Transcript Document

Networking and
Internetworking
Devices, Backbone
Networks,
and Virtual LANs
• Networking devices are used to construct networks.
• Example: A local area Network (LAN) may need to cover more
distance that its media can handle effectively. In this case, you
can use a repeater to regenerate the signal.
• Internetworking devices are used to connect networks.
• Example: If you want to link a LAN into an internet, you
need an internetworking device as a router or a gateway.
• An internet is an interconnection of individual networks.
Connecting Devices and the OSI Model
Connecting Devices
Connecting Devices
Repeaters
Hubs
Bridges
Two-Layer Switches
Connecting devices
Repeaters
• A repeater (or regenerator) is an electronic device that
operates on only the physical layer of the OSI model.
• A repeater installed on a link receives the signal before it
becomes too weak or corrupted, regenerates the original
pattern, and puts the refreshed copy back on the link.
Repeaters
• A repeater does not actually connect two LANS; it connects
two segments of the same LAN.
• A repeater forwards every frame; it has no filtering capability.
Hubs
• A Hub is a multiport repeater. It is normally used to create connections
between stations in a physical star topology.
Bridges
• Bridges operate in both the physical and the data link
layers of the OSI model.
Bridges
• Bridges can divide a large network into smaller segments. They
contain logic that allows them to keep the traffic on each segment
separate. When a frame (or packet) enters a bridge, the bridge not only
regenerates the signal but checks the destination address and forwards
the new copy only to the segment the address belong.
Bridges
• A bridge operates in both the physical and the data link layers.
• As a physical layer device, it regenerates the signal it receives.
• As a data link layer device, the bridge can check the physical
(MAC) address (source and destination) contained in the
frame.
• A bridge has filtering capability. 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 the port.
• A bridge does not change the physical (MAC) addresses in a
frame.
• A bridge has a table used in filtering decisions.
Bridge
Types of Bridges
• To select between segments, a bridge must have a look-up
table that contains the physical addresses of every station
connect to it. The table indicate to which segment each
station belongs.
Simple Bridge
• The address table must be entered manually, before a
simple bridge can be used.
• Whenever a new station is added or removed, the table
must modified.
• Installation and maintenance of simple bridges are timeconsuming and potentially more trouble than the cost
savings are worth.
Types of Bridges
Multiport bridges
A multiport bridge can be used to connect more than two
LANs.
Types of Bridges
Transparent Bridge
A transparent, or learning, bridge builds its table of station
addresses on its own as it performs its bridge functions.
Transparent Bridges
•
A transparent bridge is a bridge in which the stations are
completely unaware of the bridge’s existence.
•
If a transparent bridge is added or removed from the
system, reconfiguration of the stations is unnecessary.
•
A transparent bridge must meet three criteria:
1.
Frames must be forwarded from one station to another.
2. The forwarding table is automatically made by learning frame
movements in the network.
Learning Bridge
Loop Problem
• Transparent bridges work fine as long as there are no
redundant bridges in the system.
• Bridges are normally installed redundantly, which means that
two LANs may be connected by more than one bridge. In this
case, if the bridges are transparent bridges, the y may create a
loop, which means a packet may be going round and round,
from one LAN to another and back again to the first LAN.
• See next slide
• To solve the looping problem, the IEEE specification requires
that bridges use the spanning tree algorithm to create a
loopless topology.
Loop problem
Spanning Tree
•
•
•
A spanning tree is a graph in which there is no
loop.
In a bridged LAN, a loopless topology means a
topology in which each LAN can be reached from
any other LAN through one path only (no loop).
It is not possible to change the physical topology
of the system, but we can create a logical topology
that overlays the physical one.
Source Routing
Another solution to prevent loops in LANs connected by
bridges is source routing. In this method, the source of the
packet defines the bridges and the LANs through which the
packet should go before reaching the destination.
Routers
• Routers have access
to network layer
addresses and contain
software that enables
them to determine
which of several
possible paths
between those
addresses is the best
for a particular
transmission.
• Routers operate in the
physical, data link,
and network layers of
the OSI model.
• Routers relay packets among multiple interconnected
networks. They route packets from one network to any of a
number of potential destination networks on an internet.
Gateways
• Gateways potentially operate in all seven layers of the OSI
model.
Gateways
• A gateway is a protocol converter. A router by itself
transfers, accepts, and relays packets only across networks
using similar protocols.
A gateway can accept a packet formatted for one protocol
(e.g. AppleTalk) and convert it to a packet for another
protocol (e.g. TCP/IP).
Gateways
• A gateway is generally software installed within a router.
The gateway understands the protocols used by each
network linked into the router and is therefore able to
translate from one to another.
Backbone Networks
• A backbone network allows several LANs to be
connected.
• In a backbone network, no station is directly connected to
the backbone; the stations are part of a LAN, and the
backbone connects the LANs.
• The backbone itself is a LAN.
• The two most common architectures are the bus backbone
and the star backbone.
Bus backbone
Bus Backbone
• In a bus backbone, the topology of the backbone is a bus.
• Bus backbones are normally used as a distribution
backbone to connect different buildings in an organization.
Star Backbone
• In a star backbone, the topology of the backbone is a star;
the backbone is just a switch.
• Star backbones are mostly used as a distribution backbone
inside a building.
• In a multifloor building usually there is a LAN that serves
each particular floor. A star backbone connects these
LANs.
Star backbone
Virtual LANS
• Read section 16.3