Connecting LANs, Backbone
Networks, and Virtual LANs
Semester: 131
Course: CSET 221 Computer Networking
Instructor: Farhan Khan
Computer Science & Engineering Technology Unit
Hafr Al-Batin Community College
Outline
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Connecting devices
Backbone networks
Virtual LANs
2
Connecting Devices
3
Layer-1 Connecting Devices
4
Repeaters
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Connected segments become single
collision domain
5
Note
A repeater connects segments of a LAN.
6
Note
A repeater forwards every frame;
it has no filtering capability.
7
Note
A repeater is a regenerator,
not an amplifier.
8
Repeater Function
9
Hubs
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Similar to multiport repeaters
All segments belong to the same collision
domain
10
Layer-2 Connecting Devices
11
Bridges
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Bridges process information up to the frame-level (Layer
2)
Connecting LANs while separating collision domains
Connected segments form a single network (same
broadcast domain)
12
Main Functions of Bridges
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Three main functions:
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Forwarding / Filtering
Address Learning
Loop Avoidance
13
Forwarding/Filtering
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MAC addresses are used for forwarding / filtering
Bridging and switching devices determine if incoming
frames are destined for a device on the segment where
they were generated.
If so, the devices do not forward the frames to the other
device ports. This is an example of filtering.
If the MAC destination address is on another segment,
the devices send the frames to the appropriate segment.
This is known as forwarding.
14
Forwarding/Filtering
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If a frame destined for station 712B 13456142 arrives at port 1, the bridge
consults its table to find the departing port. According to its table, frames
for 712B 13456142 leave through port 1; therefore, there is no need for
forwarding, and the frame is dropped.
On the other hand, if a frame for 712B13456141 arrives at port 2, the
departing port is port 1 and the frame is forwarded.
15
Note
A bridge has a table used in
filtering decisions.
16
Note
A bridge does not change the physical (MAC) addresses in
a frame.
15.17
Address Learning
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In order for the bridges to begin passing information to and
from devices and segments, they must first familiarize
themselves with the addresses associated with those devices
and segments.
Initially, they must let all information pass through them,
even if that information is not intended for a device on the
opposite side of the bridges/switches. This is known as
flooding.
Once the devices have allowed the information from the
connecting segments to pass through, they can log the
address information into tables called bridge table for
further use in forwarding and filtering.
Note: Bridges and switches create bridge table based on
source Hardware address.
18
Address Learning
AD
EA
BC
19
Loop Avoidance
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Transparent bridges work fine as long as there are no
redundant bridges in the system.
Systems administrators, however, like to have redundant
bridges (more than one bridge between a pair of LANs)
to make the system more reliable.
Redundancy can create loops in the system, which is
very undesirable.
To solve the looping problem, bridges use the spanning
tree algorithm to create a loopless topology.
20
Loop Problem
21
Graph Representation
22
Creating Spanning Tree
23
Blocking Ports
24
Types of Bridging
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Transparent Bridging
Source Route Bridging
Mixed-Media Bridging
25
Transparent Bridging
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Transparent bridging is a scheme found in Ethernet
networks in which bridges pass frames along one hop at
a time based on tables associating end nodes with
bridge ports.
The operation and presence of these bridges is
transparent to network end nodes.
Transparent bridges interconnect like-media LANs (for
example, all Ethernet) to form the appearance of a
single larger network. They also isolate intra-segment
traffic, thereby reducing the traffic seen on each
individual segment.
26
Source Route Bridging
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In Source-route bridging, the entire route to a
destination is predetermined, prior to data transmission.
The source places the complete source-to-destination
route in the frame header of all frames.
Source-route bridging (SRB) was developed by IBM for
use in Token Ring networks.
27
Source Routing Bridging Example
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In the figure, the Path from A to D is L1, B1, L2, B2, L3.
If the source doesn't know the route, it sends a
"discovery frame" that goes to every LAN in the network.
The destination replies and each bridge along the way
put its ID in that reply. The source then knows all that it
needs. This discovery produces lots of excess packets.
28
Mixed Media Bridging
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Mixed-media bridging involves moving frames from one
LAN media to another, typically from
Ethernet to Token Ring (and vice versa), but might also
be from Ethernet to FDDI, and Token Ring to FDDI.
The following figure shows a mixed-media bridge
connecting Ethernet, Token Ring, and FDDI data links:
29
Switches
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Switches also referred to as multi-port bridges,
automatically learn the MAC addresses of the devices
connected to each port of the switch and builds
switching table.
The switch then examines each packet it receives to find
destination MAC address, looks up the destination
address in its switching table and determines outgoing
interface.
Like bridges, switches forward and flood traffic based on
MAC addresses. However, because switching is
performed in hardware instead of in software, it is
significantly faster.
30
Switches
31
Switching Methods
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The three types of switching methods
used by switches to forward data traffic:
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Store and Forward
Cut-Through
Fragment free
32
Store and Forward Switching
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With the store-and-forward switching method, the LAN
switch copies the entire frame into its onboard buffers
and computes the cyclic redundancy check (CRC).
The frame is discarded if it contains a CRC error, or if it
is a runt (less than 64 bytes including the CRC), or a
giant (more than 1518 bytes including the CRC).
If the frame does not contain any errors, the LAN switch
looks up the destination address in its forwarding, or
switching, table and determines the outgoing interface.
It then forwards the frame toward its destination.
33
Cut-through Switching
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With the cut-through switching method, the LAN switch
copies only the destination address (the first 6 bytes
following the preamble) into its onboard buffers.
It then looks up the destination address in its switching
table, determines the outgoing interface and forwards
the frame toward its destination.
A cut-through switch provides reduced latency because
it begins to forward the frame as soon as it reads the
destination address and determines the outgoing
interface.
34
Fragment-Free Switching
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This method works just like cut-through,
but the switch waits for 64 bytes to be
received before forwarding the first bytes
of outgoing frame.
According to Ethernet specifications,
collisions should be detected during the
first 64 bytes of the frame. Frame in error
due to collision will not be forwarded.
35
Layer-3 Connecting Devices
36
Routers
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Capable of connecting networks of
different types
Routers separate networks into different
broadcast domains
37
Routing Tables
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The routing information a router learns
from its routing sources is placed in its
routing table.
The router will rely on this table to find
which port to use when forwarding
addressed packets.
38
Routing Tables
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If the destination network is directly connected,
the router already knows the port to use when
forwarding packet
39
Routing Tables
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If destination networks are not directly attached,
then the router must learn the best route to use
when forwarding packets.
40
Static and Dynamic Routing
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The two methods for learning routes through the network are
as follows:
Static routing - Routes learned by the router when an
administrator manually establishes the route. The
administrator must manually update this static route entry
whenever a network topology change requires an update.
Dynamic Routing - Routes dynamically learned by the
router after an administrator configures a routing protocol
that helps determine routes. Unlike static routes, once the
network administrator enables dynamic routing, route
knowledge is automatically updated by a routing process
whenever new topology information is received from the
network.
41
Backbone Networks
Backbone Networks
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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.
Bus Backbone
44
Note
In a bus backbone, the topology
of the backbone is a bus.
45
Star Backbone
46
Note
In a star backbone, the topology of the backbone
is a star;
the backbone is just one switch.
47
Connecting Remote LANs
48
Note
A point-to-point link acts as a LAN in a remote
backbone connected by
remote bridges.
49
Virtual LANs
Virtual LAN
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A virtual local area network (VLAN) is a
set of workstations within a LAN that can
communicate with each other as though
they were on a single, isolated LAN.
The basic reason for splitting a network
into VLANs is to reduce data traffic
congestion on a large LAN.
51
Regular Switch
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Three LANs are connected by a regular
switch, forming one LAN (one broadcast
domain)
52
Switch with VLAN Feature
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Different VLANs belong to different broadcast domains
They are considered isolated networks
53
Multiple VLAN Switches
Trunk
Trunk
54
Note
VLANs create broadcast domains.
55
Advantages
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Performance. Workstations could be separated
off into separate LANs without being physically
divided up by routers.
Formation of virtual workgroups. Because
workstations can be moved from one VLAN to
another just by changing the configuration on
switches, it is relatively easy to put all the
people working together on a particular project
all into a single VLAN. They can then more easily
share files and resources with each other.
56
Advantages
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Greater flexibility. If users move their desks,
or just move around the place with their laptops,
then, if the VLANs are set up the right way, they
can plug their PC in at the new location, and still
be within the same VLAN.
Ease of partitioning off resources. If there
are servers or other equipment to which the
network administrator wishes to limit access,
then they can be put off into their own VLAN.
57
Routing Across VLANs
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Router is required to connect
different VLANs
Trunk
Trunk
Trunk
58
Summary
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Connecting devices
Backbone networks
Virtual LANs
59