1 chapter 8 local area networks - internetworking

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Chapter 8

Local Area Networks - Internetworking

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IntroductionMany times it is necessary to connect a local area network to another local area network or to a wide area network.

Local area network to local area network connections are often performed with a bridge-like device.

Local area network to wide area network connections are usually performed with a router.

A third device, the switch, can be used to interconnect segments of a local area network.

Chapter Eight - Local Area Networks:

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Why Interconnect?To separate / connect one corporate division with another.

To connect two LANs with different protocols.

To connect a LAN to the Internet.

To break a LAN into segments to relieve traffic congestion.

To provide a security wall between two different types of users.

Chapter Eight - Local Area Networks:

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Repeaters

Repeater

Repeaters

• Repeaters repeat a signal from one port to another. A repeater forwards every frame; • Repeaters pass all traffic through without error checking or filtering.. • Repeaters pass collisions, too.

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Repeaters extendthe maximumallowed distancebetween nodes.

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HubsAs seen earlier, a hub interconnects two or more workstations into a local area network.

When a workstation transmits to a hub, the hub immediately resends the data frame out all connecting links.

A hub can be managed or unmanaged. A managed hub possesses enough processing power that it can be managed from a remote location.

Chapter Eight - Local Area Networks: Internetworking

Hubs

– very simple devices that pass all traffic in both directions between the LAN sections they link.

– same or different cable types– pass on every message– used to connect LANs of similar technology, or to

extend the distance of one LAN

• Very similar to repeaters– Also operates at the physical layer– use physical layer protocols

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Hubs

Hub

Hubs…

• Hubs propagate a signal received on one port to all other ports.• Hubs propagate errors and collisions across ports, too. • Hubs simplify the addition and removal of nodes on a LAN.• Hubs are also used to connect network segments cabled with different media types.

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Hubs make itvery easy to addand remove hostson a network.

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Can also create multiple level of hierarchy

Hubs

Advantages :-– inexpensive– easy to Install– can connect different media– very little delay

Disadvantages :-– limited distance between devices– limited on the number of repeaters– no protocol– no error detection– does not filter

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BridgesA bridge (or bridge-like device) can be used to connect two similar LANs, such as two CSMA/CD LANs.

A bridge can also be used to connect two closely similar LANs, such as a CSMA/CD LAN and a token ring LAN.

The bridge examines the destination address in a frame and either forwards this frame onto the next LAN or does not.

The bridge examines the source address in a frame and places this address in a routing table, to be used for future routing decisions.


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Bridges• Connects two or more LANs at the link layer

– Extracts destination address from the frame.

– Looks up the destination in a table.

– Forwards the frame to the appropriate LAN segment.

– same or different cable types.

– forward only those messages that need to go out (filtering).

– “learn” whether to forward packets.

• Each segment can carry its own traffic

host host host host host

host host host host host

host

host

Bridge

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BridgesBridge

Hub

Bridges

• Bridges provide all the functionality of a hub....• Bridges filter frames by destination MAC, and segment a LAN into multiple collision domains.• Bridges filter signal and timing errors.• Bridges can be used to connect segments operating at different speeds.

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HubBridges make it possible to segmentyour network into separate collisiondomains to minimize collisions and improve performance.

Separate Collision Domains

A bridge has a table used in filtering decisions.

• It can check the destination address of a frame and decide if the frame should be forwarded or dropped.

• If the frame is to be forwarded, the decision must specify to port.

• A bridge has a table that maps address to ports.

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A bridge connecting two LANs

the bridge can check the physical/MAC addresses contained in the frame.

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Transparent BridgesA transparent bridge does not need programming but observes all traffic and builds routing tables from this observation.

This observation is called backward learning.

Each bridge has two connections (ports) and there is a routing table associated with each port.

A bridge observes each frame that arrives at a port, extracts the source address from the frame, and places that address in the port’s routing table.

A transparent bridge is found with CSMA/CD LANs.


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Transparent BridgesA transparent bridge can also convert one frame format to another, but this does not happen too often anymore since most networks are CSMA/CD.

Note that some people / manufacturers call a bridge such as this a gateway or sometimes a router.

The bridge removes the headers and trailers from one frame format and inserts (encapsulates) the headers and trailers for the second frame format.


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A learning bridge and the process of learning

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Remote BridgesA remote bridge is capable of passing a data frame from one local area network to another when the two LANs are separated by a long distance and there is a wide area network connecting the two LANs.

A remote bridge takes the frame before it leaves the first LAN and encapsulates the WAN headers and trailers.

When the packet arrives at the destination remote bridge, that bridge removes the WAN headers and trailers leaving the original frame.


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Spanning Tree AlgorithmWhat happens if you have many LANs interconnected with multiple bridges, such as shown in the next slide?

Data that leaves one workstation could travel to a bridge, across the next network, into the next bridge, and back onto the first network.

A packet may continue to cycle like this forever!


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Spanning Tree AlgorithmHow do we stop this from happening?

Disconnect one of the bridges? Maybe we want bridge redundancy in case one bridge fails.

How about applying the spanning tree algorithm.

How is the algorithm applied?


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Spanning Tree AlgorithmStep 1: Designate a Root Bridge

Step 2: Mark one port of each bridge as the Root Port. The root port is the port with the least-cost path from that bridge to the root bridge. The root ports are denoted with an asterisk in Figure 8-7b.

Step 3: The next step is to select a designated bridge for each LAN. A designated bridge has the least-cost path between that LAN and the root bridge. Mark the corresponding port that connects that LAN to its designated bridge with two asterisks (Figure 8-7b)


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Spanning Tree AlgorithmStep 4: If a port has no asterisks, that port is redundant and can be “removed”. Keep all ports with one or two asterisks. The resulting configuration is shown in Figure 8-7c.

Note there is now only one way to get to any LAN or bridge from any other LAN or bridge.


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SwitchesA switch is a combination of a hub and a bridge.

It can interconnect two or more workstations, but like a bridge, it observes traffic flow and learns.

When a frame arrives at a switch, the switch examines the destination address and forwards the frame out the one necessary connection.

Workstations that connect to a hub are on a shared segment.

Workstations that connect to a switch are on a switched segment.


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SwitchesThe backplane of a switch is fast enough to support multiple data transfers at one time.

A switch that employs cut-through architecture is passing on the frame before the entire frame has arrived at the switch.

Multiple workstations connected to a switch use dedicated segments. This is a very efficient way to isolate heavy users from the network.

A switch can allow simultaneous access to multiple servers, or multiple simultaneous connections to a single server.


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Switches

Switches

• Switches provide all the functionality of a bridge ....• Switches typically offer more ports than bridges. • Switches allow for multiple, parallel channels of communication between ports.

Switches are similar to bridges, but offer multiple parallel communication channels across ports for improved performance.

Switch

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Virtual LANsA virtual LAN, or VLAN, is a logical subgroup within a local area network that is created via switches and software rather than by manually moving wiring from one network device to another

Even though the employees and their actual computer workstations may be scattered throughout the building, LAN switches and VLAN software can be used to create a “network within a network.”


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Virtual LANsA relatively new standard, IEEE 802.1Q, was designed to allow multiple devices to intercommunicate and work together to create a virtual LAN

Instead of sending a technician to a wiring closet to move a workstation cable from one switch to another, an 802.1Q-compliant switch can be remotely configured by a network administrator


Switches

• Cut-through switches– use circuit-switching to immediately connect the port

with the incoming message to the correct outgoing port

– very fast as decisions are done in hardware– outgoing packet is lost if port is in use

• Store-and-forward switches– copy the incoming packet to memory prior to

processing the destination address -- transmit it when the outgoing port is ready

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Isolating Traffic Patterns with SwitchesWhether shared or dedicated segments are involved, the primary goal of a switch is to isolate a particular pattern of traffic from other patterns of traffic or from the remainder of the network

Switches, because of their backplane, can also allow multiple paths of communications to simultaneously occur


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Isolating Traffic Patterns with SwitchesUsing a pair of routers, it is possible to interconnect to switched segments, essentially creating one large local area network


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Full Duplex SwitchesA full duplex switch allows for simultaneous transmission and reception of data to and from a workstation.

This full duplex connection helps to eliminate collisions.

To support a full duplex connection to a switch, at least two pairs of wires are necessary - one for the receive operation and one for the transmit operation. Most people install four pairs today, so wiring is not the problem.


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Network ServersNetwork servers provide the storage necessary for LAN software.

They are usually the focal point for the network operating system.

Increasingly, network servers are functioning as bridges, switches, and routers. By adding the appropriate card, a server can assume multiple functions.


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RoutersThe device that connects a LAN to a WAN or a WAN to a WAN.

A router accepts an outgoing packet, removes any LAN headers and trailers, and encapsulates the necessary WAN headers and trailers.

Because a router has to make wide area network routing decisions, the router has to dig down into the network layer of the packet to retrieve the network destination address.


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RoutersThus, routers are often called “layer 3 devices”. They operate at the third layer, or OSI network layer, of the packet.

Routers often incorporate firewall functions.

An example of a router’s operation is shown on the next slide.


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Routers

– connect two or more LANs that use the same or different data link protocols, but the same network protocol.

– same or different cable types– operate at the network layer– forward only messages that need to go out– routers use the internetwork address– internal routing tables– only processes messages addressed to it

Routers

Advantages– can mix-in-match protocols and convert them– enable all ports to work at the same time– can be used as an extra layer of security– configurable– high speedDisadvantages– hard to configure and manage– access lists must be kept current– high maintenance/high training costs– very expensive

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LAN Internetworking In Action: A Small Office RevisitedRecall the In Action example from Chapter Seven.

A small office with 20 workstations in one room and 15 workstations in another room were connected to a server via 100BaseTX.

One hub was kept in a closet near the 20 workstations while a second hub was near the server.


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LAN Internetworking In Action: A Small Office RevisitedNow Hannah wants to connect the LAN to the Internet.

She adds a router next to the server and connects it to the hub.

She connects the router to a high-speed telephone line such as a T-1 service.

She will also have to program the router to perform IP addressing and firewall functions.


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LAN Internetworking In Action: A Small Office RevisitedNow network usage is so high that Hannah must consider segmenting the network.

She decides to install a database server near the original server and replace both hubs with switches.


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1 chapter 8 local area networks - internetworking

Documents

local area network connections

hubs hub hubs hubs

wide area network connections

network segments

frame repeaters

telnet hubs

hubs advantages

telnet repeaters