CCNA Interview QuestionsWhich layer of the OSI model is responsible for reliable connections?
The Transport layer of the OSI model is responsible for reliable connections.
What is the difference between acknowledgments and handshaking?
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.
How many VTP modes are there and what are they?
Three: Server, Client, and Transparent
What are the two types of Trunk encapsulation protocols?
IEEE 802.1Q and Cisco’s ISL
What are the four primary no routable protocols?
SNA, NetBIOS, DEC LAT, DEC MOP
What is the difference between TCP and UDP?
The primary difference between TCP and UDP is that TCP is a connection oriented protocol and UDP is a connection less protocol.
What is HSRP?
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.
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.
What does AAA stand for?
Authentication, authorization, and accounting
The H.323 protocol is used for what?
H.323 is used for multi service (multimedia) applications, usually in a Voice Over IP environment.
What type of routing protocol maintains neighbors?
what is the range of values for administrative distance?
Describe the difference between uni cast, multi cast, and broadcast traffic?
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.
What are the four different Ethernet encapsulation types?
From the Cisco IPX encapsulation command they are ARPA, NOVEL-ETHER, SAP and SNAP
What are the three main tasks of a transparent bridge?
Learning, Forwarding, Filtering
What type of routing protocol is EIGRP?
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?
The Network Layer
What algorithm does OSPF use to compute its route table?
OSPF uses the shortest path first (SPF) algorithm, which is also known as the Dijkstra algorithm.
What is a stub area?
A stub area is an area that does not accept routing updates from outside its autonomous system.
What does the TTL field of an IP packet header do?
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.
What is the primary purpose of a LAN?
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.
What is a protocol?
A protocol is an agreed-upon set of rules. In data communications, the rules usually govern a procedure or a format.
What is the purpose of a MAC protocol?
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
What is a frame?
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, anderor-checking information.
What feature is common to all frame types?
A feature common to all frame types is a format for identifying devices on the data link.
What is a MAC address or MAC identifier?
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.
Why is a MAC address not a true address?
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.
What are the three sources of signal degradation on a data link?
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.
What is the purpose of a repeater?
A repeater is a device that extends the useful range of a physical medium by reading a degraded signal and producing a “clean” copy of the signal.
What is the purpose of a bridge?
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.
What makes a transparent bridge transparent?
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.
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 from a service provider.
A LAN provides high bandwidth at a relatively cheap price. The bandwidth across a WAN is significantly more expensive.
What is the purpose of a broadcast MAC identifier? What is the broadcast MAC identifier, in hex and in binary?
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.
What is the primary similarity between a bridge and a router? What is the primary difference between a bridge and a router?
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.
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 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.
As a packet progresses across an inter network, does the source address change?
Neither the source nor the destination address of a packet changes as it progresses from the source of the packet to the destination.
What is a network address? What is the purpose of each part of a network address?
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.
What is the primary difference between a network address and a data link identifier?
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.
What are the five layers of the TCP/IP protocol suite? What is the purpose of each layer?
The five layers of the TCP/IP protocol suite are the following:
-Data link layer
-Internet (or IP) layer
What is the most common IP version presently in use?
The most common IP version now in use is version 4.
What is fragmentation? What fields of the IP header are used for fragmentation?
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.
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 decreament the integer by one. If a router decreaments the TTL to zero, it will discard the packet and send an ICMP “time exceeded” error message to the packet’s source address.
What is the first octet rule?
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.
How are class A, B, and C IP addresses recognized in dotted decimal? How are they recognized in binary?
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
What is an address mask, and how does it work?
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.
What is a subnet? Why are subnets used in IP environments?
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. Sub netting 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.
Why can’t a subnet of all zeros or all ones be used in a classful routing environment?
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.
What is ARP?
ARP, or Address Resolution Protocol, is a function that maps the IP addresses of interfaces on a data link to their corresponding MAC identifiers.
What is proxy ARP?
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.
What is a redirect?
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.
What is the essential difference between TCP and UDP?
TCP, or Transmission Control Protocol, provides a connection-oriented service over the Connectionless internet layer. UDP, or User Data gram Service, provides a connectionless service.
What mechanisms does TCP use to provide connection-oriented service?
Correct sequencing is accomplished with sequence numbers. Reliability is accomplished by using check sums, acknowledgments, timers, and re transmissions. Flow control is accomplished by windowing.
Instead of ARP, Novel Net Ware uses a network address that includes a device’s MAC address as the host portion. Why can’t IP do this?
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.
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 a connection less service on top of what is already a connectionless service?
Answer: The only purpose of the UDP header is to add fields for the source and destination port numbers.
What information must be stored in the route table?
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.
What does it mean when a route table says that an address is variably subnetted?
Variably subnetted means that the router knows of more than one subnet mask for subnets of the same major IP address.
What are discontiguous subnets?
Discontiguous subnets are two or more subnets of a major IP network address that are separated by a different major IP address.
What command is used to examine the route table in a Cisco router?
show ip route is used to examine the routing table of a Cisco router.
What are the two bracketed numbers associated with the non-directly connected routes in the route table?
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.
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?
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.
What is a summary route? In the context of static routing, how are summary routes useful?
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.
What is an administrative distance?
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.
What is a floating static route?
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.
What is the difference between equal-cost and unequal-cost load sharing?
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.
How does the switching mode at an interface affect load sharing?
If an interface is fast switched, per destination load sharing is performed. If an interface is process switched, per packet load sharing is performed.
What is a recursive table lookup?
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.
What is a routing protocol?
A routing protocol is a “language” that routers speak to each other to share information about network destinations.
What basic procedures should a routing algorithm perform?
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
Why do routing protocols use metrics?
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.
What is convergence time?
Convergence time is the time a group of routers take to complete the exchange of routing
What is load balancing? Name four different types of load balancing.
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
What is a distance vector routing protocol?
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.
Name several problems associated with distance vector protocols.
Several problems associated with distance vector protocols are:
A susceptibility to incorrect routing information because of its dependence on neighbors for correct information
Counting to infinity
What are neighbors?
Neighbors are routers connected to the same data link.
What is the purpose of route invalidation timers?
Route invalidation timers delete routes from a route table if they exceed a certain age.
Explain the difference between simple split horizon and split horizon with poisoned reverse?
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.
What is the counting-to-infinity problem, and how can it be controlled?
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.
What are hold down timers, and how do they work?
Hold down 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 inter network is re converging.
What are the differences between distance vector and link state routing protocols?
A distance vector router sends its entire route table, but it only sends the table to directly connected neighbours. A link state router sends only information about its directly connected links, but it floods the information throughout the inter networking 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.
What is the purpose of a topological database?
A topological database holds the link state information originated by all routers in the link state routing domain.
Explain the basic steps involved in converging a link state inter network.?
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 inter networking 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.
Why are sequence numbers important in link state protocols?
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 inter network.
What purpose does aging serve in a link state protocol?
Aging prevents old, possibly obsolete, link state information from residing in a topological database or from being accepted by a router.
Explain how an SPF algorithm works?
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.
How do areas benefit a link state inter network?
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.
What is an autonomous system?
Depending on the usage, an autonomous system can be defined as an inter network under a common administrative domain or a single routing domain.
What is the difference between an IGP and an EGP?
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.
What port does RIP use?
RIP uses UDP port 520.
What metric does RIP use? How is the metric used to indicate an unreachable network?
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.
What is the update period for RIP?
RIP sends periodic updates every 30 seconds minus a small random variable to prevent the updates of neighboring routers from becoming synchronized.
How many updates must be missed before a route entry will be marked as unreachable?
A route entry is marked as unreachable if six updates are missed.
What is the purpose of the garbage collection timer?
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.
Why is a random timer associated with triggered updates? What is the range of this timer?
The random timer, whose range is 1 to 5 seconds, prevents a “storm” of triggered updates during a topology change.
What is the difference between a RIP Request message and a RIP Response message?
A Request message asks a router for an update. A Response message is an update.
Which two types of Request messages does RIP use?
A Request message may either ask for a full update or in some special cases it may ask for specific routes.
Under what circumstances will a RIP response be sent?
A Response is sent when the update timer expires, or upon reception of a Request message.
Why does RIP hide subnets at major network boundaries?
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.
Which UDP port number is used to access IGRP?
IGRP does not use a UDP port. It is accessed directly from the network layer, as protocol number 9.
What is the maximum IGRP internetwork diameter, in hops?
The maximum IGRP network diameter is 255 hops.