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Chapter 10 Routing fundamentals & subnets Contents: •Routable and routed protocols •IP Routing Protocols •The Mechanics of Subnetting Routable and routed protocols •What is a protocol ? •Is a set of rules that determines how computers communicate with each other. •A protocol describes: •Message format. •The way in which computers must exchange a message. •Routed protocol allows the router to forward data between networks. •Routed (routable) protocol must provide network address and host address (IP/ IPX. •IP routed protocol requires a subnet mask. •Subnet mask is used to obtain network address from the host IP address. •Subnet mask allows groups of sequential addresses to be treated as a single unit.. •IP as a routed protocol: •Connectionless, unreliable, best-efforts delivery protocol. - Connectionless: •No dedicated circuit is established prior to transmission. - Best-effort delivery: •IP doesn’t verify that the data reached its destination. Packet propagation and switching within a router •As a packet moves through routers, frame headers and trailers are replaced at every router. •This because frames are for local addressing. •While IP is for end to end addressing. Internet protocol IP: •Two type of delivery services: - Connectionless service. - Connection – oriented service. •Connectionless service: (postal system) •No actual (physical ) circuit is established in advance. •Destination is not contacted before a packet is sent. •No dedicated path between source and destination. •Packets may follow different paths to destination. •Packets may arrive out of order. •Connectionless network processes are Packet switched •Connection oriented service: •A connection is established between the sender and the recipient before any data is transferred. (Telephone system) •Second phase: data transmission. •Third phase: circuit termination.. •Packets follow the same path to destination. •Packets arrive in order •Connection oriented network processes are called circuit switched. IP routing protocol •Routing is a layer 3 function. •Routers pass packets between networks based on network addresses. •The destination address (host address) is needed only to deliver packet to its final destination. •Routing is the process of finding the most efficient path to destination. •Two key functions of a router: •Routers build and maintain routing tables (routing protocols) and, exchange information with other routers within the network topology. •Packet switching. •Router (routing protocol) use one or more metrics to choose the best path to destination network. •Non routable protocols: Do not support layer 3 addressing. Examples : NetBUI •Routing versus switching: •Switching occurs at: Layer 2 •Routing occurs at: Layer 3 •Switch recognizes MAC addresses. •Switch keeps a table of MAC addresses •Router keeps a table of IP addresses •Switched networks don't block broadcasts. •Broadcast storms only affect the broadcast domain from which it originated. •Routers don’t pass broadcasts. •Routed versus Routing: •Routed protocol: Transfer data from one host to another. •Routing protocols: Choose the best path from source to destination. •Routed protocol functions: •provides enough information to allow routers to forward it to final destination. •Defines the format and use of the fields within a packet (IP, IPX, DECnet, ApplTalk..) •Routing protocol functions: •Build routing tables. •Exchanges routing tables with other routers. •Enable routers to route routed protocols. (RIP, IGRP, OSPF, BGR, EIGRP) •Path determination: •Path determination is to choose the best route among multiple routes available in the routing table. •Two types of routes: Static routes. Dynamic routes. •Router uses path determination to decide to which port the packet must be sent toward its destination. Routing the packet. •Each router along the path is called Hop. •Path determination: •Destination address is obtained from the packet. •The mask is applied to the address. •Network address is compared to routing table. •If matched, the packet is forwarded to the port associated with the routing table entry. •If there is no match, Default route is checked. •If there is no default route: Packet is discarded and a message of destination unreachable is sent back. •Routing tables include: •protocol type. •Destination/next-hop associations •Routing metric •Outbound interfaces •IGP and EGP •Autonomous system (AS): Is a network or set of networks under common administration control.. •Two families of routing protocols: IGP. EGP. •IGP route data within AS: - RIP and RIPv2 - IGRP - EIGRP - OSPF •EGP route data between ASs: - BGP •distance vector routing protocols: •Sends their routing tables periodically to their neighbors. •Router relies on its neighbors to get routing information. •Examples of distance-vector protocols: - RIP (hop count). -IGRP Cisco protocol to overcome RIP limitations in large networks. - EIGRP: (Cisco proprietary, hybrid protocol. •Link - state routing protocols: - Routers exchange LSAs. - Each router builds its own topology data base. - Each router runs SPF algorithm and builds its own routing tables. - Send link-state refreshes at longer time intervals. - Routers send trigger updates only when a network changes has occured. (OSPF and IS-IS) Mechanics of subnetting •Classes of IP addresses: -bits are borrowed starting from the leftmost bit of the host. - Borrowed bits are reassigned as network bits. - Subnet address include class A, B , or C plus the subnet field. •Establishing the subnet mask addressing: - Determine the maximum number of subnets. - Determine the maximum number of hosts per subnet. - Never assign the last two bits regardless of the IP class. - Use of all bits except the last two bits will result in subnets with only two usable hosts. (for serial links). - The subnet mask gives the router the information required to determine in which network and subnet a particular host resides. - The subnet mask is created by using binary ones in the positions of borrowed bits. - If 3 bits were borrowed, the subnet mask for calss C would be 255.255.255.224 - This mask can be represented as /27. Usable subnets: 2n - 2 Usable hosts: m 2 -2 •Example: