Transcript Routers
Day 12 Chapter 13 WAN Technologies and Routing Classification of Networks • LAN – spans a single building or campus • MAN – Spans a city • WAN – Spans multiple cities, countries, or continents WANs • The WAN network must allow a lot of simultaneous communication between computers • Key difference between LANs and WANs is scalability – WANS can grow as needed • The long-distance leased lines (Ch. 12) are connected to packet switches (i.e. routers) • Individual computers are then connected to the routers • Each router is a computer that can receive and send packets • See Figure 13.1, p. 202, to see logical diagram of a router Forming a WAN • See Figure 13.2, p. 203 to see how 4 routers are connected • Connections between routers represent high-speed, long-distance connections • Notice there are multiple paths that a packet may take in the network Operation of WAN • Wide area packet switching uses “store and forward” approach • The router reads the packet and stores it in memory • The packet is examined and the router determines over which interface the packet should be sent out • Packets are held in memory until the output device is available Physical addressing in WAN • WANs use a hierarchical addressing scheme • Simple scheme uses an address with two parts (IP): – First part identifies packet switch – Second part identifies computer attached to that packet switch • See Figure 13.3 for example of addresses Next-Hop Forwarding • Router selects an outgoing path over which to forward a packet • The path is selected based upon the destination address stored in the packet • The router stores information to determine the next router (next hop) the packet should be sent to • See Figure 13.4, p. 202 for example of routing table • Note- the next hop does not depend upon packet’s source or on the path it has taken so far Routing Table • The router needs to only examine the first part of the hierarchical address • The entire routing table can be shortened to contain one entry per destination packet switch • See Figure 13.5, p. 204 Routing in a WAN • Fundamental concept – switching capacity can be increased without adding individual computers • Router can be added to interior of a network to handle load • These routers do not have computers attached • These are called interior routers • Routers to which computers are attached directly are called exterior packet switches Homework • Read Chapter 13 of your text: WAN Technologies and Routing. • Study Dijkstra’s Algorithm that finds the shortest path in a WAN