Routing Fundamentals and Subnets 2003 Cisco Systems Inc

Routing Fundamentals and Subnets © 2003, Cisco Systems, Inc. All rights reserved. 1

Objectives • Routed protocol • IP routing protocols • The mechanics of subnetting © 2003, Cisco Systems, Inc. All rights reserved. 2

Routed Protocol © 2003, Cisco Systems, Inc. All rights reserved. 3

Routable and Routed Protocols • A routed protocol allows the router to forward data between nodes on different networks. • In order for a protocol to be routable, it must provide the ability to assign a network number and a host number to each individual device. • These protocols also require a network mask in order to differentiate the two numbers. • The reason that a network mask is used is to allow groups of sequential IP addresses to be treated as a single unit. © 2003, Cisco Systems, Inc. All rights reserved. 4

IP as a Routed Protocol • IP is a connectionless, unreliable, best-effort delivery protocol. • As information flows down the layers of the OSI model; the data is processed at each layer. • IP accepts whatever data is passed down to it from the upper layers. © 2003, Cisco Systems, Inc. All rights reserved. 5

Packet Propagation and Switching Within a Router © 2003, Cisco Systems, Inc. All rights reserved. 6

Packet Propagation and Switching Within a Router • As a frame is received at a router interface. • The MAC address is checked to see if the frame is directly addressed to the router interface, or a broadcast. • The frame header and trailer are removed and the packet is passed up to Layer 3. • The destination IP address is compared to the routing table to find a match. • The packet is switched to the outgoing interface and given the proper frame header. • The frame is then transmitted. © 2003, Cisco Systems, Inc. All rights reserved. 7

Internet Protocol (IP): Connectionless • The Internet is a gigantic, connectionless network in which all packet deliveries are handled by IP. • TCP adds Layer 4, connection-oriented reliability services to IP. © 2003, Cisco Systems, Inc. All rights reserved. 8

Telephone Calls: Connection-oriented A connection is established between the sender and the recipient before any data is transferred. © 2003, Cisco Systems, Inc. All rights reserved. 9

Anatomy of an IP Packet • While the IP source and destination addresses are important, the other header fields have made IP very flexible. • The header fields are the information that is provided to the upper layer protocols defining the data in the packet. © 2003, Cisco Systems, Inc. All rights reserved. 10

IP Routing Protocols © 2003, Cisco Systems, Inc. All rights reserved. 11

Routing Overview • A router is a network layer device that uses one or more routing metrics to determine the optimal path. • Routing metrics are values used in determining the advantage of one route over another. • Routing protocols use various combinations of metrics for determining the best path for data. © 2003, Cisco Systems, Inc. All rights reserved. 12

Routing Versus Switching • This distinction is routing and switching use different information in the process of moving data from source to destination. © 2003, Cisco Systems, Inc. All rights reserved. 13

Routing Versus Switching © 2003, Cisco Systems, Inc. All rights reserved. 14

Routed Versus Routing • A routed protocol: Includes any network protocol suite that provides enough information in its network layer address to allow a router to forward it to the next device and ultimately to its destination. Defines the format and use of the fields within a packet. • A routing protocol: Provides processes for sharing route information. Allows routers to communicate with other routers to update and maintain the routing tables. © 2003, Cisco Systems, Inc. All rights reserved. 15

Path Determination • Path determination enables a router to compare the destination address to the available routes in its routing table, and to select the best path. © 2003, Cisco Systems, Inc. All rights reserved. 16

Routing Tables • Routers keep track of the following: Protocol type Destination/next-hop associations Routing metric Outbound interfaces © 2003, Cisco Systems, Inc. All rights reserved. 17

Routing Algorithms and Metrics • Routing protocols have one or more of the following design goals: Optimization Simplicity and low overhead Robustness and stability Flexibility Rapid convergence © 2003, Cisco Systems, Inc. All rights reserved. 18

IGP and EGP • IGPs route data within an autonomous system. RIP, RIPv 2, IGRP, EIGRP, OSPF, IS-IS • EGPs route data between autonomous systems Border Gateway Protocol (BGP) © 2003, Cisco Systems, Inc. All rights reserved. 19

Link State and Distance Vector • Examples of distance-vector protocols: Routing Information Protocol (RIP) Interior Gateway Routing Protocol (IGRP) Enhanced IGRP (EIGRP) • Examples of link-state protocols: Open Shortest Path First (OSPF) Intermediate System-to-Intermediate System (IS-IS) © 2003, Cisco Systems, Inc. All rights reserved. 20

Routing Protocols • RIP v 2 • IGRP • EIGRP • OSPF • IS-IS • BGP © 2003, Cisco Systems, Inc. All rights reserved. 21

Mechanics of Subnetting © 2003, Cisco Systems, Inc. All rights reserved. 22

Classes of Network IP Addresses © 2003, Cisco Systems, Inc. All rights reserved. 23

Introduction to Subnetting • Host bits must are reassigned (or “borrowed”) as network bits. • The starting point is always the leftmost host bit. 3 bits borrowed allows 23 -2 or 6 subnets 5 bits borrowed allows 25 -2 or 30 subnets 12 bits borrowed allows 212 -2 or 4094 subnets © 2003, Cisco Systems, Inc. All rights reserved. 24

Reasons for Subnetting • Provides addressing flexibility for the network administrator. Each LAN must have its own network or subnetwork address. • Provides broadcast containment and lowlevel security on the LAN. • Provides some security since access to other subnets is only available through the services of a router. © 2003, Cisco Systems, Inc. All rights reserved. 25

Establishing the Subnet Mask Address • Determines which part of an IP address is the network field and which part is the host field. • Follow these steps to determine the subnet mask: 1. Express the subnetwork IP address in binary form. 2. Replace the network and subnet portion of the address with all 1 s. 3. Replace the host portion of the address with all 0 s. 4. Convert the binary expression back to dotteddecimal notation. © 2003, Cisco Systems, Inc. All rights reserved. 26

Establishing the Subnet Mask Address • To determine the number of bits to be used, the network designer needs to calculate how many hosts the largest subnetwork requires and the number of subnetworks needed. • The “slash format” is a shorter way of representing the subnet mask: /25 represents the 25 one bits in the subnet mask 255. 128 © 2003, Cisco Systems, Inc. All rights reserved. 27

Establishing the Subnet Mask Address © 2003, Cisco Systems, Inc. All rights reserved. 28

Subnetting Class A and B Networks • The available bits for assignment to the subnet field in a Class A address is 22 bits while a Class B address has 14 bits. © 2003, Cisco Systems, Inc. All rights reserved. 29

Calculating the Subnetwork With ANDing • ANDing is a binary process by which the router calculates the subnetwork ID for an incoming packet. 1 AND 1 = 1; 1 AND 0 = 0; 0 AND 0 = 0 • The router then uses that information to forward the packet across the correct interface. Packet Address 192. 168. 10. 65 11000000. 10101000. 00001010. 010 00001 Subnet Mask 255. 224 11111111. 111 00000 Subnetwork Address 192. 168. 10. 64 11000000. 10101000. 00001010. 010 00000 © 2003, Cisco Systems, Inc. All rights reserved. 30