ch. 15 connecting lans
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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
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