Chapter 11 Layer 3 Protocols

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Chapter 11 Layer 3 Protocols
Paul Flynn
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Layer 2 Devices Bridges and Switches

• uses physical, or MAC addresses (assigned by NIC
  card manufacturer), to make data forwarding
  decisions
• primarily used to connect segments of a network.

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Layer 3 Devices - Routers

• passes data packets between networks, based on
  Layer 3 addresses (IP, protocol addresses,
  logical addresses or network addresses).
• has the ability to make intelligent decisions
  regarding the best path for delivery of data on
  the network
• Assigned by the Network Administrator

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Routers and Data Relaying
Interface

• Routers connect two or more networks, each of
  which must have a unique network number in order
  for routing to be successful.
• Each interface must have a separate, unique
  network (or subnetwork) address that is part of
  the network attached to that interface. 

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Addressing Schemes

• Static
• Network administrator must go to each individual
  device and configure it with an IP address. This
  method requires you to keep very meticulous
  records, because problems can occur on the
  network if you use duplicate IP addresses
• Dynamic
• Reverse Address Resolution Protocol (RARP)binds
  MAC addresses to IP addresses (Diskless
  workstations)
• BOOTstrap Protocol (BOOTP)used by a device when
  it starts up, to obtain an IP address.
• Dynamic Host Configuration Protocol (DHCP)has
  been proposed as a successor to BOOTP. Unlike
  BOOTP, DHCP allows a host to obtain an IP address
  quickly and dynamically.

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Address Resolution Protocol (ARP)

• In order for devices to communicate, the sending
  devices need both the IP addresses and the MAC
  addresses of the destination devices.
• When they try to communicate with devices whose
  IP addresses they know, they must determine the
  MAC addresses. ARP enables a computer to find the
  MAC address of the computer that is associated
  with an IP address.
• Some keep tables that contain all the MAC
  addresses and IP addresses of other devices that
  are connected to the same LAN. They are called
  Address Resolution Protocol (ARP) tables, and
  they map IP addresses to the corresponding MAC
  addresses
• If it lacks one or the other, the data will not
  pass from Layer 3 to the upper layers. In this
  way, MAC addresses and IP addresses act as checks
  and balances for each other.

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Default Gateway

• IP address of the interface on the router that
  connects to the network segment on which the
  source host is located.
• The default gateways IP address must be in the
  same network segment as the source host.
• Without a default gateway, the source computer
  has no way to look up destination MAC addresses
  on other segments of the network, and the message
  is undeliverable

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Proxy ARP

• variation of the ARP protocol
• Basically, a router acts as the default gateway,
  allowing a device on one subnet to obtain the MAC
  address of a host on another subnet.

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Routed vs. Routing Protocols
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Routed Protocols

• Characteristics
• provide support for the network layer
• Have the ability to assign a network number as
  well as the host number
• IP, IPX/SPX, AppleTalk
• IP relies on the Transport layer protocol to
  determine whether packets have been lost, and to
  request retransmission

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Routing Protocols

• Determines the paths that routed protocols (IP,
  etc) follow to their destinations.
• Used to exchange routing tables and share routing
  information by enabling routers that are
  connected to create a map, internally, of other
  routers in the network or on the Internet
• Routing Information Protocol (RIP)
• (Most Common used between routers on the same
  network)
• Interior Gateway Routing Protocol (IGRP)
• Developed by CISCO
• Enhanced Interior Gateway Routing Protocol
  (EIGRP)
• Developed by CISCO
• Open Shortest Path First (OSPF).

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Routing contd

• In addition to IP addresses and MAC addresses of
  devices located on networks to which it connects,
  a router also possesses IP addresses and MAC
  addresses of other routers.
• It uses these addresses to direct data toward its
  final destination.
• If a router receives a packet whose destination
  address is not in its routing table, it forwards
  it to the address of another router that most
  likely does contain information about the
  destination host in its routing table.
• This DOES NOT change the hosts destination IP
  address in the packet.

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Routing Metrics

• Bandwidth (Data capacity of a link)
• Delay (length of time required to move a packet
  from source to destination)
• Load (Amount of activity on a network resource)
• Reliability (refers to error rate of each network
  link)
• Hop Count (No. of passages of a packet)
• Ticks (delay on a data link approx. 35
  milliseconds)
• Cost (or path cost - arbitrary value assigned by
  an NA)

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RIP

• Distance vector routing protocol
• Only metric is number of hops
• Max. hops is 15
• Updates every 30 seconds
• Doesnt always select fastest path
• Generates lots of network traffic with constant
  updates
• Most commonly used routing protocol for routers
  on same network

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Interior Gateway Protocols (IGP)

• Route data in an autonomous system.
• RIP (See previous slide)
• IGRP
• a distance-vector protocol however, when
  determining the best path, it also takes into
  consideration such things as bandwidth, load,
  delay, and reliability. Network administrators
  can determine the importance given to any one of
  these metrics, or, allow IGRP to automatically
  calculate the optimal path.
• EIGRP
• provides superior operating efficiency and
  combines the advantages of link-state protocols
  with those of distance-vector protocols
• OSPF
• means "open shortest path first". A better
  description, however, might be "determination of
  optimum path", because this Interior Gateway
  Protocol actually uses several criteria to
  determine the best route to a destination. These
  criteria include cost metrics, which factor in
  such things as route speed, traffic, reliability,
  and security

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Exterior Gateway Protocols (EGP)

• Route data between autonomous systems.
• An example of an EGP is BGP (Border Gateway
  Protocol), the primary exterior routing protocol
  of the Internet. 

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Static Routing

• Manual entries in the routing table
• Requires less overhead than dynamic routing
• Uses
• For hiding parts of an internetwork
• To test a particular link in a network
• It is the preferred method for maintaining
  routing tables whenever there is only one path to
  a destination network (stub network)

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Dynamic Routing

• routers send periodic routing update messages to
  each other
• Relies on routing protocol to share knowledge
• Allows updates to occur for changing network
  conditions
• Routing protocol defines the set of rules used by
  router when it communicates with neighboring
  routers.

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Other Network Services

• Connectionless Network Services (Packet switched)
• In a connectionless system the destination is
  not contacted before a packet is sent (Postal
  Service analogy)
• Connection-Oriented
• (Circuit switched)
• a connection is established between the sender
  and the recipient before any data is transferred
  (Phone system analogy)