Ch. 15 Connecting LANs

15.1 Connecting Device

• Five different categories of connecting devices– Passive hub, Repeater, Bridge, Router, Gateway

Passive Hubs and Repeaters

• Passive hubs– Connects wires– Can be considered as a part of transmission medium

• Repeaters– Operates only in the physical layer– Connects “segments of a LAN”

-- segments are considered as a single LAN– Forwards every frame by regenerating signals (not by

amplifying)

Function of Repeater

• Active hub = multiport repeater (often used to create a physical star topology)

Single LAN

Bridges• Operate both in the physical and data link layer• Can check the physical (MAC) addresses– Decide if the received frame should be forwarded to

the other side or dropped Filtering

Transparent Bridges

• Stations are completely unaware of the bridge’s existence– Frame must be forwarded Forwarding – Forwarding table is automatically updated

Learning– Only one path exists between two stations No looping

Learning

Loop Problem

Spanning Tree

• Loop-free graph that connects all stations– A LAN can be reached through one path only– Often, find a spanning tree that minimizes some cost (e.g., #

hop distance, delay, bandwidth, etc)• Steps to find a spanning tree:– Elect a root station (e.g., station with smallest ID)– Find the shortest (= minimum cost) path from each station to

the root– Based on the found paths, set forwarding ports and blocking

ports• Can be found automatically by dynamic algorithm

Finding a Spanning Tree

Forwarding and Blocking Ports

Bridge

• An alternative way to prevent loop: Source routing– A sending station defines the bridges that the frame must visit– The addresses of all the bridges are included in the frame

• Issues in bridging different LANs– Frame format– Frame size– Data rate– Bit order– Security– Multimedia support

Switches

• Two-layer switch (or Layer-two switch)– E.g., bridges

• Three-layer switch (or Layer-three switch)– Often called “router”– Uses network layer info:

logical address (IP)• Gateway

– Some use gateway and router interchangeably

– Gateway usually operates in “ALL layers” including application layer

15.2 Backbone Networks

• A backbone network connects several LANs– Bus, or star backbone

– Connecting remote LANs:use point-to-point links

15.3 Virtual LANs

• Virtual local area network (VLAN)– A local area network configured by software (not by

physical wiring)– VLANs group stations that connected to different switches,

and create broadcast domain (or membership for grouping)– Stations can be re-grouped by software configuration

• Advantages– Cost and time reduction– Creating virtual work groups– Security

Example

Homework

• Exercise– 12, 16, 20