Chapter 8
Local Area Networks Internetworking
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Chapter Eight - Local Area Networks:
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Introduction
Many 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.
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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.
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Repeaters
Repeater
Repeaters extend
the maximum
allowed distance
between nodes.
telnet
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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Chapter Eight - Local Area Networks: Internetwork
Hubs
As 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.
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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
Hubs
Hub
Hubs make it
very easy to add
and remove hosts
on a network.
telnet
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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Can also create multiple level of hierarchy
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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
Chapter Eight - Local Area Networks: Internetwork
Bridges
A 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
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–
–
–
–
–
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
Bridge
host
host
host
host
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Bridges
Bridge
Hub
telnet
Hub
Bridges make it possible to segment
your network into separate collision
domains to minimize collisions and
improve performance.
telnet
Separate Collision Domains
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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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.
A bridge connecting two LANs
the bridge can check the
physical/MAC addresses
contained in the frame.
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Chapter Eight - Local Area Networks: Internetwork
Transparent Bridges
A 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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Chapter Eight - Local Area Networks: Internetwork
Transparent Bridges
A 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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Chapter Eight - Local Area Networks: Internetwork
Remote Bridges
A 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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Chapter Eight - Local Area Networks: Internetwork
Spanning Tree Algorithm
What 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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Chapter Eight - Local Area Networks: Internetwork
Spanning Tree Algorithm
How 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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Chapter Eight - Local Area Networks: Internetworki
Spanning Tree Algorithm
Step 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
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(Figure 8-7b)
Chapter Eight - Local Area Networks: Internetworki
Spanning Tree Algorithm
Step 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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Chapter Eight - Local Area Networks: Internetwork
Switches
A 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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Chapter Eight - Local Area Networks: Internetwork
Switches
The 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
Switch
Switches are
similar to bridges,
but offer multiple parallel
communication channels across
ports for improved
performance.
telnet
telnet
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.
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Chapter Eight - Local Area Networks: Internetwork
Virtual LANs
A 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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Chapter Eight - Local Area Networks: Internetwork
Virtual LANs
A 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 802.1Qcompliant switch can be remotely configured by a network
administrator
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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
Chapter Eight - Local Area Networks: Internetwork
Isolating Traffic Patterns with Switches
Whether 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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Chapter Eight - Local Area Networks: Internetwork
Isolating Traffic Patterns with Switches
Using a pair of routers, it is possible to interconnect to switched
segments, essentially creating one large local area network
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Chapter Eight - Local Area Networks: Internetwork
Full Duplex Switches
A 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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Chapter Eight - Local Area Networks: Internetwork
Network Servers
Network 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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Chapter Eight - Local Area Networks: Internetwork
Routers
The 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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Chapter Eight - Local Area Networks: Internetwork
Routers
Thus, 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 speed
Disadvantages
– hard to configure and manage
– access lists must be kept current
– high maintenance/high training costs
– very expensive
Chapter Eight - Local Area Networks: Internetwork
LAN Internetworking In Action: A Small
Office Revisited
Recall 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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Chapter Eight - Local Area Networks: Internetwork
LAN Internetworking In Action: A Small
Office Revisited
Now 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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Chapter Eight - Local Area Networks: Internetwork
LAN Internetworking In Action: A Small
Office Revisited
Now 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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