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Computer Science BSc - Year 2 > Networking Systems > Flashcards

Networking Systems Flashcards

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1
Q

What is the second packet forwarding mechanism?

A
  • Fast switching
  • common mechanism which uses a fast switching cache to store the next-hop information
  • packet arrives on an interface and is forwarded to the control plane where the CPU searches for a match in the fast-switching cache
  • if no match, it is process-switched and forwarded to next interface
  • packet flow information stored in fast-switching cache for quick lookup
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2
Q

What happens when a router receives a packet from a network that is destined for another network?

A

Step 1; de-encapsulates the layer 2 frame header and trailer to expose the layer 3 packet

Step 2; Examines the destination IP address of the IP packet to find the best path in the routing table

Step 3; if the router finds a path to the destination, it encapsulates the layer 3 packet into a new layer 2 frame and forwards it to the sit interface

-After each hop between routers, the layer 3 IP address does not change but the data link address does

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3
Q

What is quad zero?

A
  • 0.0.0.0
  • if a packet has no destination it can be sent here
  • can also be used to route data that is internet bound
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4
Q

What are some port roles on a switch?

A
  • Root ports – ports closets to the root bridge
  • Designated ports – non-root ports permitted to forward traffic
  • Alternate and backup ports – blocking state to prevent loops
  • Disabled ports – switch port that is shut down
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5
Q

What is the first type of dynamic routing?

A

Distance vector - means that routes are advertised by providing 2 characteristics;

  • Distance = ID’s how far is it to the destination networks based on hop count
  • Vector = specifies the direction of the next hop router or exit interface to reach the destination
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6
Q

Four other commands that link to OSPF?

A

1) “Show ip OSPF neighbour” to verify the router has formed an adjacency with a directly connected router
2) “show ip protocols” to verify the OSPFv2 process ID, router ID, networks being advertised by the router and other things
3) “show ip OSPF interface” to see details for every OSPFv2 enabled interface, especially to see if the network statements were correctly composed
4) “show ip interface brief” command to see key information about OSPFv2 enabled interfaces on a particular router

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7
Q

What are administrative distances?

A
  • possible to have multiple routes to same destination network
  • each routing protocol might prefer a different path so router choses by using administrative distance
  • represents the trustworthiness of the route, the lower the better
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8
Q

Cons of link-state

A
  • Memory and processing consumption high
  • the flooding of link state packets can adversely affect bandwidth
  • using multiple areas can reduce the size of the link state DB
  • Multiple areas can limit amount of link-state information flooding and send LSPs only to those routers that need them
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9
Q

How does topology relate to OSPF?

A
  • topology used to describe OSPF configuration; defines the area of range of coverage for this protocol
  • shows links to external networks
  • WAN links
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10
Q

What is the OSPF metric?

A
  • cost = reference bandwidth / interface bandwidth
  • lower the better
  • can be seen in the routing table in brackets
  • instead of manually setting the interface bandwidth, the OSPF cost can be manually configured using the “ip OSPF cost value” command
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11
Q

Draw a diagram showing the communication and transmission of a packet between two nodes A and B when sent using TCP

A
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12
Q

What is load balancing?

A
  • if router has 2 or more paths with ID metrics to the same destination network the router will forward the packet using both paths equally
  • routing table contains a single destination but has multiple exit interfaces
  • one for each equal cost path; known as Equal Cost Load Balancing
  • increase the effectiveness and performance of the network
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13
Q

Difference between a straight-through cable and cross over cable?

A
  • straight-through is between a computer and a modem, router, switch or other dissimilar networking equipment
  • Crossover is between two computers or two similar networking devices
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14
Q

What is DHCP?

A
  • Dynamic host configuration protocol
  • responsible for assigning IP address to network nodes
  • IP addresses are leased to the node
  • can change at anytime between 24 hours to 30 days
  • node requests a new IP when lease is up
  • very often its the same IP address
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15
Q

What is effective routing based on?

A
  • Adequate bandwidth = must ensure bandwidth can cope with the intended volume of data
  • Speed of data transmission = determined by the bandwidth, speed of operation of the NIC and the number of routers and switches involved in the route, causes latency
  • Network card speed, number of devices across the network, the volume of traffic, good design
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16
Q

What is latency?

A
  • time it takes to get from source to destination
  • could be as result of problems with the transmission medium itself or errors with the routers/switches as each device takes time to examine and change the packet header
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17
Q

What is the 4th layer of the OSI?

A
  • Transport manages end to end message delivery in the network
  • Provides both reliable and sequential packet delivery through error recovery and flow control mechanisms (TCP)
  • provides unreliable transport as well (UDP)
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18
Q

What is a VLAN trunk?

A
  • Point to point link that carries more than one VLAN
  • Links between S1 and S2, and S1 and S3 are configured to transmit traffic coming from VLANs 10, 20, 30 and 99 across the network
  • This network could not function without VLAN trunks
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19
Q

What is router on a stick?

A
  • The alternative to inter-VLAN routing
  • only one port is configured and operates as a trunk link, connected to a trunked switch port
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20
Q

How can a router learn about the network?

A
  • can learn about indirectly connected router through its connected routers
  • information about indirectly connected routers appear as protocol
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21
Q

What is an IPv6 link local address?

A
  • enables a device to communicate with other IPv6 devices on the same link and only on that link (subnet)
  • packet with a source or destination link local address cannot be routed beyond the link from where the packet originated -used to exchange OSPFv3 messages
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22
Q

Why cant we end an IP with 255?

A

-reserved as broadcast address

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23
Q

What is the first IP class?

A
  • Class A
  • Start address is 0.0.0.0, end address is 127.255.255.255
  • Number of networks is 2^7 -addresses per network is 2^24
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24
Q

What is the physical layer responsible for?

A
  • bit-by-bit delivery = symbol or baud rate is used here to measure how fast something happens (symbol is a pile or tone that represents data)
  • Modulation = the process of modulating a signal onto a carrier
  • providing a standardisation interface to the medium
  • circuit switching
  • forward error recovery
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25
Q

What is the second type of dynamic routing?

A
  • Link-state = uses the link state information received from other routers…
  • ….To create a topology map and select the best path to all destination networks in the topology
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26
Q

Draw a diagram showing the communication and transmission of a packet between two nodes A and B when sent using UDP

A
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27
Q

What is the first packet forwarding mechanism?

A
  • process switching, slower and older than others
  • the packet arrives on an interface, it is forwarded to the control lane where the CPU then matches the destination address with an entry in its routing table in order to determine the exit interface
  • slow as does this for every packet in stream
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28
Q

What are some uses for dynamic routing protocol?

A
  • best choice for large networks
  • protocols help administrator manage the network
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29
Q

What are some problems with switches?

A
  • They broadcast, eg; when a new device is connected an ARP is sent out to request info to add to the CAM table
  • the request is passed out of every port except the one the packet arrived on
  • So, for a 24-port switch, 23 ports will be broadcast
  • If your network had 20 switches, that’s a lot of broadcasting (Broadcast domains)
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30
Q

What is VSLM?

A
  • Variable length subnet masks
  • more flexible approach than subnetting
  • allows for multiple sub network to be addressed out of same address space
  • eg; in the same network you may have some /25 addresses and some /27
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31
Q

What is an APIPA address?

A
  • If your device is assigned an IP address that starts 169.254.x.x then it is unable to contact a DHCP server
  • it will not be able to communicate with other devices
  • automatic private IP addressing
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32
Q

How are router IDS used with OSPF?

A
  • used to uniquely ID an OSPF router
  • 32 bits long in both v2 (IPv4) and v3 (IPv6)
  • used in the election of the DR if a priority number is not configured
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33
Q

What is BGP

A
  • border gateway protocol
  • exterior routing protocol -highly scalable
  • Mainly used to connect LAN to the internet or to other networks
  • provides loop free routing between autonomous systems
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34
Q

What is the 3rd layer of OSI?

A
  • network manages the transfer of packets that are to be forwarded on to different networks and between nodes on the same network
  • routers
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35
Q

What are the two sublayers of the data link layer?

A
  • LLC = logical link control layer
  • MAC = media access control layer
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36
Q

What are dead intervals?

A
  • by default are 4 times hello interval
  • if the dead interval expires before the router receives a hello packet, OSPF removes that neighbour from its link state DB with info about the down neighbour
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37
Q

What does the structuce of an ethernet header look like?

A
  • Preamble
  • Destination MAC Address
  • Source MAC Address
  • Ethertype
  • Data
  • FCS
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38
Q

What are some benefits of the OSI?

A
  • reduced complexity = complex problem decomposed into 7 layer
  • increased evolution = tech can advance and still integrate with old tech
  • guarantees interoperability = ensures data can be transferred between differing computer types, software OS, network and computer hardware
  • allows modular engineering = hardware and software can interface well with each other
  • standardised interfaces = products can be designed to easily plug into one or more layers of the model
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39
Q

What is a CDN?

A

-content delivery network refers to the geographically distributed group of servers which work together to provide fast delivery of internet connection, doesnt host the site just caches information

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40
Q

What is ethernet?

A
  • Ethernets LAN’s consist of network nodes and interconnecting media
  • Network nodes fall into two main classes;

>Data terminal equipment (DTE) = devices that are either the source or the destination of data frames (PC’s, printers, file servers )

>Data communication equipment (DCE) = intermediate network devices that receive and forward frames across the network (NIC’s, modems, repeaters)

-Ineracts with layers 1, 2 and 3 of the OSI model

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41
Q

What is the CAM?

A
  • Content addressable memory table
  • has a physical address mapped to the port the destination device is attached to
  • when a switch receives an incoming frame with a MAC address that is not found in the CAM table, it floods it to all ports, except the one that received the frame
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42
Q

What is fibre optic

A
  • uses light pulses to transmit data
  • Operates over large distances
  • has two modes; single (transmit data at 100 GBPs for 100km without signal repeat) and multi-mode (100mbps for 2km)
  • immune to electrical interference and cross talk
  • each fibre can carry many independent channels with each using a different wavelength of light making it hard to hack
  • can bend around 3cm before fractures appear
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43
Q

What is an easier way to work out network address?

A
  • just look at whole address
  • 10.10.10.10 255.0.0.0 has to be 10.0.0.0 as the first bits are all turned to 1 (two 1’s means 1 is carried down) and the rest are 0’s (0 carried with anything is 0)
  • 172.16.25.14 255.255.0.0 has to be 172.16.0.0
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44
Q

What is the 2nd layer of the OSI?

A
  • data link provides a reliable link between two directly connected devices
  • switches
  • uses MAC addresses
  • error detection = prevents collisions
  • PPP = point to point protocol
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45
Q

What is a routing table?

A
  • stored in RAM it contains all the available routes
  • ID’s the next hop for remote networks
  • directly connected = obtained from the active router interfaces
  • remote networks = statically configured or learned from dynamic routing protocols
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46
Q

What are the 3 ways signals can get damaged?

A

Attenuation - data loss (usually as a result of the cable going beyond its effective range)

Dispersion - signal flattens out and widens due to interference

Distortion - signal completely changes due to interferences

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47
Q

Why do we have DR/BDR election?

A
  • reduce the number of LSAs sent
  • the DR is the only router used to send LSAs for the shared network
  • reduce the number of advances over a multi-access network like ethernet
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48
Q

What is IS-IS

A
  • creates a map of a network and uses it to calculate shortest path to destination
  • uses areas
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49
Q

What is the 5th layer of the OSI?

A
  • Manages communication between applications after a connection is made
  • session inter-host communication = sets up, maintains and closes a session
  • provides an open communication path with another system
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50
Q

What is routing protocol?

A
  • AIDS routers by dynamically learning paths to routes and other networks
  • OSPF = open shortest path first
  • RIP = Routing Information protocol
  • BGP = border gateway protocol
  • IS-IS = intermediate system to intermediate system
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51
Q

What are standards?

A
  • Define the rules for the protocols to communicate effectively
  • Required to ensure full compatibility and interoperability between devices
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52
Q

What is the process of ARP?

A
  • switch receives a packet which contains an un ID MAC address
  • ARP then broadcasts an ARP request to request information about the known IP address (we have the IP, so whoever’s IP this is has the MAC)
  • all hosts receive the broadcasted frame
  • if there is a match the matching device will send the required information; unicast
  • MAC information is updated in the MAC address table and that port is associated with that device If no match the ARP is discarded
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53
Q

What is bandwidth? What is Broadband?

A
  • A measure of the amount of data that can be sent over a connection; max transmission capacity
  • Allows data to be sent on multiple channels simultaneously. Broadens the available bandwidth (date/second)
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54
Q

Pros and cons of TCP?

A

Pros;

  • Reliable
  • Guarantees data will be sent and received
  • Makes sure correct ordering is maintained

Cons;

-Slower

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55
Q

what is a loop?

A
  • when a signal path has the same source and destination address
  • result in signal amplifying itself rather than stopping at a destination
  • causes network performance issues due to the constant amplification
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56
Q

why do we use private IP addresses?

A
  • Instead of having devices inside a home network each use a public IP address, of which there’s a limited supply, private IP addresses provide a separate set of addresses that still allow access on a network but without taking up a public IP address space
  • Private IP addresses also provide a way for devices that don’t need contact with the internet, like file servers and printers, to still communicate with other devices on a network without being exposed to the public.
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57
Q

What are hello packet intervals?

A

-hello and dead intervals must be the same interval setting on neighbouring routers on the same link -

transited to multicast address 224.0.0.5 in IPv4 and FF02::5 in IPv6

  • sent every 10 seconds by default on multi access networks like ethernet and point-point links
  • sent every 30 seconds by default on non-broadcasts multiple access networks like frame relay
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58
Q

what are the ranges for private IP addresses?

A
  • 10.0.0.0 to 10.255.255.255
  • 172.16.0.0 to 172.31.255.255
  • 192.168.0.0 to 192.168.255.255
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59
Q

What does the OSI model address?

A
  • To interconnect equipment made by different manufacturers through a standard interface
  • To integrate software and hardware and to be portable on differing systems
  • create a model which will be adhered to by all countries of the world
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60
Q

What is copper cable? Difference between twisted pair and coaxial cable?

A
  • copper is most common and cheapest form of network cable
  • it is easy to install modify and manage
  • coaxial is much slower, less flexible and used for TV Ariels
  • Twisted pair has less insulation and shielding, comes in categories with the higher categories being better (Cat I and II are used for telephone lines, cat VI is used for high speed internet)
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61
Q

When do routers use routing tables?

A

-router only uses routing table if destination IP not directly connected

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62
Q

What is the OSI model?

A
  • Theoretical 7 layer model
  • headed up by international organisation for standardisation
  • We are concerned with layers 1-4 and a little bit of 5
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63
Q

Pros of VLAN’s?

A
  • Slightly improved security – internally
  • Reduced cost
  • Better performance
  • Smaller broadcast domains
  • IT efficiency
  • Management efficiency
  • Simpler project and application management
64
Q

Methods of modulation?

A
  • PSK = Phase Shift keying, a finite number of phases are used
  • FSK = Frequency Shift keying, a finite number of frequencies are used
  • ASK = Amplitude Shift keying, a finite number of amplitudes are used
65
Q

What are the three databases associated with OSPFv2?

A

Adjacency (neighbour table) - lists all neighbour routers to which a router has established a bidirectional communication, unique for each router

Link state (LSDB) (topology table) - lists all info about other routers, contains the same LSDB info as other routers in the same area

Forwarding (routing table) - list routes generated when the SPF algorithm is run on the link-state DB, unique to each router and contains info on how and where to send packets destined for remote networks

66
Q

What is subnetting?

A
  • when we run out of IP addresses for connecting devices, we subnet our network
  • process of dividing the default IP address range to reduce the number of wasted IP addresses and increase the number of devices on the network
  • allows us to make use of previously wasted IP’s
  • for example; if we allocate 192.168.10.0 for 47 devices, we will waste 207 IP addresses as this is a class C address with a max of 254 addresses
67
Q

What are the OSPF operational states?

A
  • progress through several states while attempting to reach convergence -down = no hello packets received; router sends hello packets
  • init = hello packets are received that contain the sending routers router ID
  • two way = used to elect DR and BDR on an ethernet link
  • exstart = negotiate a master/slave relationship and DBD packet sequence number; the master initiates the DBD exchange
68
Q

what is a collision domain?

A

-A Collision Domain consists of all the devices connected using a Shared Media (Bus Topology or using Ethernet Hubs) where a Collision can happen between any device at any time.

69
Q

what is a broadcast domain?

A
  • A Broadcast Domain consists of all the devices that will receive any broadcast packet originating from any device within the network segment.
  • more devices leads to more broadcasts, which leads to the quality of the network decreasing thanks to the following reasons.
    1) A decrease in available Bandwidth
    2) A decrease in the processing power of computers: Computers need to process all the broadcast packets they receive, a portion of the computer CPU power is spent on processing the broadcast packets.
70
Q

what is sliding window?

A
  • Alternative to stop and wait
  • The sender and receiver agree on the number of data frames to be sent before an acknowledgement is sent
  • far better utilisation of resources than stop and wait
  • tells exactly which frame didn’t get received and resends all frames
71
Q

Switch vs router

A
  • By default, all ports on a router are turned off
  • By default, all ports on a switch are turned on
  • The expectation is that the switch ports will be used
  • The unused ports should be turned off so rouge devices cannot be attached to the network
72
Q

Types of VLAN?

A
  • Data – user generated traffic
  • Default – all switch ports become part of this VLAN until switch is configured
  • Native – used for untagged traffic
  • Management – used to access management capabilities
  • VLANs can be configured to include voice traffic
  • Voice traffic is prioritised over all other traffic to ensure quality
73
Q

What is special about IP address 127.0.0.1

A
  • reserved as loopback address
  • helps troubleshooting connectivity issues
  • ping loopback will tell you if your NIC is functioning correctly or not
74
Q

What are;

1) single bit errors
2) Burst errors

and what leads to them?

A

Attenuation and interference in signals lead to…

1) Single-bit errors occur when a single bit gets changed during transmission of data due to interference in network communication - this can be detected by parity checking
2) Burst errors will effect a strings of bits over time and much more complex error methods are needed to detect and correct these - more likely to occur, as the duration of noise is generally longer than the duration of transferring 1 bit

75
Q

What are the two ways to use the network command

A

1) advertise the network, calculate wildcard mask
2) advertise the IP address on the router interface with 0.0.0.0

76
Q

How does protocol relate to physical media?

A
  • although given media is used as the transfer agent, it is the media protocol that defines the process and characteristics of the data transfer
  • in other words; some media types can be used to support multiple different media protocols
77
Q

What is the third and preferred packet forwarding mechanism?

A
  • Cisco Express Forwarding, CEF, fastest and most recent
  • builds a forwarding information base (FIB) and an adjacency table
  • table entries are not packet triggered but change triggered when something changes in the network topology
  • when a network has converged the FIB and adjacency tables contain all the information a router would have to consider when forwarding a packet
78
Q

What is OSPFv3?

A
  • used to exchange IPv6 prefixed and build an IPv6 routing table
  • builds three OSPF tables; neighbour, topology and routing
  • link-local address are used to exchange OSPFv3 messages
  • OSPFv2 uses plain text and MD5 authentication, OSPFv3 uses IPv6 authentication
79
Q

What is the 1st layer of the OSI?

A
  • Physical layer is responsible for transmission of binary data via a medium
  • defines the protocol to establish and terminate a connection between two directly connected nodes
  • NIC converts binary data into electrical voltage
80
Q

What are static routes?

A
  • Manually configured routes between two points that define an explicit path
  • if the topology changes the routes must be changed manually
  • requires less bandwidth
  • routers do not have to calculate the route so no CPU cycles are required
  • good for point-point and used for backup in event of failures
  • indicated by S in a routing table, S* shows it is a candidate for default route
81
Q

What is STP?

A
  • STP ensures there is only one logical path between all destinations on the network
  • Blocks redundant paths that could cause a loop
  • Unknown unicast frames sent onto a looped network can result in duplicate frames arriving at the destination device
  • Use STP in situations where you want redundant (backup) links, but not loops
  • Redundant links are as important as backups in the case of a failover in a network.
82
Q

What is the link-state process? (LONG)

A
  • Each router learns about each of its own directly connected networks
  • “” Responsible for saying hello to its neighbours on directly connected networks
  • “” builds a link-state packet (LSP) containing the state of each directly connected link
  • “” floods the LSP to all neighbours who then store all LSP’s received in a DB
  • “” uses the DB to construct a complete map of the topology and computes the best path to each destination network; best paths are added to routing table
83
Q

What is packet switching?

A
  • Delivers messages sent from one node to another by splitting them into small, limited size packets
  • each packet has source and destination address
  • travels to its destination by being stored and forwarded from node to node
  • may travel different paths but must be reassembled into a message as the destination
84
Q

What are OSPFv3 packet types?

A
  • Hello - discovers neighbours and builds adjacencies between them
  • Database description (DBD) - Checks for DB synchronisation between routers
  • Link state request (LSR) - requests specific link-state records from router to router
  • link state update (LSU) - sends specifically requested link-state records
  • link state acknowledgement (LSAck) - acknowledges other packet types
85
Q

what is the root bridge?

A
  • The root bridge serves as a reference point for all STP calculations
  • The switch with the lowest BID will become the root bridge
  • can be configured manually according to network requirements
  • Works out the port costs based on the speed the port operates at
  • Formula for working out STP path cost is; >1 Gb/s / bandwidth
86
Q

Why are Anycast flips a problem?

A
  • changes in routing shifts a client to a new server without any notification to either
  • If the client and server have some shared states, such as active TCP connections, these will break because of a TCP reset and need to be restarted
87
Q

What are switches?

A
  • connect devices together
  • connect peripheral devices to the network
  • only used for wired networks
  • Makes decisions based on the physical address (MAC addresses which are attached to the port the destination device is attached to) in the packet header
  • To find devices on the network the switch will send out an ARP request
  • Port and MAC address information are stored in the CAM table
  • Use cut-through and store-and-forward methods of switching
  • multiple ports to allow numerous devices to be connected
88
Q

What is cut-through

A
  • The second switch forwarding method
  • forwards the frame before it is entirely received
  • At a minimum, the destination address of the frame must be read before the frame can be forwarded
  • faster method
  • No buffering, no FCS check, can start forwarding the packet within 10 microseconds
89
Q

How do we manage congestion?

A
  • FIFO
  • WFQ = Fair, dynamic queuing that divides bandwidth across queues of traffic based on weights
90
Q

What are ACL’s?

A
  • Permit or deny network traffic
  • can be done by IP address or protocol
  • filter internal and external traffic
  • provides a basic level of security
  • can be used to prevent users from misusing the system
  • associated with the interface, so placement during configuration is key
91
Q

What is circuit switching?

A
  • between 2 devices
  • all links to form a source-to-destination circuit path are selected and dedicated to one set of communication transmissions for its full duration
  • full bandwidth given
  • remains connected for duration of communication
  • designed for voice traffic -minimal delays
92
Q

How does a router work?

A
  • connects networks together regardless of the type of network or traffic being transmitted
  • determines the best and most efficient path to forward data between routers
  • when a router receives a packet, it examines the destination address of the packet and uses a routing table to look for the best path to that network
93
Q

What is a passive interface?

A
  • does not send OSPF messages
  • best practice for interfaces that have users attached (security)
  • doesn’t waste bandwidth sending messages out OSPF enabled interfaces that don’t have another router attached
  • configured using the “passive interface” command
94
Q

What is TCP/IP?

A
  • IP purpose is to deliver packets with as little per-packet effort as possible (no error checking)
  • TCP performs the useful networking functions; error recovery, resending lost packets
95
Q

What is the third IP class?

A
  • Class C
  • Start address is 192.0.0.0, end address is 223.255.255.255
  • Number of networks is 2^21 -addresses per network is 2^8
96
Q

Difference between digital and analogue signals?

A
  • Analogue signals vary continuously and can take any value within some given range
  • digital signals are chosen from a discrete range of possibilities (0-9, A-Z)
97
Q

What is a network?

A
  • A large system consisting of many smaller parts connected to allow movement or communication between or along these parts
  • combines; computers, switches, routers, bridges, cables etc
  • purpose is to get info from A-B as effectively, quickly and reliably as possible
98
Q

What is multiplexing?

A

-multiple cables combined into one cable at one end and then unpacked into multiple cables at the other end

99
Q

What is structure and how does it relate with protocol?

A
  • physical media can impose structure
  • structure and media protocol often define limitations and expectations of a network in terms of;

>The number of nodes that can operate within the network

>distance over which the network will operate

>bandwidth that the network will support

100
Q

What is a single area OSPF?

A
  • all routers contained in one area
  • called the backbone area
  • known as area 0
  • used in small network with few routers
101
Q

What are hardware failures?

A

Devices fail

  • this causes links to become unavailable and causes routes to be re-routed
  • causes a temporary increase in network traffic as topologies are updated in the routing table
  • causes disruption and some delay in the end-end delivery
102
Q

what is store and forward

A
  • One of the two switch forwarding methods
  • receives an entire frame and computes the CRC
  • If the CRC is valid, the switch looks up the destination address
  • The frame is then forwarded out the correct the port
  • Error checking performed on the packet
103
Q

What are OSPFv2 packet types?

A

-Hello, Database description, link state request, link state update, link state acknowledgment

104
Q

How do I enable OSPF on interfaces?

A
  • use the network command to specify which interfaces participate in the OSPFv2 area
  • router OSPF x THEN
  • network x.x.x.x wildcard_mask area area-id
  • To workout the wildcard mask subtract the normal mask from 255.255.255.255 Eg; 255.255.255.255 - 255.255.255.000 = 000.000.000.255
105
Q

What is default static route?

A
  • like default gateway
  • specifies an exit point when the routing table does not have a path for the destination network
106
Q

What are hello packets?

A
  • used to discover neighbours, establish neighbour adjacencies, advertise parameters both routers must agree upon in order to become neighbours
  • also electrics the designated router and backup designated router
107
Q

What is a router?

A
  • Small computers which have memory, CPU and I/O devices
  • Contain RAM, ROM, NVRAM and Flash memory
  • Modular devices so different types of media can be attached by adding the correct module
  • Can be configured to provide some security using access control lists (ACLs)
  • Routers are not interested in specific devices, but are interested in the network
  • Routers use the network portion of the IP address to work out whether a packet is destined for that network or should be passed on to other devices
  • Does not have any peripheral devices attached to them directly.
108
Q

What do routers act as?

A
  • default gateway; exit to other networks
  • move data between networks
  • restrict network broadcasts (routers do not broadcast)
  • router acts as a dispatcher, choosing the best path for information to travel so it’s received
109
Q

What is dynamic routing?

A
  • share information about the status and reachability of remote networks
  • use network discovery to share network information with other routers using the same protocol
  • all this info is added to the routing table
  • once all the topology info has been exchanged the network has converged
  • if there are changes to the topology dynamic routing protocols will calculate a new route automatically
110
Q

What is Anycast?

A
  • network addressing and routing method in which incoming requests can be routed to a variety of different nodes
  • in the case of CDN, Anycast routes incoming traffic to the nearest data centre
  • Anycast instability occurs when traffic silently shifts from one site to another, each one of these instabilities is called a flip
111
Q

What is ARP?

A
  • how switches learn about the devices attached to each port
  • Address resolution protocol
112
Q

What is APARnet?

A
  • worlds first packet switching network
  • first to use TCP/IP -created as experiment by US military during Cold War
113
Q

What are VLAN’s?

A
  • Virtual local area network
  • Define groups by the floors in a building or by groups or use types = departments in a business that may have devices on multiple floors can be split into their own VLAN group
  • Logical partition of layer 2 network
  • Enable the implementation of security and access policies according to the specifics of groups
  • Multiple VLAN’s can exist on the same network
  • Each VLAN is a broadcast domain
  • mutually isolated and packets can only pass between them via a router
  • invisible to the end user
114
Q

Pros and cons of packet switching?

A

Pros;

  • Most efficient method of communication
  • Multiple people can communicate at the same time
  • large files easier to send as broken down into small packets

Cons;

  • Duplicate and out of sequence packets may arrive at the destination node
  • no guarantee the message will reach destination undamaged or the sender will be informed of its failure
115
Q

What is anding?

A
  • helps find out network address using subnet mask
  • where there are two 1’s, 1 is carried down
  • where there are 0’s, the 0 is carried down
  • where there is a 1 and a 0 the 0 is carried down
116
Q

What are the two types of IP design topologies?

A
  • Physical = the intended location of the devices (floor plan)
  • Logical = the IP addresses for each device in that location
117
Q

what is lag?

A
  • Lag is a slang term for a noticeable decrease in application speed, due to extreme network congestion or insufficient processing power
  • When traffic is heavier than network capacity, the network requires that a program waits before sending or receiving data.
118
Q

Pros of link-state?

A
  • each router builds its own topological map of the network to determine shortest path
  • immediate flooding of LSPs achieves faster convergence
  • LSPs are sent only when there is a change in the topology and contain only the information regarding that change
  • Hierarchical design used when implementing multiple areas
119
Q

What are some of the symbols on routes to remote networks?

A
  • Directly connected networks are added auto and are ID by C and L
  • S = static
  • D = EIGRP
  • O = OSPF
  • R = RIP
120
Q

What is the best path?

A

-the route in the routing table that has the greatest number of far left matching bits with the destination IP address of the packet

121
Q

What is OSPF?

A
  • Open shortest Path first
  • Uses Dijkstra’s algorithm to compute the shortest path
  • Scalable, so is able to manage large networks
  • Operates within a single autonomous systems
  • Link-state protocol
  • Uses the cost of the bandwidth as it’s metric
  • It is an interior gateway protocol.
122
Q

What is the logical link control layer?

A
  • LLC provides flow control, acknowledgement and error notification
  • controls data exchanged between the source and destination machines
  • takes packets from the network layer above; creates frames to send to physical layer
  • error detection and correction; also responsible for detecting and correcting errors in receiving data
123
Q

What is Time to Live (TTL)?

A

A value in an IP packet that tells a network router whether or not the packet has been in the network for too long and should be discarded

124
Q

what is inter-VLAN routing

A

-Layer 2 switches cannot forward traffic between VLANs without assistance from a router, known as inter-VLAN routing

125
Q

What is Transmission Control Protocol?

A
  • TCP is connection orientated, tracks all data sent and requires an acknowledgement for each octet allowing the sender and receiver to have a two-way communication
  • guarantees that data will be sent, received and constructed in the correct order
  • considered more reliable than UDP, ensures no data is sent to the upper layer that is out of order, duplicated or has missing pieces
  • allows for lost data to be resent if needed.
126
Q

What is user Datagram protocol?

A
  • Connection-less and very lightweight
  • main use include streaming applications
  • data once lost is not resent.
127
Q

Features of OSPF

A
  • complex, open source
  • ability to load balance
  • perofmans well
  • easy to troubleshoot
  • uses Dijkstra’s shortest algorithm to find best path
128
Q

One way to solve some problems with switches?

A

-Routers do not broadcast so the easiest way to alleviate the issue is to segment the network into separate collision domains

129
Q

What are link state updates?

A
  • a link state update (LSU) contains one or more LSAs; LSAs contain route info for destination network
  • routers initially send type 2 DBD packet
  • an abbreviated list of the sending routers LSDB
  • type 3 LSR is used by the receiving router to request more information about an entry in the DBD
  • Type 4 LSU is used to reply to an LSR packet
130
Q

Common layer 2 attacks?

A

CDP reconnaissance attack

telnet attacks

MAC address table flooding attack

VLAN attacks

DHCP attacks

131
Q

How do subnet masks relate to classes?

A
  • Each class has own default subnet mask
  • Class A; 255.0.0.0 (this is /8 as first 8 bits in subnet mask turned to 1)
  • Class B; 255.255.0.0 (/16 as first 16 bits are turned to 1)
  • Class C; 255.255.255.0 (/24)
132
Q

What are the fourth and fifth IP classes?

A
  • Class D and E
  • only used for experiment purposes
133
Q

What is a MAC?

A
  • determines who is allowed to access the media at any one time CSMA/CD
  • determines where one frame ends and the other starts (frame synchronisation)
134
Q

What information do routes to remote networks contain?

A
  • Route source; how the route was learned
  • Destination network
  • Administrative distance
  • Metric; lower the better
  • Next hop IP -Route timestamp
  • Outgoing interface
  • the exit interface used to forward the packet
135
Q

How does DHCP work?

A
  • works in client/server mode
  • has a pool of iP addresses
  • shares workload during busy periods and provides redundancy
136
Q

What is CSMA/CD?

A
  • Carrier sense multiple access with collision detection
  • modification of CSMA
  • improves performance by terminating transmission as soon as a collision is detected
  • determine if another transmission is in progress if there is a carrier wait for it to end, ‘listen before talking’, each device has a timeout period which is different per device
  • keep checking for collisions, has one occurred?, do nothing, if one has occurred stop transmission and put devices into a timeout
  • it then waits for a few seconds before resending the data
137
Q

What is the second IP class?

A
  • Class B
  • Start address is 128.0.0.0, end address is 191.255.255.255
  • Number of networks is 2^14 -addresses per network is 2^16
138
Q

What is an RFC?

A
  • request for comment
  • standardisation documents -detail how a tech will work
  • any interested parties can comment on the content of the RFC and suggest changes
139
Q

What is an IP address?

A

-Unique ID for a device on a network

140
Q

What is RIP?

A
  • Routing Information Protocol
  • Simplistic protocol used to connect small networks.
  • Uses hop count as it’s metric
  • Sends updates every 30 seconds
  • Directly connected networks are manually added
  • Other routes can be learned dynamically through the topology updates.
141
Q

How is the performance of physical channels affected?

A
  • the properties of different kinds of physical channels determine the performance
  • throughput, latency, error rate etc
142
Q

What are shortest path first protocols?

A
  • Link-state protocols also known as shortest path first protocols
  • built around Dijkstra’s shortest path first algorithm
  • OSPF, IS-IS
143
Q

What is a multi area OSPF?

A
  • all areas connect to area 0
  • more commonly seen with numerous areas around area 0 (like a daisy)
  • routers that connect area 0 to another area are known as area border router’s (ABR)
  • used in large networks
  • multiple areas reduce processing and memory overhead A failure in one does not affect other areas
144
Q

What is the TCP 3-way handshake?

A

Designed to ensure tat data sent using TCP for transport is received and acknowledged by the receiver;

1) Host A sends a TCP SYNchronize packet to Host B
2) Host B receives A’s SYN
3) Host B sends a SYNchronize-ACKnowledgement
4) Host A receives B’s SYN-ACK
5) Host A sends ACKnowledge
6) Host B receives ACK.
7) TCP socket connection is ESTABLISHED.

145
Q

What is encapsulating in OSPF?

A
  • OSPF adds its own layer 3 header after the IP layer 3 header
  • OSPF packet header IDs the type of OSPF packet, the router ID, and the area ID
  • OSPF packet type, contains the specific OSPF packet type information router ID and area ID
146
Q

what is tagging frames?

A
  • Frame tagging is the process of adding a VLAN ID header to the frame
  • It is used to properly transmit multiple VLAN frames through a trunk link
  • Switches tag frames to ID the VLAN to which they belong
  • Different tagging protocols exist; defines the structure of the tagging header
147
Q

What are some components of dynamic routing protocols?

A
  • date structures = tables or databases kept in RAM
  • routing protocol messages = to discover neighbouring routers, exchange routing information and maintain accurate information about the network
  • algorithms = to facilitate learning routing information and for best path determination
148
Q

1) How would I configure an OSPF router ID 2) how would I modify a router ID

A

1) router id x.x.x.x
2) clear ip OSPF process

149
Q

What is flow control?

A
  • Manages the data rate between sender and receiver
  • prevents a slow receiver being overloaded by a faster sender
  • stop and wait approach (receiver says ready for each frame to be sent, must be received before timeout)
  • the sender transmits a single frame, the receiver transmits acknowledgement as it receives the frame, the sender receives an acknowledgement within timeout, repeat
150
Q

What is the physical layer?

A
  • involves the actual physical medium
  • used in the transfer of messages
  • most basic network layer
  • actual signals used varies depending on types of medium (radio waves, optical signals)
  • gets data converts into electrical signals, gets it onto the wire
  • It works point-to-point and is connectionless, providing no error correction other than allowing FEC
  • Carrier Sense takes place at this layer.
151
Q

Pros and cons of UDP?

A

Pros;

  • Faster; there’s no need to wait for connection establishment, just send frames as soon as you are able to. Great for live streaming multicast services.
  • When there’s congestion in the network and packets get dropped, one can gracefully degrade service to use lower bandwidth when using UDP. With TCP, it will retry and always send all packets. Graceful degradation of service requires complex out of band exchanges between sender and reciever.

Cons;

  • Unreliable; can lose packets
  • ordering; doesnt care about odering
152
Q

What is hop-by-hop? What does .0 mean at the end of an IP address?

A
  • lists the next device in the path to the destination
  • fundamental in the IP/Network layer
  • network address
  • it is revered
153
Q

Difference between IPv4 and IPv6?

A
  • IPv4 is older and Being replaced as we are running out of addresses -
  • being replaced by IPv6 which has many more location available to it, uses 128-bit addresses to support 340 trillion addresses as opposed to IPv4 32 bit addresses which support 4.3 billion
154
Q

what are the layers of the TCP/IP stack? How do they relate to OSI model?

A
  • Application layer = Application, presentation and session layers
  • Transport layer = transport layer
  • Network layer = network layer
  • Network interface layer= data link layer, physical layer
155
Q

What is the DHCP 4 step process?

A

1) DHCP DISCOVER broadcast to find DHCP server
2) server receives message and reserves address, ARP entry created of the MAC address requesting node, DHCP OFFER sent
3) node receives offer and sends back DHCP REQUEST message, offer accepted
4) The server verifies lease info and replies with DHCP PACK, node logs info and issues an ARP to see if address already in use, if not node starts using it

156
Q

What is link state operation?

A

1) Use hello packets to form neighbour adjacencies
2) LSAs flood info about the state of links
3) each router builds a topology table
4) routers run SPF algorithm resulting in SPF tree
5) router builds routing table that includes each path and its cost

Computer Science BSc - Year 2 (3 decks)

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