Transcript Lans-4-wb
CS3502: Data and Computer Networks Local Area Networks - 4 Bridges / LAN internetworks Bridges : connecting LANs together why do we need to connect LANs what is a bridge? types of bridges routing in LAN internetworks comparison: bridges, routers, repeaters connecting similar LANs connecting dissimilar LANs why LANs need to be connected 1. connect 2 existing LANs (CS, math) -- different organizations want to be connected 2. LAN too big; need to split it, but stay connected -- too many stations or traffic for one LAN 3. connect geographically separate LANs. -- eg, 2 offices in different towns need connecting 4. reduce collisions -- help increase efficiency 5. security --help restrict traffic to one LAN bridge : what is it? low level “switch” that connects two or more LANs. “low level” => “MAC layer” transparent : there is no change in the LANs or in the protocols of the networks must be able to do simple routing retains the simplicity and flexibility of the LANs it connects faster than “software” switches (routers) reasonable cost; generally cheaper than routers types of bridges and LAN connectors local bridge remote same (2 half bridges) LAN, different LAN two port, multiport hub (not a bridge) repeater (not a bridge) router(not a bridge) bridge : basic function suppose a bridge B connects networks X and Y. Then B : 1. reads all packets on X and Y, noting the destination, source addresses (DA, SA) 2. each packet on Y with DA on Y is copied and transmitted on X. 3. each packet on X with DA on Y is copied, transmitted on Y. the bridge operates on X, Y using the MAC protocol of those LANs. bridge connecting 2 ethernets notes on bridges all stations have unique MAC addresses bridge must “know” which LAN station is on multi-port bridges - similar; extends to multiple LANs no change or adjustment in NIC needed; bridge completely transparent bridge operates on each LAN using the MAC protocol remote (half bridge) - may use another protocol between the 2 half bridges, while using MAC on each LAN LAN internet half - bridge, connecting 2 LANs 2 halves communicate through some other protocol, eg, PPP, HDLC. bridges - routing how do bridges “know” which packets to forward, and in which direction? 2 basic techniques : 1. fixed routing - the information is loaded manually into the bridge (typing it in, etc.). This info is then stored in a routing table. 2. dynamic routing: “learning bridges” the bridge “learns” where the stations are by watching the traffic on its ports bridges - routing for fixed routing, many topologies possible dynamic routing - the internet must be configured as a tree; this simplifies routing tree : LANs and bridges are the nodes, and the links between them are the edges, and - LANs can be connected only to bridges, not (directly) to other LANs if a cycle exists, the bridges will detect it and remove one from the active network, so that a tree structure is maintained bridges - dynamic routing bridge has a routing table, 3 fields : [ dest.address | next port | time] when bridge receives a packet [DA,SA] on port X: 1. if SA found in table, reset timer, else add [SA, time, X ] to table 2. if [DA] found in table send packet out on next port indicated; else send packet on all ports except X. bridge dynamic routing timer: typical value : 300 seconds (why have the timer? is this a good default value?) given the tree structure, bridges will learn a station’s direction (explain how?) MAC addresses could be divided into (network, station) parts. If so, tables can be made smaller, but same algorithm applies bridges, routers, repeaters, hubs repeaters : simply connect 1 cable to another, repeat the bits. No routing decisions or filtering. hubs : serve to extend the ethernet. No routing or filtering of messages. bridges - connect LANs together at the MAC layer; filter and rout messages at the MAC layer. routers - connect LANs to LANs and/or the Internet. Layer 3/3.5 (internet). Software, IP protocol. Considerably more “intelligent” than above, but also in software so run slower. Usually more expensive. Discussed in CS4550 ....