1. CS3502: Data and Computer Networks Local Area Networks - 4 Bridges / LAN internetworks

2. Bridges : connecting LANs together u why do we need to connect LANs u what is a bridge? u types of bridges u routing in LAN internetworks u comparison: bridges, routers, repeaters u connecting similar LANs u connecting dissimilar LANs

3. 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

4. bridge : what is it? u low level “switch” that connects two or more LANs. “low level” => “MAC layer” u transparent : there is no change in the LANs or in the protocols of the networks u must be able to do simple routing u retains the simplicity and flexibility of the LANs it connects u faster than “software” switches (routers) u reasonable cost; generally cheaper than routers

5. types of bridges and LAN connectors u local bridge u remote (2 half bridges) u same LAN, different LAN u two port, multiport u hub (not a bridge) u repeater (not a bridge) u router(not a bridge)

6. 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. u the bridge operates on X, Y using the MAC protocol of those LANs.

7. bridge connecting 2 ethernets

8. notes on bridges u all stations have unique MAC addresses u bridge must “know” which LAN station is on u multi-port bridges - similar; extends to multiple LANs u no change or adjustment in NIC needed; bridge completely transparent u bridge operates on each LAN using the MAC protocol u remote (half bridge) - may use another protocol between the 2 half bridges, while using MAC on each LAN

9. LAN internet

10. half - bridge, connecting 2 LANs u 2 halves communicate through some other protocol, eg, PPP, HDLC.

11. bridges - routing u 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

12. bridges - routing u for fixed routing, many topologies possible u dynamic routing - the internet must be configured as a tree; this simplifies routing u tree : LANs and bridges are the nodes, and the links between them are the edges, and -- u LANs can be connected only to bridges, not (directly) to other LANs u if a cycle exists, the bridges will detect it and remove one from the active network, so that a tree structure is maintained

13. bridges - dynamic routing u 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.

14. bridge dynamic routing u timer: typical value : 300 seconds (why have the timer? is this a good default value?) u given the tree structure, bridges will learn a station’s direction (explain how?) u MAC addresses could be divided into (network, station) parts. If so, tables can be made smaller, but same algorithm applies

15. bridges, routers, repeaters, hubs u repeaters : simply connect 1 cable to another, repeat the bits. No routing decisions or filtering. u hubs : serve to extend the ethernet. No routing or filtering of messages. u bridges - connect LANs together at the MAC layer; filter and rout messages at the MAC layer. u 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....