Lecture 2 Connecting LANs, Backbone Networks, and Virtual LANs.ppt
1. Lecture 2
Connecting LANs, Backbone
Networks, and Virtual LANs
A. Prof. Dr. Salah Abdulghani
Computer Engineering Department
University of Mosul / March 2022
Computer Networks II
Third Level / Second Semester
15.1
2. Connecting Devices
In this section, we divide connecting devices into five
different categories based on the layer in which they
operate in a network.
Passive Hubs
Active Hubs
Bridges
Two-Layer Switches
Routers
Three-Layer Switches
Gateways
2
3. Figure 1 A repeater connecting two segments of a LAN
A repeater connects segments of a LAN.
A repeater forwards every frame; it has no filtering
capability.
A repeater is a regenerator, not an amplifier.
3
6. Bridge: Layer 2 device used to create two or more LAN segments,
each of which is a collision domain
Bridging was developed to help ease the collision problem
Bridge filters traffic to keep local traffic local, yet allow connectivity
to other segments
Bridges build tables of MAC addresses located on network
segment and other networks and maps them to ports
A bridge has a table used in filtering decisions
A bridge does not change the physical (MAC) addresses in a frame
Bridging and switching decrease congestion on LANs and increase
bandwidth
Switches and Bridges
Work at Layer 2 of OSI
Forward frames based on MAC Address
Forward all broadcast frames, which can result in traffic being
looped endlessly
6
7. A bridge compares destination MAC address in frame
to MAC addresses in its table
If destination MAC is on same network segment as the
source, it does not forward the frame, which is called
filtering
If destination MAC is not on same network segment, but
bridge knows where the destination segment it, it copies
(forwards) the data to the appropriate segment
If the bridge does not know the destination address, the
bridge broadcasts the frame to all segments except the
one from which it originates, which is called flooding
Layer 2 Bridging.
How Bridge Works ?
7
15. Spanning Tree Protocol (STP)
Switched networks are often designed with redundant paths to
provide for reliability and fault tolerance
However, switching loops can occur by design or by accident,
and they can lead to broadcast storms that will rapidly
overwhelm a network
15
16. Spanning Tree Protocol
To counteract loops, switches are provided with a
standards-based protocol called the Spanning-Tree
Protocol (STP)
Each switch using STP sends special messages called Bridge
Protocol Data Units (BPDUs) out all its ports to let other
switches know of its existence and to elect a root bridge for the
network
The Spanning-Tree Algorithm (STA) is used to resolve and
shut down the redundant paths
16
17. Spanning Tree Protocol
Each port on a switch using Spanning-Tree Protocol
exists in one of the following five states:
Blocking – sends and listens to BPDU but does not
forward frames, default
Listening – listens to BPDUs to make sure there are no
loops, frames are not forwarded
Learning – learns MAC addresses and builds address
table, frames are not forwarded
Forwarding – forwards frames, BPDU are sent and
listened to
Disabled – does not participate in STP, does not listen or
forward frames
17
19. Spanning Tree Protocol
A port moves through these five states as follows:
1. From initialization to blocking
2. From blocking to listening or to disabled
3. From listening to learning or to disabled
4. From learning to forwarding or to disabled
5. From forwarding to disabled
The result of resolving and eliminating loops using STP
is to create a logical hierarchical tree with no loops
19
20. Layer 2 Switching
Generally, a bridge has only two ports limiting the division into two
collision domains.
All decisions made by a bridge are based on MAC or Layer 2
addressing and do not affect the logical or Layer 3 addressing. Thus, a
bridge will divide a collision domain but has no effect on a logical or
broadcast domain.
On the other hand a switch is essentially a fast, multi-port bridge.
Rather than accommodate only two collision domains, each port
creates its own collision domain.
In a network of twenty nodes, twenty collision domains exist if each
node is plugged into its own switch port.
A switch dynamically builds and maintains a Content-Addressable
Memory (CAM) table, holding all of the necessary MAC information for
each port.
20
22. Microsegmentation
A switch is simply a bridge with many ports. The two nodes in this
small segment, or collision domain, consist of the two switch ports and
the host connected to each. These small physical segments are called
microsegments.
22
23. Switching Modes
synchronous switching
The source and destination ports are operating at the same bandwidth
asynchronous switching
The source and destination ports are operating at different bandwidths
cut-through
A switch starts to transfer a frame as soon as the destination MAC address is
received. No error checking is available. Must use synchronous switching.
Lowest latency
store-and-forward
At the other extreme, the switch must receive the entire frame before
sending it out the destination port. This gives the switch software an
opportunity to verify the Frame Check Sum (FCS) to ensure that the frame
was reliably received before sending it to the destination. If invalid, frame is
discarded. Must be used with asynchronous switching.
fragment-free
A compromise between the cut-through and store-and-forward modes.
Fragment-free reads the first 64 bytes, which includes the frame header, and
switching begins forwarding before the entire data field and checksum are
read. Verifies the reliability of the addressing 23
24. Data Flow
Layer 1 devices do no filtering, so everything that is received is
passed on to the next segment
The frame is simply regenerated and retimed and thus returned to
its original transmission quality
Any segments connected by Layer 1 devices are part of the same
domain, both collision and broadcast
Layer 2 devices filter data frames based on the destination MAC
address
A frame is forwarded if it is going to an unknown destination
outside the collision domain
The frame will also be forwarded if it is a broadcast, multicast, or
a unicast going outside of the local collision domain
The only time that a frame is not forwarded is when the Layer 2
device finds that the sending host and the receiving host are in
the same collision domain
A Layer 2 device, such as a bridge, creates multiple collision
domains but maintains only one broadcast domain
24
25. Layer 3 devices filter data packets based on IP destination address
The only way that a packet will be forwarded is if its destination IP
address is outside of the broadcast domain and the router has an
identified location to send the packet
A Layer 3 device creates multiple collision and broadcast domains
Data flow through a Layers 1, 2, and 3 of the OSI model
Layer 1 is used for transmission across the physical media
Layer 2 for collision domain management
Layer 3 for broadcast domain management.
25
27. 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.
Bus Backbone
Star Backbone
Connecting Remote LANs
Topics discussed in this section:
27
28. Figure 7 Bus backbone
In a bus backbone, the topology of the backbone is a bus.
28
29. Figure 8 Star backbone
In a star backbone, the topology of the backbone is a star;
the backbone is just one switch.
29
31. VIRTUAL LANs
We can roughly define a virtual local area network
(VLAN) as a local area network configured by
software, not by physical wiring.
Membership
Configuration
Communication between Switches
IEEE Standard
Advantages
Topics discussed in this section:
VLANs create broadcast domains.
31
34. Figure 12 Two switches in a backbone using VLAN software
34
35. A station is considered part of a LAN if it physically belongs to that LAN. The
standard of membership is geographic. What happens if we need a virtual
connection between two stations belonging to two different physical LANs? We
can roughly define a virtual local area network (VLAN) as a local area
network configured by software, not by physical wiring.
The whole idea of VLAN technology is to divide a LAN into logical, instead of physical,
segments. A LAN can be divided into several logical LANs, called VLANs. Each VLAN
is a work group in the organization. If a person moves from one group to another, there
is no need to change the physical configuration. The group membership in VLANs is
defined by software, not hardware. Any station can be logically moved to another
VLAN. All members belonging to a VLAN can receive broadcast messages
sent to that particular VLAN. This means that if a station moves from VLAN 1 to VLAN
2, it receives broadcast messages sent to VLAN 2, but no longer receives broadcast
messages sent to VLAN 1.
35
36. Membership
Vendors use different characteristics such as interface numbers, port
numbers, MAC addresses, IP addresses, IP multicast addresses, or a
combination of two or more of these.
Interface Numbers
Some VLAN vendors use switch interface numbers as a membership
characteristic. For example, the administrator can define that stations
connecting to ports 1, 2, 3, and 7 belong to VLAN 1, stations connecting to
ports 4, 10, and 12 belong to VLAN 2, and so on.
MAC Addresses
Some VLAN vendors use the 48-bit MAC address as a membership
characteristic. For example, the administrator can specify as a condition that
stations having MAC addresses E2:13:42:A1:23:34 and F2:A1:23:BC:D3:41
belong to VLAN 1.
36
37. IP Addresses
Some VLAN vendors use the 32-bit IP address as a membership characteristic.
For example, the administrator can specify as a condition that stations having
IP addresses 181.34.23.67, 181.34.23.72, 181.34.23.98, and 181.34.23.112
belong to VLAN 1.
Multicast IP Addresses
Some VLAN vendors use the multicast IP address as a membership
characteristic. Multicasting at the IP layer is now translated to multicasting at
the datalink layer.
37
38. Manual Configuration
In a manual configuration, the network administrator uses the VLAN software
to manually assign the stations into different VLANs at setup. Later migration
from one VLAN to another is also done manually. Note that this is not a
physical configuration; it is a logical configuration. The term manually here
means that the administrator types the port numbers, the IP addresses, or
other characteristics, using the VLAN software.
Automatic Configuration
In an automatic configuration, the stations are automatically connected or
disconnected from a VLAN using criteria defined by the administrator. For
example, the administrator can define the project number as the condition for
being a member of a group. When a user changes projects, he or she
automatically migrates to a new VLAN.
Configuration
How are the stations grouped into different VLANs? Stations are configured in
one of two ways: manually, and automatically.
38
39. Communication between Switches
In a multi-switched backbone, each switch must know not only which
station belongs to which VLAN, but also the membership of stations
connected to other switches. For example, in Figure 17.12, switch A must
know the membership status of stations connected to switch B, and switch
B must know the same about switch A. Three methods have been devised
for this purpose: table maintenance, frame tagging, and time division
multiplexing.
Table Maintenance
In this method, when a station sends a broadcast frame to its group
members, the switch creates an admission in a table and records station
membership. The switches send their tables to one another periodically for
updating.
Frame Tagging
In this method, when a frame is traveling between switches, an extra
header is added to the MAC frame to define the destination VLAN. The
frame tag is used by the receiving switches to determine the VLANs to be
receiving the broadcast message. 39
40. Time-Division Multiplexing (TDM)
In this method, the connection (trunk) between switches is divided into
time-shared channels. For example, if the total number of VLANs in a
backbone is five, each trunk is divided into five channels. The traffic
destined for VLAN 1 travels in channel 1, the traffic destined for VLAN 2
travels in channel 2, and so on. The receiving switch determines the
destination VLAN by checking the channel from which the frame arrived.
40
41. Advantages
There are several advantages to using VLANs.
Cost and Time Reduction
VLANs can reduce the migration cost of stations going from one group to
another. Physical reconfiguration takes time and is costly. Instead of
physically moving one station to another segment or even to another
switch, it is much easier and quicker to move it by using software.
Creating Virtual Work Groups
VLANs can be used to create virtual work groups. For example, in a
campus environment, professors working on the same project can send
broadcast messages to one another without the necessity of belonging
to the same department. This can reduce traffic if the multicasting
capability of IP was previously used.
Security
VLANs provide an extra measure of security. People belonging to the
same group can send broadcast messages with the guaranteed
assurance that users in other groups will not receive these messages.
41
