CS3502:CS3502:

Data and Computer NetworksData and Computer Networks

Local Area Networks - 4Local Area Networks - 4

Bridges / LAN internetworksBridges / LAN internetworks

Bridges : connecting LANs Bridges : connecting LANs together 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 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? 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 types of bridges and LAN connectors connectors

local bridge

remote (2 half bridges)

same LAN, different LAN

two port, multiport

hub (not a bridge)

repeater (not a bridge)

router(not a bridge)

bridge : basic functionbridge : 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 ethernetsbridge connecting 2 ethernets

notes on bridgesnotes 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 LAN internet

half - bridge, connecting 2 LANshalf - bridge, connecting 2 LANs

2 halves communicate through some other protocol, eg, PPP, HDLC.

bridges - routingbridges - 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 - routingbridges - 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 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 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, bridges, routers, repeaters, hubshubs

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 ....