Name several problems associated with distance vector protocols.
Ans: Several problems associated with distance vector protocols are
A susceptibility to incorrect routing information because of its dependence on neighbors for
correct information
Slow convergence
Route loops
Counting to infinity
Q109: What are neighbors?
Ans: Neighbors are routers connected to the same data link.
Q110: What is the purpose of route invalidation timers?
Ans: Route invalidation timers delete routes from a route table if they exceed a certain age.
Q111: Explain the difference between simple split horizon and split horizon with poisoned reverse.
Ans: Simple split horizon does not send route information back to the source of the route information. Split horizon with poisoned reverse sends the information back to the source but sets the metric to unreachable.
Q112: What is the counting-to-infinity problem, and how can it be controlled?
Ans: Counting to infinity occurs when routes update a route over a loop; each router increases the metric of the route until the metric reaches infinity. The effects of counting to infinity are controlled by defining infinity as a fairly low metric so that infinity is reached fairly quickly and the route is declared unreachable.
Q113: What are holddown timers, and how do they work?
Ans: Holddown timers help prevent routing loops. If a route is declared unreachable or if the metric increases beyond a certain threshold, a router will not accept any other information about that route until the hold down timer expires. This approach prevents the router from accepting possibly bad
routing information while the internetwork is reconverging
Q114: What are the differences between distance vector and link state routing protocols?
Ans: A distance vector router sends its entire route table, but it only sends the table to directly connected neighbors. A link state router sends only information about its directly connected links, but it floods the information throughout the internetworking area. Distance vector protocols usually use a variant of the Bellman-Ford algorithm to calculate routes, and link state protocols usually use a variant of the Dijkstra algorithm to calculate routes.
Q115: What is the purpose of a topological database?
Ans: A topological database holds the link state information originated by all routers in the link state routing domain.
Q116: Explain the basic steps involved in converging a link state internetwork.
Ans: Each router floods a link state information advertisement describing its links, the states of its links,
and any neighboring routers connected to those links, throughout the internetworking area. All
routers store all received copies of the link state advertisement in a link state database. Each router calculates a shortest path tree from the information in the topological database and enters routes in its routing tables based on the shortest path tree.
Q117: Why are sequence numbers important in link state protocols?
Ans: Sequence numbers help a router differentiate between multiple copies of the same link state
advertisement and also prevent flooded link state advertisements from circulating endlessly
throughout the internetwork.
Q118: What purpose does aging serve in a link state protocol?
Ans: Aging prevents old, possibly obsolete, link state information from residing in a topological
database or from being accepted by a router.
Q119: Explain how an SPF algorithm works.
Ans: A router builds a shortest path tree by first adding itself as the root. Using the information in the topological database, the router creates a list of all of its directly connected neighbors. The lowest cost link to a neighbor becomes a branch of the tree, and that router's neighbors are added to the list. The list is checked for duplicate paths, and if they exist, the higher-cost paths are removed from the list. The lowest-cost router on the list is added to the tree, that router's neighbors are added to the list, and the list is again checked for duplicate paths. This process continues until no routers remain on the list.
Q120: How do areas benefit a link state internetwork?
Ans: Within a routing domain, areas are subdomains. They make link state routing more efficient by limiting the size of the link state database of each router in the area.
Q121: What is an autonomous system?
Ans: Depending on the usage, an autonomous system can be defined as an internetwork under a common administrative domain or a single routing domain.
Q122: What is the difference between an IGP and an EGP?
Ans: An Interior Gateway Protocol is a routing protocol that routes within an autonomous system. An Exterior Gateway Protocol is a routing protocol that routes between autonomous systems.
Q123: What port does RIP use?
Ans: RIP uses UDP port 520.
Q124: What metric does RIP use? How is the metric used to indicate an unreachable network?
Ans: RIP uses a hop count metric. An unreachable network is indicated by setting the hop count to 16, which RIP interprets as an infinite distance.
Q125: What is the update period for RIP?
Ans: RIP sends periodic updates every 30 seconds minus a small random variable to prevent the updates of neighboring routers from becoming synchronized.
Q126: How many updates must be missed before a route entry will be marked as unreachable?
Ans: A route entry is marked as unreachable if six updates are missed.
Q127: What is the purpose of the garbage collection timer?
Ans: The garbage collection timer, or flush timer, is set when a route is declared unreachable. When the
timer expires, the route is flushed from the route table. This process allows an unreachable route to
remain in the routing table long enough for neighbors to be notified of its status.
Q128: Why is a random timer associated with triggered updates? What is the range of this timer?
Ans: The random timer, whose range is 1 to 5 seconds, prevents a "storm" of triggered updates during a topology change.
Q129: What is the difference between a RIP Request message and a RIP Response message?
Ans: A Request message asks a router for an update. A Response message is an update.
Q130: Which two types of Request messages does RIP use?
Ans: A Request message may either ask for a full update or in some special cases it may ask for specific routes.
Q131: Under what circumstances will a RIP response be sent?
Ans: A Response is sent when the update timer expires, or upon reception of a Request message.
Q132: Why does RIP hide subnets at major network boundaries?
Ans: RIP updates do not include the subnet mask of the destination address, so a RIP router depends on the subnet masks of its own interfaces to determine how an attached major network address is subnetted. If a router does not have an attachment to a particular major network address, it has no way to know how that major network is subnetted. Therefore, no subnets of a major network address can be advertised into another major network.
Q133: Which UDP port number is used to access IGRP?
Ans: IGRP does not use a UDP port. It is accessed directly from the network layer, as protocol number 9.
Q134: What is the maximum IGRP internetwork diameter, in hops?
Ans: The maximum IGRP network diameter is 255 hops.
Q135: What is the default update period for IGRP?
Ans: The default IGRP update period is 90 seconds.
Q136: Why does IGRP specify an autonomous system number?
Ans: IGRP specifies an autonomous system number so that multiple IGRP processes can be enabled within the same routing domain and even on the same router.
Q137: Referring to
Ans: McCloy will advertise 192.168.1.0 to Acheson as a system route because the address is being
advertised into another major network. Acheson will advertise 172.16.0.0 as a system route to
McCloy, and as an interior route to Kennan.
Q138: What is the default IGRP holddown time?
Ans: The default IGRP holddown time is 280 seconds.
Q139: Which variables can IGRP use to calculate its composite metric?
Ans: IGRP can use bandwidth, delay, load, and reliability to calculate its metric. By default, it uses only bandwidth and delay.
Q140: How many entries can be carried within a single IGRP update packet?
Ans: An IGRP update packet can carry up to 104 route entries.
Q141: Which three fields are new to the RIPv2 message format?
Ans: The Route Tag field, the Subnet Mask field, and the Next Hop field are RIPv2 extensions that do not exist in RIPv1 messages. The basic format of the RIP message remains unchanged between the two versions; version 2 merely uses fields that are unused in version 1.
Q142: Besides the extensions defined by the three fields of question 1, what are the other two major
changes from RIPv1?
Ans: In addition to the functions that use the new fields, RIPv2 supports authentication and multicast updates.
Q143: What is the multicast address used by RIPv2? What is the advantage of multicasting messages overbroadcasting them?
Ans: RIPv2 uses the multicast address 224.0.0.9. Multicasting of routing messages is better than
broadcasting because hosts and non-RIPv2 routers will ignore the multicast messages.
Q144: What is the purpose of the Route Tag field in the RIPv2 message?
Ans: When another routing protocol uses the RIPv2 domain as a transit domain, the protocol external to RIPv2 can use the Route Tag field to communicate information to its peers on the other side of the RIPv2 domain.
Q145: What is the purpose of the Next Hop field?
Ans: The Next Hop field is used to inform other routers of a next-hop address on the same multi-access network that is metrically closer to the destination than the originating router.
Q146: What is the UDP port number used by RIPv2?
Ans: RIPv2 uses the same UDP port number as RIPv1, port number 520.
Q147: Which one feature must a routing protocol have to be a classless routing protocol?
Ans: A classless routing protocol does not consider the major network address in its route lookups, but just looks for the longest match.
Q148: Which one feature must a routing protocol have to use VLSM?
Ans: To support VLSM, a routing protocol must be able to include the subnet mask of each destination address in its updates.
Q149: Which two types of authentication are available with Cisco's RIPv2? Are they both defined in RFC 1723?
Ans: Cisco's implementation of RIPv2 supports clear-text authentication and MD5 authentication. Only clear-text authentication is defined in RFC 1723.
Q150: Is EIGRP a distance vector or a link state routing protocol?
Ans: EIGRP is a distance vector protocol.
Q151: What is the maximum configured bandwidth EIGRP will use on a link? Can this percentage be changed?
Ans: By default, EIGRP uses no more than 50% of the link's bandwidth, based on the bandwidth
configured on the router's interface. This percentage to be changed with the command ip
bandwidth-percent eigrp.
Q152: How do EIGRP and IGRP differ in the way they calculate the composite metric?
Ans: EIGRP and IGRP use the same formula to calculate their composite metrics, but EIGRP scales the metric by a factor of 256.
Q153: What are the four basic components of EIGRP?
Ans: The four basic components of EIGRP are
The Protocol Dependent Modules
The Reliable Transport Protocol
The Neighbor Discovery and Recovery Module
The Diffusing Update Algorithm
Q154: In the context of EIGRP, what does the term reliable delivery mean? Which two methods ensure reliable delivery of EIGRP packets?
Ans: Reliable delivery means EIGRP packets are guaranteed to be delivered, and they are delivered in order. RTP uses a reliable multicast, in which received packets are acknowledged, to guarantee delivery; sequence numbers are used to ensure that they are delivered in order.
Q155: Which mechanism ensures that a router is accepting the most recent route entry?
Ans: Sequence numbers ensure that a router is receiving the most recent route entry.
Q156: What is the multicast IP address used by EIGRP?
Ans: EIGRP uses the multicast address 224.0.0.10.
Q157: What are the packet types used by EIGRP?
Ans: The packet types used by EIGRP are
Hellos
Acknowledgments
Updates
Queries
Replies
Q158: At what interval, by default, are EIGRP Hello packets sent?
Ans: The default EIGRP Hello interval is 5 seconds, except on some slow-speed (T1 and below)
interfaces, where the default is 60 seconds.
Q159: What is the default hold time?
Ans: The EIGRP default hold time is three times the Hello interval.
Q160: What is the difference between the neighbor table and the topology table?
Ans: The neighbor table stores information about EIGRP-speaking neighbors; the topology table lists all known routes that have feasible successors.
Q161: What is a feasible distance?
Ans: The feasible distance to a destination is a router's lowest calculated distance to the destination.
Q162: What is the feasibility condition?
Ans: The feasibility condition is the rule by which feasible successors are chosen for a destination. The feasibility condition is satisfied if a neighbor's advertised distance to a destination is lower than the receiving router's feasible distance to the destination. In other words, a router's neighbor meets the feasibility condition if the neighbor is metrically closer to the destination than the router. Another way to describe this is that the neighbor is "downstream" relative to the destination.
Q163: What is a feasible successor?
Ans: A feasible successor to a destination is a neighbor that satisfies the feasibility condition for that destination.
Q164: What is a successor?
Ans: A successor to a destination is a feasible successor that is currently being used as the next hop to the destination.
Q165: What is the difference between an active route and a passive route?
Ans: A route is active on a particular router if the router has queried its neighbors for a feasible
successor and has not yet received a reply from every queried neighbor. The route is passive when there are no outstanding queries.
Q166: What causes a passive route to become active?
Ans: A route becomes active when no feasible successor exists in its topology table.
Q167: What causes an active route to become passive?
Ans: An active route becomes passive when a reply has been received from every queried neighbor.
Q168: What does stuck-in-active mean?
Ans: If a router does not receive a reply from a queried neighbor within the active time (3 minutes, by default), the route is declared stuck-in-active. A response with an infinite metric is entered on the neighbor's behalf to satisfy DUAL, and the neighbor is deleted from the neighbor table.
Q169: What is the difference between subnetting and address aggregation?
Ans: Subnetting is the practice of creating a group of subnet addresses from a single IP network address.Address aggregation is the practice of summarizing a group of network or subnet addresses with a single IP network address.
Q170: What is an OSPF neighbor?
Ans: From the perspective of an OSPF router, a neighbor is another OSPF router that is attached to oneof the first router's directly connected links.
Q171: What is an OSPF adjacency?
Ans: An OSPF adjacency is a conceptual link to a neighbor over which LSAs can be sent.
Q172: What are the five OSPF packet types? What is the purpose of each type?
Ans: The five OSPF packet types, and their purposes, are
Hellos - which are used to discover neighbors, and to establish and maintain adjacencies
Updates - which are used to send LSAs between neighbors
Database Description packets - which a router uses to describe its link state database to a
neighbor during database synchronization
Link State Requests - which a router uses to request one or more LSAs from a neighbor's
link state database
Link State Acknowledgments - used to ensure reliable delivery of LSAs
Q173: What is an LSA? How does an LSA differ from an OSPF Update packet?
Ans: A router originates a link state advertisement to describe one or more destinations. An OSPF
Update packet transports LSAs from one neighbor to another. Although LSAs are flooded
throughout an area or OSPF domain, Update packets never leave a data link.
Q174: What are LSA types 1 to 5 and LSA type 7? What is the purpose of each type?
Ans: The most common LSA types and their purposes are
Type 1 (Router LSAs) are originated by every router and describe the originating router, the
router's directly connected links and their states, and the router\xd5 s neighbors.
o Type 2 (Network LSAs) are originated by Designated Routers on multiaccess links
and describe the link and all attached neighbors.
o Type 3 (Network Summary LSAs) are originated by Area Border Routers and
describe inter-area destinations.
o Type 4 LSAs (ASBR Summary LSAs) are originated by Area Border Routers to
describe Autonomous System Boundary Routers outside the area.
o Type 5 (AS External LSAs) are originated by Autonomous System Boundary
Routers to describe destinations external to the OSPF domain.
o Type 7 (NSSA External LSAs) are originated by Autonomous System Boundary
Routers within not-so-stubby areas.
Q175: What is a link state database? What is link state database synchronization?
Ans: The link state database is where a router stores all the OSPF LSAs it knows of, including its own. Database synchronization is the process of ensuring that all routers within an area have identical link state databases.
Q176: What is the default HelloInterval?
Ans: The default OSPF HelloInterval is 10 seconds.
Q177: What is the default RouterDeadInterval?
Ans: The default RouterDeadInterval is four times the HelloInterval.
Q178: What is a Router ID? How is a Router ID determined?
Ans: A Router ID is an address by which an OSPF router identifies itself. It is either the numerically highest IP address of all the router's loopback interfaces, or if no loopback interfaces are configured, it is the numerically highest IP address of all the router's LAN interfaces.
Q179: What is an area?
Ans: An area is an OSPF sub-domain, within which all routers have an identical link state database.
Q180: What is the significance of area 0?
Ans: Area 0 is the backbone area. All other areas must send their inter-area traffic through the backbone.
Q181: What is MaxAge?
Ans: MaxAge, 1 hour, is the age at which an LSA is considered to be obsolete.
Q182: What are the four OSPF router types?
Ans: The four OSPF router types are
Internal Routers, whose OSPF interfaces all belong to the same area
Backbone Routers, which are Internal Routers in Area 0
Area Border Routers, which have OSPF interfaces in more than one area
Autonomous System Boundary Routers, which advertise external routes into the OSPF
Domain
Q183: What are the four OSPF path types?
Ans: The four OSPF path types are
Intra-area paths
Inter-area paths
Type 1 external paths
Type 2 external paths
Q184: What are the five OSPF network types?
Ans: The five OSPF network types are
Point-to-point networks
Broadcast networks
Non-broadcast multi-access (NBMA) networks
Point-to-multipoint networks
Virtual links
Q185: What is a Designated Router?
Ans: A Designated Router is a router that represents a multiaccess network, and the routers connected to the network, to the rest of the OSFP domain.
Q186: How does a Cisco router calculate the outgoing cost of an interface?
Ans: Cisco IOS calculates the outgoing cost of an interface as 108/BW, where BW is the configured bandwidth of the interface.
Q187: What is a partitioned area?
Ans: An area is partitioned if one or more of its routers cannot send a packet to the area's other routers without sending the packet out of the area.
Q188: What is a virtual link?
Ans: A virtual link is a tunnel that extends an OSPF backbone connection through a non-backbone area.
Q189: What is the difference between a stub area, a totally stubby area, and a not-so-stubby area?
Ans: A stub area is an area into which no type 5 LSAs are flooded. A totally stubby area is an area into which no type 3, 4, or 5 LSAs are flooded, with the exception of type 3 LSAs to advertise a default route. Not-so-stubby areas are areas through which external destinations are advertised into the OSPF domain, but into which no type 5 LSAs are sent by the ABR.
Q190: What is the difference between OSPF network entries and OSPF router entries?
Ans: OSPF network entries are entries in the route table, describing IP destinations. OSPF router entries are entries in a separate route table that record only routes to ABRs and ASBRs.
Q191: Why is type 2 authentication preferable over type 1 authentication?
Ans: Type 2 authentication uses MD5 encryption, whereas type 1 authentication uses clear-text
passwords.
Q192: Which three fields in the LSA header distinguish different LSAs? Which three fields in the LSA header distinguish different instances of the same LSA?
Ans: The three fields in the LSA header that distinguish different LSAs are the Type, Advertising
Router, and the Link State ID fields. The three fields in the LSA header that distinguish different
instances of the same LSA are the Sequence Number, Age, and Checksum fields.
Q193: What is an intermediate system?
Ans: An Intermediate System is the ISO term for a router.
Q194: What is a network protocol data unit?
Ans: A Network Protocol Data Unit is the ISO term for a packet.
Q195: What is the difference between an L1, an L2, and an L1/L2 router?
Ans: An L1 router has no direct connections to another area. An L2 router only routes inter-area traffic.
An L1/L2 router routes both inter-area and intra-area traffic and acts as an inter-area gateway for
L1 routers.
Q196: Explain the basic difference between an IS-IS area and an OSPF area.
Ans: The borders of IS-IS areas are between routers, on links. The borders of OSPF areas are defined by the routers themselves.
Q197: What is a network entity title (NET)?
Ans: The Network Entity Title is an address by which a router identifies both itself and the area in which it resides.
Q198: To what value must the NSAP Selector be set in a NET?
Ans: The NSAP Selector should be set to 0x00 in a NET.
Q199: What is the purpose of a System ID?
Ans: The System ID uniquely identifies a router within an IS-IS domain.
Q200: How does a router determine what area it is in?
Ans: The portion of the NET preceding the last seven octets is the area address.
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