institute of technology sligo - dept of computing chapter 11 layer 3 protocols paul flynn

Institute of Technology Sligo - Dept of Computing

ChChapter 11apter 11 Layer 3 Protocols Layer 3 Protocols

Paul Flynn


Layer 2 Devices – Layer 2 Devices – Bridges and SwitchesBridges and Switches

uses physical, or MAC addresses uses physical, or MAC addresses (a(assigned by NIC card manufacturer)ssigned by NIC card manufacturer), to , to make data forwarding decisions make data forwarding decisions

primarily used to connect segments of a primarily used to connect segments of a network.network.


Layer 3 Devices - RoutersLayer 3 Devices - Routers• passes data packets between networks, based on Layer 3 passes data packets between networks, based on Layer 3

addresses (IP, protocol addresses, logical addresses or addresses (IP, protocol addresses, logical addresses or network addresses). network addresses).

• has the ability to make intelligent decisions regarding the has the ability to make intelligent decisions regarding the best path for delivery of data on the networkbest path for delivery of data on the network

• Assigned by the Network AdministratorAssigned by the Network Administrator

Institute of Technology Sligo - Dept of ComputingRouters and Data RelayingRouters 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.


Addressing SchemesAddressing Schemes StaticStatic

Network administrator must go to each individual device and Network administrator must go to each individual device and configure it with an IP address. This method requires you to configure it with an IP address. This method requires you to keep very meticulous records, because problems can occur on keep very meticulous records, because problems can occur on the network if you use duplicate IP addressesthe network if you use duplicate IP addresses

DynamicDynamic Reverse Address Resolution Protocol (RARP)Reverse Address Resolution Protocol (RARP)

binds MAC addresses to IP addresses (Diskless workstations)binds MAC addresses to IP addresses (Diskless workstations) BOOTstrap Protocol (BOOTP)BOOTstrap Protocol (BOOTP)

used by a device when it starts up, to obtain an IP address. used by a device when it starts up, to obtain an IP address. Dynamic Host Configuration Protocol (DHCP)Dynamic Host Configuration Protocol (DHCP)

has been proposed as a successor to BOOTP. Unlike BOOTP, has been proposed as a successor to BOOTP. Unlike BOOTP, DHCP allows a host to obtain an IP address quickly and DHCP allows a host to obtain an IP address quickly and dynamically. dynamically.


Address Resolution Protocol (ARP)Address Resolution Protocol (ARP)

In order for devices to communicate, the sending devices need In order for devices to communicate, the sending devices need both the IP addresses and the MAC addresses of the both the IP addresses and the MAC addresses of the destination devices. destination devices.

When they try to communicate with devices whose IP When they try to communicate with devices whose IP addresses they know, they must determine the MAC addresses. addresses they know, they must determine the MAC addresses. ARP enables a computer to find the MAC address of the ARP enables a computer to find the MAC address of the computer that is associated with an IP address.computer that is associated with an IP address.

Some keep tables that contain all the MAC addresses and IP Some keep tables that contain all the MAC addresses and IP addresses of other devices that are connected to the same addresses of other devices that are connected to the same LAN. They are called LAN. They are called Address Resolution ProtocolAddress Resolution Protocol ( (ARP)ARP) tablestables, , and they map IP addresses to the corresponding MAC and they map IP addresses to the corresponding MAC addresses addresses

If it lacks one or the other, the data will not pass from Layer 3 to 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 the upper layers. In this way, MAC addresses and IP addresses act as checks and balances for each other.act as checks and balances for each other.


Default GatewayDefault Gateway

IP address of the interface on the router that IP address of the interface on the router that connects to the network segment on which connects to the network segment on which the source host is located.the source host is located.

The default gateway’s IP address must be in The default gateway’s IP address must be in the same network segment as the source the same network segment as the source host. host.

Without a default gateway, the source Without a default gateway, the source computer has no way to look up destination computer has no way to look up destination MAC addresses on other segments of the MAC addresses on other segments of the network, and the message is undeliverable;network, and the message is undeliverable;


Proxy ARPProxy ARP

variation of the ARP protocolvariation of the ARP protocol Basically, a router acts as the default Basically, a router acts as the default

gateway, allowing a device on one gateway, allowing a device on one subnet to obtain the MAC address of a subnet to obtain the MAC address of a host on another subnet.host on another subnet.


Routed vs. Routing ProtocolsRouted vs. Routing Protocols


Routed ProtocolsRouted ProtocolsCharacteristicsprovide 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


Routing ProtocolsRouting Protocols Determines the paths that routed protocols (IP, etc) Determines the paths that routed protocols (IP, etc)

follow to their destinations. follow to their destinations. Used to exchange routing tables and share routing Used to exchange routing tables and share routing

information by enabling routers that are connected information by enabling routers that are connected to create a map, internally, of other routers in the to create a map, internally, of other routers in the network or on the Internetnetwork or on the Internet Routing Information Protocol (RIP) Routing Information Protocol (RIP)

(Most Common used between routers on the same network)(Most Common used between routers on the same network)

Interior Gateway Routing Protocol (IGRP)Interior Gateway Routing Protocol (IGRP) Developed by CISCODeveloped by CISCO

Enhanced Interior Gateway Routing Protocol (EIGRP) Enhanced Interior Gateway Routing Protocol (EIGRP) Developed by CISCODeveloped by CISCO

Open Shortest Path First (OSPF). Open Shortest Path First (OSPF).


Routing cont’dRouting cont’d In addition to IP addresses and MAC In addition to IP addresses and MAC

addresses of devices located on networks to addresses of devices located on networks to which it connects, a router also possesses IP which it connects, a router also possesses IP addresses and MAC addresses of other addresses and MAC addresses of other routers. routers.

It uses these addresses to direct data toward It uses these addresses to direct data toward its final destination. its final destination.

If a router receives a packet whose If a router receives a packet whose destination address is not in its routing table, destination address is not in its routing table, it forwards it to the address of another router it forwards it to the address of another router that most likely does contain information that most likely does contain information about the destination host in its routing table.about the destination host in its routing table.

This DOES NOT change the hosts This DOES NOT change the hosts destination IP address in the packet.destination IP address in the packet.


Routing MetricsRouting Metrics Bandwidth Bandwidth (Data capacity of a link)(Data capacity of a link)

DelayDelay (length of time required to move a packet from source to (length of time required to move a packet from source to

destinationdestination)) Load Load (Amount of activity on a network resource)(Amount of activity on a network resource)

Reliability Reliability (refers to error rate of each network link)(refers to error rate of each network link)

Hop Count Hop Count (No. of passages of a packet)(No. of passages of a packet)

Ticks Ticks (delay on a data link – approx. 35 milliseconds)(delay on a data link – approx. 35 milliseconds)

Cost Cost (or path cost - arbitrary value assigned by an NA)(or path cost - arbitrary value assigned by an NA)


RIPRIP Distance vector routing protocolDistance vector routing protocol Only metric is number of hopsOnly metric is number of hops Max. hops is 15Max. hops is 15 Updates every 30 secondsUpdates every 30 seconds Doesn’t always select fastest pathDoesn’t always select fastest path Generates lots of network traffic with constant Generates lots of network traffic with constant

updatesupdates Most commonly used routing protocol for Most commonly used routing protocol for

routers on same networkrouters on same network


Interior Gateway Protocols (IGP)Interior Gateway Protocols (IGP)

Route data Route data inin an autonomous system. an autonomous system. RIPRIP (See previous slide) (See previous slide) IGRP IGRP

a distance-vector protocol; however, when determining the best a distance-vector protocol; however, when determining the best path, it also takes into consideration such things as bandwidth, path, it also takes into consideration such things as bandwidth, load, delay, and reliability. Network administrators can determine load, delay, and reliability. Network administrators can determine the importance given to any one of these metrics, or, allow IGRP to the importance given to any one of these metrics, or, allow IGRP to automatically calculate the optimal path.automatically calculate the optimal path.

EIGRPEIGRP provides superior operating efficiency and combines the provides superior operating efficiency and combines the

advantages of link-state protocols with those of distance-vector advantages of link-state protocols with those of distance-vector protocolsprotocols

OSPFOSPF means "open shortest path first". A better description, however, means "open shortest path first". A better description, however,

might be "determination of optimum path", because this Interior might be "determination of optimum path", because this Interior Gateway Protocol actually uses several criteria to determine the Gateway Protocol actually uses several criteria to determine the best route to a destination. These criteria include cost metrics, best route to a destination. These criteria include cost metrics, which factor in such things as route speed, traffic, reliability, and which factor in such things as route speed, traffic, reliability, and securitysecurity


Exterior Gateway Protocols Exterior Gateway Protocols (EGP)(EGP)

Route data Route data betweenbetween autonomous systems. autonomous systems. An example of an EGP is BGP (Border An example of an EGP is BGP (Border

Gateway Protocol), the primary exterior Gateway Protocol), the primary exterior routing protocol of the Internet. routing protocol of the Internet.


Static RoutingStatic 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)


Dynamic RoutingDynamic 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.


Other Network ServicesOther Network Services Connectionless Network Services Connectionless Network Services (Packet switched)(Packet switched) In a connectionless system the destination is not contacted before a packet In a connectionless system the destination is not contacted before a packet

is sentis sent (Postal Service analogy) (Postal Service analogy)

Connection-OrientedConnection-Oriented (Circuit switched)(Circuit switched) a connection is established between the sender and the recipient before any data a connection is established between the sender and the recipient before any data

is transferredis transferred (Phone system analogy) (Phone system analogy)

institute of technology sligo - dept of computing chapter 11 layer 3 protocols paul flynn

Documents

addresses ip

network addresses

protocol addresses

ip addresses act

logical addresses

corresponding mac addresses

default gateways ip

network segment