Q-1 Which layer of the OSI model is responsible for reliable connections?
Answer: The Transport layer of the OSI model is responsible for reliable connections.
Q-2 What is the difference between acknowledgments and handshaking?
Answer: Handshaking is used to negotiate the properties of a connection that is being established. Acknowledgments are used to tell the sender that data has been successfully received by the destination during the use of a connection.
Q-3 How many VTP modes are there and what are they?
Answer: Three: Server, Client, and Transparent
Q-4 What are the two types of Trunk encapsulation protocols?
Answer: IEEE 802.1Q and Cisco’s ISL
Q-5 What are the four primary no routable protocols?
Answer: SNA, NetBIOS, DEC LAT, DEC MOP
Q-6 What is the difference between TCP and UDP?
Answer: The primary difference between TCP and UDP is that TCP is a connection oriented protocol and UDP is a connectionless protocol.
Answer: HSRP, or the Hot Standby Routing Protocol, is a Cisco proprietary protocol that brings routing functionality to end devices that would otherwise not be capable of taking advantage of redundant network connections. HSRP enables a pair of Cisco routers to work together to present the appearance of a single virtual default-gateway to end devices on a LAN segment.
Q-8 What is the difference between a Public IP address and a Private IP address?
Answer: Public address space is a unique address that is assigned to a company. Private address space is not recognized by the Internet and can be used by anyone
within their private network.
Q-9 What does AAA stand for?
Answer: Authentication, authorization, and accounting
Q-10 The H.323 protocol is used for what?
Answer: H.323 is used for multiservice (multimedia) applications, usually in a Voice Over IP environment.
Q-1 What type of routing protocol maintains neighbors?
Q-2 what is the range of values for administrative distance?
Q-3 Describe the difference between unicast, multicast, and broadcast traffic?
Answer: Unicast traffic flows from a single source to a single destination MAC address. Multicast traffic flows from a single source MAC address to many destinations and uses a functional MAC address. Broadcast traffic is from a single source to all devices on the Ethernet segment. This is specified by a destination MAC address of all ones.
Q-4 What are the four different Ethernet encapsulation types?
Answer: From the Cisco IPX encapsulation command they are ARPA, NOVELL-ETHER, SAP and SNAP
Q-5 What are the three main tasks of a transparent bridge?
Answer: Learning, Forwarding, Filtering
Q-6 What type of routing protocol is EIGRP?
Q-7 While troubleshooting a connectivity problem on the network, you issue the ping command from your PC command prompt, but the output shows "request times out." At which OSI layer is this problem associated with?
Answer: The Network Layer
Q-8 What algorithm does OSPF use to compute its route table?
Answer: OSPF uses the shortest path first (SPF) algorithm, which is also known as the Dijkstra algorithm.
Answer: A stub area is an area that does not accept routing updates from outside its autonomous system.
Q-10 What does the TTL field of an IP packet header do?
Answer: The TTL field indicates the maximum time that a packet can be on the network. Each router that processes this packet decrements the TTL value by 1. If the value reaches zero, the packet is discarded from the network. The purpose of this field is to eliminate the possibility of a packet endlessly traversing the network.
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1 What is the primary purpose of a LAN?
A: The primary purpose of a local-area network is to allow resource sharing. The resources may be devices, applications, or information. Examples of shared resources are files, databases, e-mail, modems, and printers.
A: A protocol is an agreed-upon set of rules. In data communications, the rules usually govern a
3: What is the purpose of a MAC protocol?
A: A Media Access Control protocol defines how a given LAN medium is shared, how LAN devices
connected to the medium are identified, and how frames transmitted onto the medium are
A: A frame is a digital "envelope" that provides the information necessary for the delivery of data across a data link. Typical components of a frame are identifiers (addresses) of the source and destination devices on the data link, an indicator of the type of data enclosed in the frame, anderror-checking information.
5: What feature is common to all frame types?
A: A feature common to all frame types is a format for identifying devices on the data link.
6: What is a MAC address or MAC identifier?
A: A Media Access Control address or identifier is a means by which individual devices connected to a data link are uniquely identified for the purpose of delivering data.
7: Why is a MAC address not a true address?
A: An address specifies a location. A MAC address is not a true address because it is permanently associated with the interface of a specific device and moves whenever the device moves. A MAC identifies the device, not the location of the device.
8: What are the three sources of signal degradation on a data link?
A: The three sources of signal degradation on a data link are attenuation, interference, and distortion. Attenuation is a function of the resistance of the medium. Interference is a function of noise entering the medium. Distortion is a function of the reactive characteristics of the medium, which react differently to different frequency components of the signal.
9: What is the purpose of a repeater?
A: A repeater is a device that extends the useful range of a physical medium by reading a degradedsignal and producing a "clean" copy of the signal.
10: What is the purpose of a bridge?
A: A bridge is a device that increases the capacity of a LAN. A bridge divides the data link into
segments, forwarding only traffic that is generated on one segment and is destined for another
segment. By controlling and limiting the traffic on a data link, more devices may be attached to the LAN.
11: What makes a transparent bridge transparent?
A: A transparent bridge "listens promiscuously" on each of its ports. That is, it examines all frames on all media to which it is attached. It records the source MAC identifiers of the frames, and the ports on which it learns the identifiers, in a bridging table. It can then refer to the table when deciding whether to filter or forward a frame. The bridge is transparent because it performs this learning function independently of the devices that originate the frames. The end devices themselves have no knowledge of the bridge.
12: Name three fundamental differences between LANs and WANs.
A: Three fundamental differences between local-area and wide-area networks are:
LANs are limited to a small geographic area, such as a single building or small campus.
WANs cover a large geographic area, from citywide to worldwide.
LANs usually consist entirely of privately owned components. Some components of a
WAN, such as a packet switching network or point-to-point serial links, are usually leased
A LAN provides high bandwidth at a relatively cheap price. The bandwidth across a WAN
is significantly more expensive.
13: What is the purpose of a broadcast MAC identifier? What is the broadcast MAC identifier, in hex and in binary?
A: A broadcast MAC identifier, when used as the destination address of a frame, signifies that the data is for all devices attached to the data link. In binary, the broadcast MAC identifier is all ones. In hex, it is ffff.ffff.ffff.
14: What is the primary similarity between a bridge and a router? What is the primary difference
between a bridge and a router?
A: The primary similarity between a bridge and a router is that both devices increase the number of hosts that may be interconnected into a common communications network. The difference is that a bridge works by interconnecting separate segments of a single network, whereas a router
interconnects separate networks.
15: What is a packet? What is the primary similarity between a frame and a packet? What is the
primary difference between a frame and a packet?
A: A packet is the means by which data is transported from one network to another. The similarity between a frame and a packet is that they both encapsulate data and provide an addressing scheme for delivering the data. The difference between a frame and a packet is that the frame delivers data between two devices sharing a common data link, whereas a packet delivers data across a logical pathway, or route, spanning multiple data links.
16: As a packet progresses across an internetwork, does the source address change?
A: Neither the source nor the destination address of a packet changes as it progresses from the source of the packet to the destination.
17: What is a network address? What is the purpose of each part of a network address?
A: Network addresses are the addresses used in packets. Each network address has a network part, which identifies a particular data link, and a host or node part, which identifies a specific device on the data link identified by the network part.
18: What is the primary difference between a network address and a data link identifier?
A: A packet identifies a device from the perspective of the entire internetwork. A frame identifies a device from the perspective of a single data link. Because the connection between two devices across an internetwork is a logical path, a network address is a logical address. Because the connection between two devices across a data link is a physical path, a data link identifier is a physical address.
1: What are the five layers of the TCP/IP protocol suite? What is the purpose of each layer?
A: The five layers of the TCP/IP protocol suite are the following:
2: What is the most common IP version presently in use?
A: The most common IP version now in use is version 4.
3: What is fragmentation? What fields of the IP header are used for fragmentation?
A: Routers perform fragmentation when a packet is longer than the maximum packet length
(Maximum Transmission Unit, or MTU) supported by a data link onto which the packet must be
transmitted. The data within the packet will be broken into fragments, and each fragment will be
encapsulated in its own packet. The receiver uses the Identifier and Fragment Offset fields and the
MF bit of the Flags field to reassemble the fragments.
4: What is the purpose of the TTL field in the IP header? How does the TTL process work?
The Time to Live (TTL) field prevents "lost" packets from being passed endlessly through the IP internetwork. The field contains an 8-bit integer that is set by the originator of the packet. Each router through which the packet passes will decrement the integer by one. If a router decrements the TTL to zero, it will discard the packet and send an ICMP "time exceeded" error message to the packet's source address.
5: What is the first octet rule?
A: The first octet rule determines the class of an IP address as follows:
Class A: The first bit of the first octet is always 0.
Class B: The first two bits of the first octet are always 10.
Class C: The first three bits of the first octet are always 110.
Class D: The first four bits of the first octet are always 1110.
Class E: The first four bits of the first octet are always 1111.
6: How are class A, B, and C IP addresses recognized in dotted decimal? How are they recognized inbinary?
A: The A, B, C IP addresses are recognized in dotted decimal and binary as follows:
Class Binary Range of First Octet Decimal Range of First Octet
A 0000000 - 01111110 1 - 126
B 10000000 - 10111111 128 - 191
C 11000000 - 11011111 192- 223
7: What is an address mask, and how does it work?
A: An IP address mask identifies the network part of an IP address. Each one in the 32-bit mask marks the corresponding bit in the IP address as a network bit. A zero in the mask marks the
Corresponding bit in the IP address as a host bit. A Boolean AND is performed in all 32 bits of the address and the mask; in the result, all network bits of the mask will be repeated, and all host bits will be changed to zero.
8: What is a subnet? Why are subnets used in IP environments?
A: A subnet is a sub grouping of a class A, B, or C IP address. Without subletting, the network part of a major class A, B, or C IP address can only identify a single data link. Subnetting uses some of the host bits of a major IP address as network bits, allowing the single major address to be “Subdivided" into multiple network addresses.
9: Why can't a subnet of all zeros or all ones be used in a classful routing environment?
A: A classful routing protocol has no way to differentiate between the all-zeroes subnet and the major IP address, and between the all-ones subnet and the all-hosts, all-subnets broadcast address of the major IP address.
A: ARP, or Address Resolution Protocol, is a function that maps the IP addresses of interfaces on a data link to their corresponding MAC identifiers.
A: Proxy ARP is a function of an IP router. If the router hears an ARP request, and
The destination network or subnet is in the router's routing table, and
The table indicates that the destination is reachable via a different router interface than the
one on which the ARP request was received, The router will respond to the ARP request with its own MAC address.
A: A redirect is an IP router function. If a device has sent a packet to the router and the router must forward the packet to a next-hop router on the same data link, the router will send a redirect to the originating device. The redirect will inform the device that it can reach the next-hop router directly.
13: What is the essential difference between TCP and UDP?
A: TCP, or Transmission Control Protocol, provides a connection-oriented service over the
Connectionless internet layer. UDP, or User Datagram Service, provides a connectionless service.
14: What mechanisms does TCP use to provide connection-oriented service?
A: Correct sequencing is accomplished with sequence numbers. Reliability is accomplished by using checksums, acknowledgments, timers, and retransmissions. Flow control is accomplished by windowing.
15: Instead of ARP, Novell NetWare uses a network address that includes a device's MAC address as the host portion. Why can't IP do this?
A: A MAC identifier is a fixed-length binary integer. If IP used MAC identifiers as the host part of the IP address, subnetting would not be possible because there would be no flexibility in using some of the host bits as network bits.
16: NetWare has a transport layer service similar to TCP called Sequenced Packet Exchange (SPX), but no service similar to UDP. Applications requiring connectionless service directly access the connectionless IPX at the network layer. What purpose does UDP serve by providing aconnectionless service on top of what is already a connectionless service?
A: The only purpose of the UDP header is to add fields for the source and destination port numbers.
1: What information must be stored in the route table?
A: At a minimum, each entry of the routing table must include a destination address and the address of a next-hop router or an indication that the destination address is directly connected.
2: What does it mean when a route table says that an address is variably subnetted?
A: Variably subnetted means that the router knows of more than one subnet mask for subnets of the same major IP address.
3: What are discontiguous subnets?
A: Discontiguous subnets are two or more subnets of a major IP network address that are separated by a different major IP address.
4: What command is used to examine the route table in a Cisco router?
A: show ip route is used to examine the routing table of a Cisco router.
5: What are the two bracketed numbers associated with the non-directly connected routes in the route table?
A: The first bracketed number is the administrative distance of the routing protocol by which the route was learned. The second number is the metric of the route.
6: When static routes are configured to reference an exit interface instead of a next-hop address, in what way will the route table be different?
A: When a static route is configured to reference an exit interface instead of a next-hop address, the destination address will be entered into the routing table as directly connected.
7: What is a summary route? In the context of static routing, how are summary routes useful?
A: A summary route is a single route entry that points to multiple subnets or major IP addresses. In the context of static routes, summary routes can reduce the number of static routes that must be configured.
8: What is an administrative distance?
A: An administrative distance is a rating of preference for a routing protocol or a static route. Every routing protocol and every static route has an administrative distance associated with it. When a router learns of a destination via more than one routing protocol or static route, it will use the route with the lowest administrative distance.
9: What is a floating static route?
A: A floating static route is an alternative route to a destination. The administrative distance is set high enough that the floating static route is used only if a more-preferred route becomes unavailable.
10: What is the difference between equal-cost and unequal-cost load sharing?
A: Equal-cost load sharing distributes traffic equally among multiple paths with equal metrics.
Unequal-cost load sharing distributes packets among multiple paths with different metrics. The
traffic will be distributed inversely proportional to the cost of the routes.
11: How does the switching mode at an interface affect load sharing?
A: If an interface is fast switched, per destination load sharing is performed. If an interface is process switched, per packet load sharing is performed.
12: What is a recursive table lookup?
A: 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.
1: What is a routing protocol?
A: A routing protocol is a "language" that routers speak to each other to share information about
2: What basic procedures should a routing algorithm perform?
A: 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
3: Why do routing protocols use metrics?
A: 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.
4: What is convergence time?
A: Convergence time is the time a group of routers take to complete the exchange of routing
5: What is load balancing? Name four different types of load balancing.
A: Load balancing is the process of sending packets over multiple paths to the same destination. Four types of load balancing are:
Equal cost, per destination
Unequal cost, per destination
6: What is a distance vector routing protocol?
A: 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.
7: Name several problems associated with distance vector protocols.
A: Several problems associated with distance vector protocols are:
A susceptibility to incorrect routing information because of its dependence on neighbors for
A: Neighbors are routers connected to the same data link.
9: What is the purpose of route invalidation timers?
A: Route invalidation timers delete routes from a route table if they exceed a certain age.
10: Explain the difference between simple split horizon and split horizon with poisoned reverse.
A: 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.
11: What is the counting-to-infinity problem, and how can it be controlled?
A: 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.
12: What are holddown timers, and how do they work?
A: 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
13: What are the differences between distance vector and link state routing protocols?
A: 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.
14: What is the purpose of a topological database?
A: A topological database holds the link state information originated by all routers in the link state routing domain.
15: Explain the basic steps involved in converging a link state internetwork.
A: 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.
16: Why are sequence numbers important in link state protocols?
A: 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.
17: What purpose does aging serve in a link state protocol?
A: Aging prevents old, possibly obsolete, link state information from residing in a topological
database or from being accepted by a router.
18: Explain how an SPF algorithm works.
A: 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.
19: How do areas benefit a link state internetwork?
A: 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.
20: What is an autonomous system?
A: Depending on the usage, an autonomous system can be defined as an internetwork under a common administrative domain or a single routing domain.
21: What is the difference between an IGP and an EGP?
A: 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.
1: What port does RIP use?
A: RIP uses UDP port 520.
2: What metric does RIP use? How is the metric used to indicate an unreachable network?
A: 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.
3: What is the update period for RIP?
A: RIP sends periodic updates every 30 seconds minus a small random variable to prevent the updates of neighboring routers from becoming synchronized.
4: How many updates must be missed before a route entry will be marked as unreachable?
A: A route entry is marked as unreachable if six updates are missed.
5: What is the purpose of the garbage collection timer?
A: 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.
6: Why is a random timer associated with triggered updates? What is the range of this timer?
A: The random timer, whose range is 1 to 5 seconds, prevents a "storm" of triggered updates during a topology change.
7: What is the difference between a RIP Request message and a RIP Response message?
A: A Request message asks a router for an update. A Response message is an update.
8: Which two types of Request messages does RIP use?
A: A Request message may either ask for a full update or in some special cases it may ask for specific routes.
9: Under what circumstances will a RIP response be sent?
A: A Response is sent when the update timer expires, or upon reception of a Request message.
10: Why does RIP hide subnets at major network boundaries?
A: 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.
1: Which UDP port number is used to access IGRP?
A: IGRP does not use a UDP port. It is accessed directly from the network layer, as protocol number 9.
2: What is the maximum IGRP internetwork diameter, in hops?
A: The maximum IGRP network diameter is 255 hops.
3: What is the default update period for IGRP?
A: The default IGRP update period is 90 seconds.
4: Why does IGRP specify an autonomous system number?
A: 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.
A: 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.
6: What is the default IGRP holddown time?
A: The default IGRP holddown time is 280 seconds.
7: Which variables can IGRP use to calculate its composite metric?
A: IGRP can use bandwidth, delay, load, and reliability to calculate its metric. By default, it uses only bandwidth and delay.
8: How many entries can be carried within a single IGRP update packet?
A: An IGRP update packet can carry up to 104 route entries.
1: Which three fields are new to the RIPv2 message format?
A: 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.
2: Besides the extensions defined by the three fields of question 1, what are the other two major
A: In addition to the functions that use the new fields, RIPv2 supports authentication and multicast updates.
3: What is the multicast address used by RIPv2? What is the advantage of multicasting messages overbroadcasting them?
A: 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.
4: What is the purpose of the Route Tag field in the RIPv2 message?
A: 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.
5: What is the purpose of the Next Hop field?
A: 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.
6: What is the UDP port number used by RIPv2?
A: RIPv2 uses the same UDP port number as RIPv1, port number 520.
7: Which one feature must a routing protocol have to be a classless routing protocol?
A: A classless routing protocol does not consider the major network address in its route lookups, but just looks for the longest match.
8: Which one feature must a routing protocol have to use VLSM?
A: To support VLSM, a routing protocol must be able to include the subnet mask of each destination address in its updates.
9: Which two types of authentication are available with Cisco's RIPv2? Are they both defined in RFC 1723?
A: Cisco's implementation of RIPv2 supports clear-text authentication and MD5 authentication. Only clear-text authentication is defined in RFC 1723.
1: Is EIGRP a distance vector or a link state routing protocol?
A: EIGRP is a distance vector protocol.
2: What is the maximum configured bandwidth EIGRP will use on a link? Can this percentage be changed?
A: 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
3: How do EIGRP and IGRP differ in the way they calculate the composite metric?
A: EIGRP and IGRP use the same formula to calculate their composite metrics, but EIGRP scales the metric by a factor of 256.
4: What are the four basic components of EIGRP?
A: 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
5: In the context of EIGRP, what does the term reliable delivery mean? Which two methods ensure reliable delivery of EIGRP packets?
A: 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.
6: Which mechanism ensures that a router is accepting the most recent route entry?
A: Sequence numbers ensure that a router is receiving the most recent route entry.
7: What is the multicast IP address used by EIGRP?
A: EIGRP uses the multicast address 224.0.0.10.
8: What are the packet types used by EIGRP?
A: The packet types used by EIGRP are:
9: At what interval, by default, are EIGRP Hello packets sent?
A: The default EIGRP Hello interval is 5 seconds, except on some slow-speed (T1 and below)
interfaces, where the default is 60 seconds.
10: What is the default hold time?
A: The EIGRP default hold time is three times the Hello interval.
11: What is the difference between the neighbor table and the topology table?
A: The neighbor table stores information about EIGRP-speaking neighbors; the topology table lists all known routes that have feasible successors.
12: What is a feasible distance?
A: The feasible distance to a destination is a router's lowest calculated distance to the destination.
13: What is the feasibility condition?
A: 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.
14: What is a feasible successor?
A: A feasible successor to a destination is a neighbor that satisfies the feasibility condition for that destination.
A: A successor to a destination is a feasible successor that is currently being used as the next hop to the destination.
16: What is the difference between an active route and a passive route?
A: 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.
17: What causes a passive route to become active?
A: A route becomes active when no feasible successor exists in its topology table.
18: What causes an active route to become passive?
A: An active route becomes passive when a reply has been received from every queried neighbor.
19: What does stuck-in-active mean?
A: 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.
20: What is the difference between subnetting and address aggregation?
A: 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.
1: What is an OSPF neighbor?
A: 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.
2: What is an OSPF adjacency?
A: An OSPF adjacency is a conceptual link to a neighbor over which LSAs can be sent.
3: What are the five OSPF packet types? What is the purpose of each type?
A: 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 Acknowledgments - used to ensure reliable delivery of LSAs
4: What is an LSA? How does an LSA differ from an OSPF Update packet?
A: 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.
5: What are LSA types 1 to 5 and LSA type 7? What is the purpose of each type?
A: 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.
6: What is a link state database? What is link state database synchronization?
A: 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.
7: What is the default HelloInterval?
A: The default OSPF HelloInterval is 10 seconds.
8: What is the default RouterDeadInterval?
A: The default RouterDeadInterval is four times the HelloInterval.
9: What is a Router ID? How is a Router ID determined?
A: 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.
A: An area is an OSPF sub-domain, within which all routers have an identical link state database.
11: What is the significance of area 0?
A: Area 0 is the backbone area. All other areas must send their inter-area traffic through the backbone.
A: MaxAge, 1 hour, is the age at which an LSA is considered to be obsolete.
13: What are the four OSPF router types?
A: 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
14: What are the four OSPF path types?
A: The four OSPF path types are:
15: What are the five OSPF network types?
A: The five OSPF network types are:
Non-broadcast multi-access (NBMA) networks
Point-to-multipoint networks
16: What is a Designated Router?
A: 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.
17: How does a Cisco router calculate the outgoing cost of an interface?
A: Cisco IOS calculates the outgoing cost of an interface as 108/BW, where BW is the configured bandwidth of the interface.
18: What is a partitioned area?
A: 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.
19: What is a virtual link?
A: A virtual link is a tunnel that extends an OSPF backbone connection through a non-backbone area.
20: What is the difference between a stub area, a totally stubby area, and a not-so-stubby area?
A: 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.
21: What is the difference between OSPF network entries and OSPF router entries?
A: 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.
22: Why is type 2 authentication preferable over type 1 authentication?
A: Type 2 authentication uses MD5 encryption, whereas type 1 authentication uses clear-text
23: Which three fields in the LSA header distinguish different LSAs? Which three fields in the LSA header distinguish different instances of the same LSA?
A: 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.
1: What is an intermediate system?
A: An Intermediate System is the ISO term for a router.
2: What is a network protocol data unit?
A: A Network Protocol Data Unit is the ISO term for a packet.
3: What is the difference between an L1, an L2, and an L1/L2 router?
A: 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
4: Explain the basic difference between an IS-IS area and an OSPF area.
A: The borders of IS-IS areas are between routers, on links. The borders of OSPF areas are defined by the routers themselves.
5: What is a network entity title (NET)?
A: The Network Entity Title is an address by which a router identifies both itself and the area in which it resides.
6: To what value must the NSAP Selector be set in a NET?
A: The NSAP Selector should be set to 0x00 in a NET.
7: What is the purpose of a System ID?
A: The System ID uniquely identifies a router within an IS-IS domain.
8: How does a router determine what area it is in?
A: The portion of the NET preceding the last seven octets is the area address.
9: Does IS-IS elect a Backup Designated Router on a broadcast subnetwork?
A: IS-IS does not elect a BDR.
10: What is the purpose of the Pseudonode ID?
A: The Pseudonode ID is the last octet of a LAN ID. Its purpose is to distinguish LAN IDs which are originated by a single router which is the DR on multiple LANs.
11: What is the maximum age (MaxAge) of an IS-IS LSP?
A: The MaxAge of an IS-IS LSP is 1200 seconds (20 minutes).
12: What is the basic difference between the way OSPF ages its LSAs and the way IS-IS ages its
A: OSPF increments the age up to MaxAge; IS-IS decrements the age down to 0. A new OSPF LSA has an age of 0, whereas a new IS-IS LSP has an age of MaxAge.
13: How often does an IS-IS router refresh its LSPs?
A: The refresh rate of an IS-IS router is 900 seconds (15 minutes).
14: What is a Complete Sequence Number Packet (CSNP)? How is it used?
A: A Complete Sequence Number Packet contains a full listing of all LSPs in a database. A CSNP is periodically sent by the Designated Router on a broadcast network to maintain database
15: What is a Partial Sequence Number Packet (PSNP)? How is it used?
A: A Partial Sequence Number Packet contains a listing of one or more LSPs. It has two uses: On point-to-point networks, it is used to acknowledge the receipt of LSPs. On broadcast networks, it is used to request LSPs.
16: What is the purpose of the Overload (OL) bit?
A: An IS-IS router uses the Overload bit to inform its neighbors that it is experiencing a memory
overload and cannot store the entire link state database.
17: What is the purpose of the Attached (ATT) bit?
A: The Attached bit is used by L1/L2 routers to inform L1 routers that it is attached to the L2
18: What metrics are specified by the ISO for IS-IS? How many of these metrics does the Cisco IOS support?
A: The ISO specifies four metrics: Default, Expense, Delay, and Error. Cisco supports only the
19: What is the maximum value of the IS-IS default metric?
A: The maximum value of any of the IS-IS metrics is 63.
20: What is the maximum metric value of an IS-IS route?
A: The maximum metric value of an IS-IS route is 1023.
21: What is the difference between a level 1 IS-IS metric and a level 2 IS-IS metric?
A: L1 IS-IS metrics apply to intra-area routes, and L2 IS-IS metrics apply to inter-area routes.
22: What is the difference between an internal IS-IS metric and an external IS-IS metric?
A: Internal metrics apply to routes to destinations within the IS-IS domain. External metrics apply to routes to destinations external to the IS-IS domain.
1: From what sources can a route be redistributed?
A: Routes that are learned from another routing protocol, static routes, or a direct connection to the destination network can be redistributed into a routing domain.
2: What is the purpose of an administrative distance?
A: In contrast to metrics, which are used to determine the best path among multiple routes to the same destination discovered by the same routing protocol, administrative distances are used to determine the best path among multiple routes to the same destination discovered by different routing protocols.
3: How can administrative distances cause problems when redistributing?
A: A route to a destination within a routing domain with a higher administrative distance can be
redistributed into a routing domain with a lower administrative distance. If that route is redistributed back into the higher-distance domain, packets might be misrouted into the lower-distance domain.
4: How can redistribution from a classless to a classful routing protocol cause problems?
A: Redistributing variably subnetted destination addresses from a classless domain into a classful domain can cause problems.
5: Which IP IGPs can use the default redistribution metric, and which IGPs must have a metric
configured in order for redistribution to work?
A: OSPF and IS-IS understand the default metric. RIP, IGRP, and EIGRP do not.
6: What is the difference between using the metric keyword with the redistributecommand and using the default-metric command?
A: The metric command assigns a metric to specific redistribution statements. The default-metric command assigns a metric to all redistribution commands that do not include the metric command.
7: What is the purpose of the subnets keyword when redistributing OSPF?
A: Without the subnets keyword, only major network addresses that are not directly connected to the router will be redistributed.
8: How is the null interface useful when summarizing routes?
A: A router that originates a summary route should use the null interface as the next hop of the
summary route. Any packets that match the summary route, but for which there is no more-specific route to the packet's destination address, will be dropped. This prevents the router from forwarding "lost" packets.
1: What is the destination address of default routes used by the open protocols?
A: The default route address is 0.0.0.0.
2: How are default routes identified and advertised by IGRP and EIGRP?
A: IGRP and EIGRP advertise a default address as an external address type.
3: Can a static route to 0.0.0.0 be used as the default route on a router running IGRP?
4: What is a stub router? What is a stub network?
A: A stub router is a router with only a single link to another router. A stub network is a network with only one attached router.
5: What is an advantage of using default routes instead of a full routing table?
A: Using a default route rather than a full routing table can conserve router memory by keeping the table small and can save router processing cycles by limiting the routing information that must be processed.
6: What is an advantage of using a full routing table instead of a default route?
A: Using a full routing table rather than a default route can make routing more accurate.
7: What data link protocol does On-Demand Routing use to discover routes?
A: ODR uses Cisco Discovery Protocol (CDP) to discover routes.
8: What IOS restrictions are placed on ODR?
A: ODR is available in IOS 11.2 and later.
9: What media restrictions are placed on ODR?
A: The medium over which ODR is to run must support SNAP.
1: How are route maps similar to access lists? How are they different?
A: Route maps are similar to access lists in that they define match criteria and an action to take in theevent of a match. Route maps are different from access lists in that they not only specify matchcriteria but also specify set criteria. The set action can modify a route or route a packet according tothe parameters of the packet.
2: What are policy routes?
A: Policy routes are static routes that use route maps to determine which packets should be routed andwhere the packets should be routed.
A: Route tags are fields within routing information packets that allow external information to be carried through the routing domain.
4: In what way do route tags affect routing protocols?
A: Route tags have no effect on the routing protocols that carry them.
