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.Switches also can support higher port densities than bridges.Some switches support cut-through switching, which reduces latency and delays in the network, whereas bridges supportonly store-and-forward traffic switching.Switches also support virtual LANs (VLANs).A VLAN is a logical, rather than physical,grouping of devices.The devices are grouped using switch management software so that theycan communicate as if they were attached to the same wire, when in fact they might be locatedon a number of different physical LAN segments.Cisco switches are known as Catalyst switches (because Cisco bought a company calledCatalyst).Catalyst switches include the following series:Catalyst 1900 Catalyst 4000Catalyst 2820 Catalyst 5000Catalyst 2900 Catalyst 5500Catalyst 2900XL Catalyst 6000Catalyst 3000 Catalyst 3900Catalyst 8500 multiservice switch routersGenerally the bigger the series number, the more LAN ports the switch has. CH01.book Page 520 Friday, January 7, 2000 5:35 PM520 Appendix C: Internetworking Technology ReviewRoutersA router connects devices on LANs to devices on other LANs, usually via WANs, as shown inFigure C-4.Figure C-4 A Router Connects Devices on LANs to Devices on Other LANs, Usually Via WANsToronto OttawaLAN LANA router has a lot of smarts. When companies started deploying PCs and connecting them viaLANs, they soon wanted to go one step further and interconnect LANs and PCs located atgeographically separate locations.The router provides this facility.The router will connect to alocal LAN and then connect over a longer distance to another router, which in turn is connectedto the remote LAN.Two PCs located hundreds of miles apart can now exchange data.A router s job is comprised of the following tasks:" Segment LANs and WANs" Figure out the best way to send data to its destination" Talk to other routers to learn from them and tell them what it knows" Send the data the best way, over a LAN or a WANWhen devices are connected via a router, a device will hear only the following:" Everything that the other devices on its port send" Any information from devices on other ports that was meant for devices on its portA device connected to a router will not hear any of the information meant just for devices onother ports, nor any information from devices on other ports that was meant for everyone.Cisco has a large selection of routers, including the following series:Cisco 700 Cisco 3600Cisco 800 Cisco MC3810 multiservice concentratorCisco 1000 Cisco 4000Cisco 1600 Cisco AS5200/AS5300/AS5800 access serversCisco 1720 Cisco 7200Cisco 2500 Cisco 12000Cisco 2600Cisco 7500 CH01.book Page 521 Friday, January 7, 2000 5:35 PMRouting 521Generally the bigger the series number, the more LAN and WAN ports the router has and thebetter performance it provides.RoutingRouting is the act of moving information across an internetwork from a source to a destination.Along the way, at least one intermediate node typically is encountered.Routing occurs at Layer3, the network layer.The book Introduction to Cisco Router Configuration (Cisco Press) defines routed protocolsand routing protocols:" A routed protocol is a protocol that contains enough network-layer addressing information foruser traffic to be directed from one network to another network.Routed protocols define theformat and use of the fields within a packet.Packets that use a routed protocol are conveyedfrom end system to end system through an internetwork." A routing protocol supports a routed protocol by providing mechanisms for sharing routinginformation.Routing protocol messages move between the routers.A routing protocol allowsthe routers to communicate with other routers to update and maintain routing tables.Routingprotocol messages do not carry end-user traffic from network to network.A routing protocoluses the routed protocol to pass information between routers.(pg.79)A metric is a standard of measurement, such as path length, that is used by routing algorithmsto determine the optimal path to a destination.To aid the process of path determination, routingalgorithms initialize and maintain routing tables, which contain route information.Routeinformation varies depending on the routing algorithm used.Routing algorithms can be classified by type.Key differentiators include:" Static versus dynamic Static routing algorithms are hardly algorithms at all, butare table mappings established by the network administrator prior to the beginning ofrouting.These mappings do not change unless the network administrator alters them." Single-path versus multipath Some sophisticated routing protocols supportmultiple paths to the same destination." Flat versus hierarchical In a flat routing system, the routers are peers of all others.In a hierarchical routing system, some routers form what amounts to a routing backbone.Routing systems often designate logical groups of nodes, called domains, autonomoussystems, or areas." Host-intelligent versus router-intelligent Some routing algorithms assumethat the source end-node will determine the entire route.This is usually referred to assource routing.Other algorithms assume that hosts know nothing about routes.In thesealgorithms, routers determine the path through the internetwork based on their owncalculations. CH01.book Page 522 Friday, January 7, 2000 5:35 PM522 Appendix C: Internetworking Technology Review" Intradomain versus interdomain Some routing algorithms work only withindomains; others work within and between domains." Link-state versus distance-vector versus hybrid Link-state algorithms (alsoknown as shortest path first algorithms) flood routing information to all nodes in theinternetwork.Each router, however, sends only the portion of the routing table thatdescribes the state of its own links
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