Institute of Technology Sligo - Dept of Computing Chapter 11 Layer 3 Protocols Paul Flynn

Institute of Technology Sligo - Dept of Computing 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.

Institute of Technology Sligo - Dept of Computing Layer 3 Devices - Routers passes data packets between networks, based on Layer 3 addresses (IP, protocol addresses, logical addresses or network addresses).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 networkhas the ability to make intelligent decisions regarding the best path for delivery of data on the network Assigned by the Network AdministratorAssigned by the Network Administrator

Institute of Technology Sligo - Dept of Computing 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.

Institute of Technology Sligo - Dept of Computing 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.

Institute of Technology Sligo - Dept of Computing 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.

Institute of Technology Sligo - Dept of Computing 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 gateway’s 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;

Institute of Technology Sligo - Dept of Computing 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.

Institute of Technology Sligo - Dept of Computing Routed vs. Routing Protocols

Institute of Technology Sligo - Dept of Computing 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

Institute of Technology Sligo - Dept of Computing 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).

Institute of Technology Sligo - Dept of Computing Routing cont’d  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.

Institute of Technology Sligo - Dept of Computing 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)

Institute of Technology Sligo - Dept of ComputingRIP  Distance vector routing protocol  Only metric is number of hops  Max. hops is 15  Updates every 30 seconds  Doesn’t always select fastest path  Generates lots of network traffic with constant updates  Most commonly used routing protocol for routers on same network

Institute of Technology Sligo - Dept of Computing 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

Institute of Technology Sligo - Dept of Computing 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.  An example of an EGP is BGP (Border Gateway Protocol), the primary exterior routing protocol of the Internet.

Institute of Technology Sligo - Dept of Computing 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)

Institute of Technology Sligo - Dept of Computing 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.

Institute of Technology Sligo - Dept of Computing 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)