Yu Cheng (Jade) ICS 351 Prelab Report 6 December 04, 2008 (late) [Question 1] Describe the difference between a LAN switch/bridge and a router The LAN switch is a box like device that connect multiple ethernet segments. The router is a more sophisticated network device. It is designed to join muliple area networks. It is a combination of ethernet switch and Network Address Translator (NAT). They usually include a Dynamic Host Configuration Prototol (DHCP) server, Domain Name Service (DNS), and a hardware firewall to protect the LAN from mallicious intrusion from the internet. It runs routing protocols. It routes TCP/IP packets between multiple PCs on LAN and a WAN, and much more. References: http://www.duxcw.com/faq/ics/diffrout.htm http://compnetworking.about.com/od/homenetworkhardware/f /routervsswitch.htm [Question 2] What is the difference between an Ethernet switch and an Ethernet hub? Which is more suitable for a network with a high traffic load, a switch or a hub? Why? A hub is less expensive, less intelligent, and less complicated of the two. Its job is simple: anything that comes in one port is sent out to the others. PCs connected with a hub can "see" everything transmitted through the hub. A switch does the what a hub does. It "learns" where particular addresses are. It remembers the port that is used for transmission to a certain PC. Later, it knows that this PC is connected to this port and the traffic to this PC nneds to only ben sent to that port and not to any of the others. The broadcasting happens at the beginning. Most of the traffic only goes where it needs to go rather than to every port. So a switch is faster. When there's high traffic load, we want to use a switch. References: http://ask-leo.com/whats_the_difference_between_a_hub_a_switch_and _a_router.html http://compnetworking.about.com/od/homenetworkhardware/f /routervsswitch.htm [Question 3] What motivates the use of the term transparent in transparent bridges? Transparent bridge is a paticular case of network bridge. The network bridge simply enables local networks to communicate with each other, but forwards the tranfic to all ports. The transparent bridge is capable of redirecting the packets to the proper port, hence it can isolate the networks from broadcast traffic. The transparent bridge keeps a forwarding table that associates addresses to ports. The table is built by learning the network topology from the analysis of the incoming traffic. Transparent bridge is named this way because its presence and operation are transparent to network hosts. Reference: http://en.wikipedia.org/wiki/Transparent_bridge http://www.pulsewan.com/data101/transparent_bridging_basics.htm [Question 4] Which role does the spanning tree protocol play when interconnecting LAN switches/bridges? STP is a link management protocol that provides path redundancy while preventing undesireable loops in the network. When loops occur, some switches see sending and receiving stations appear on both sides of the switch. This confusion make the forwarding algorithm to send duplicate frames. STP prevent this confusion from happening. It defines a tree and forces certain redundant data paths into a standby state. If one network segment in the STP becomes unreachable, or if STP costs changes, the STP algorithm reconfigures the spanning tree topoloty and re-establishes the link by activating standby paths. Reference: http://www.cisco.com/univercd/cc/td/doc/product/rtrmgmt/sw_ntman /cwsimain/cwsi2/cwsiug2/vlan2/stpapp.htm [Question 5] In the context of the IEEE 802.Id specificatoin of the spanning tree protocol, define the following terms: (a) Root bridge: In the spanning tree, each bridge has a unique ID and a configurable priority number. The root bridge of the spanning tree is the bridge with the smallest bridge ID. It continuously transmits network topology information to other bridges using STP, in order to notify all other bridges on the network when topoloty changes are required. The root bridges is where the paths that frames take through the network they are assigned. It should be located centrally on the network to provide the shortest path to other links on the network. Unlike other bridges, the root bridge always forwards frames out over all of its ports. (b) Root port: After the root bridge has been chosen, each bridge determines the cost of each possible path from itself to the root. The bridge picks the one with the smallest cost. The port connecting to that path becomes the root port of this bridge. (c) Designated bridge After the root bridge has been chosen, the bridges on a network segment collectively determine which bridge has the least-cost path from the network segment to the root. This bridge is called the designated bridge. So, for a bridge on a network or a LAN, its designated bridge is the bridge that provides the minimum root path cost. The designated bridge is the only bridge allowed to forward frames to and from the this bridge or the LAN. (d) Designated port The port that connects the designated bridge to the network segment is the designated port for the segment. (e) Blocked port Any active port that is not a root port or a designated port is a blocked port. A blocked port is the port that will not forward frames, in order to prevent loops. However, a blocked port will alwyas listen to frames. All ports are in blocking state by default when the switch is powered up. The decisions of which port is to be blocked and which port is to be put in forwarding mode are made from the perspective of the root bridge. References: http://en.wikipedia.org/wiki/Spanning_tree_protocol http://encyclopedia2.thefreedictionary.com/root+bridge http://harrychanputra.wordpress.com/2007/05/06/spanning-tree- protocol-stp/ [Question 6] In the spanning tree protocol, how does a LAN switch/bridge decide which ports are in a blocking state This is the same procedure of selecting the designated port and root port because when those two are decided, the rest of the them are set to blocking state. The bridge ID is used to elect the root bridge in the STP domain as well as to determine the root port. The priority of each bridge is combined with its MAC address. If two switches or bridges happen to have the same priority value, the MAC address becomes the tie breaker for figuring out which one has the lowest ID. If only one link is connected to the root bridge, then it needs to be a designated port. If otherwise, port cost becomes the factor used to determine the root port/designated port. The path that has a smaller total cost wins. Other than the root port and the designated port, the rest of them are set to blocking state.