Transcript WAN Technologies & Topologies

WAN Technologies & Topologies
Lecture 8
October 4, 2000
WAN Background
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WAN - Wide Area Network, spans multiple
cities/states.
MAN - Metropolitan Area Networks exist in a
single city.
LAN – Single building environment.
Multiple MANs can make a WAN, multiple
LANs can make a MAN, etc.
WAN Example
MAN As A Building Block
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As I mentioned earlier, multiple MANs usually
make up the WAN.
Each MAN has control of its own domain, but
it’s uplink to the other MANs is considered the
WAN (backbone).
Using Verio Boston’s example…
The Interconnects
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MAN & WAN circuits must terminate in some
form of a packet switch.
Using the UNet example from the other day,
the packet switches were the Cisco Catalyst
6509 switches.
But keep in mind, the telephone company also
has some switches in the field that handle even
more traffic than what you’re getting!
Switch Interconnects – The “Cloud”
WAN Switch Functionality
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WAN switches use store and forward
technology.
The store operation occurs when the packet
arrives: the I/O hardware copies the packet,
sticks it in memory, and signals the processor
to forward the packet.
The forward operation is the act of removing
the packet from memory, and sends it to the
appropriate interface.
WAN Switch Functionality (cont.)
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Storing the packets also leads to a form of
queuing for each interface.
If the destination interface is busy, the packet is
queued until the destination interface is idle,
then the forward occurs.
The store and forward paradigm allows to
handle the maximum bandwidth of the WAN
connection, since all data is buffered!
Physical Addressing in the WAN
Environment
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A hierarchical scheme is used with WAN
addressing.
The simplest form of this scheme: The first part
of the address holds the destination switch, the
second part holds the specific machine that the
packet is destined for on that switch.
This is scheme is used in many WAN
environments.
WAN Addressing (cont.)
Next-Hop Forwarding
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In order for networking to occur, each device
must have some knowledge of the devices
which it is connected to.
Next-hop forwarding is a scheme where
devices know their neighbors, but don’t know
the specifics of what is connected to each
neighbor.
The Airline Example
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Suppose a passenger is traveling from San
Francisco to Miami. Only one flight is listed, with
three legs: Dallas, Atlanta, Miami.
From San Francisco, his next destination is Dallas.
From Dallas, his next destination is Atlanta.
From Atlanta, his last destination is Miami.
But all along, the LAST destination was Miami, even
though the next hop was changing at each leg.
Next-Hop Diagram
Source Independence
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The next hop does not depend on the direction
that the packet came from.
This is referred to source independence.
Source independence is a fundamental
concept in data networks. It allows for lowoverhead, efficient networks.
Hierarchical Addressing & Routing
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Heirarchical addressing is almost routing…
The act of forwarding a packet to the next
address is dubbed routing.
Routing uses a table format to determine the
next hop of the communication, and since it
only needs to inspect the first part of the
address, it is efficient!
A Simplified Routing Table
Routing (cont.)
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The two part addressing scheme provides us
with the following:
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Switches along the path of the transmission need to
inspect the first part of the hierarchical address.
The last switch in the transmission must inspect the
last part of the address.
Really efficient network transport! Not much
overhead.
Routing in the WAN
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There are LOTS of routing algorithms that are
commonly used in the WAN environment.
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RIP: Router Information Protocol
OSPF: Open Shortest Path First
BGP: Border Gateway Protocol
IGRP: Interior Gateway Routing Protocol
I’m not going to focus on the different
algorithms, but please read up and understand
the differences between them!
Routing in the WAN (cont.)
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The best way to visualize routing is to imagine
how all of the networks are connected together.
Each node in the network is a packet switch.
Each connection between switches is a link or
an edge.
A Network Diagram
Routing Table for the Example
Default Routes
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Default routes allow for the simplification of
routing tables.
Since many packets would have the same
routes, a default route would reduce the
amount of work the router/switch would have to
do.
Simplified Routing Table
Default Routing (cont.)
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Only one default route per device is allowed.
The default route has lower priority to other
entered routes.
If a transmission does not find a valid route, it
will send the packet down the default route.
Determination of the Routing Table
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Two ways exist for route determination:
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Static routing
Dynamic routing
Why have different options?
Static Routing
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Static routing is the most straightforward of the
two schemes.
Pros:
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Simple to visualize
Low overhead on devices which perform routing
Cons:
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Static, inflexible
Dynamic Routing
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Dynamic routing can be very confusing.
Multiple types of dynamic routing exist:
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OSPF
IGRP
BGP
RIP
Main disadvantage of dynamic routing: difficult
to understand.
OSPF
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OSPF – Open Shortest Path First
Uses a system of metrics to determine route
preference.
The entire route preference is the sum of the
individual metrics of the links between the
computers.
The routers send their routing tables out
periodically to their neighbors.
Common WAN Technologies
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ATM- Asynchronous Transfer Mode
Frame Relay
SONET
ATM
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Data is divided into small, fixed packets called cells.
Each cell is 53 octets:
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5 octets for header info
48 octets for data transmission
ATM originally designed for simultaneous transmission
of data, voice, and video.
Quality of Service (QOS) is adjustable with bandwidth
needs: higher for video, lower for data and video.
ATM (cont).
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Connections are usually 155Mbps OC-3.
Uses the “cloud” concept.
Frame Relay
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Data is divided into small, variable sized cells,
up to 16,000 octets!
Up to 1.544Mbps transmission rate.
Uses “cloud” concept – shared bandwidth with
other connections.
OSPF Example