Thanks to Cisco
for their support in creating this article!
If you
have read other HowStuffWorks articles on networking
or the Internet,
then you know that a typical network consists of nodes
(computers), a connecting medium (wired or wireless) and
specialized network equipment like routers or
hubs. In the case of the Internet, all of these pieces work
together to allow your computer to send information to another
computer that could be on the other side of the world!
Switches are a fundamental part of most networks.
They make it possible for several users to send information
over a network at the same time without slowing each other
down. Just like routers allow different networks to
communicate with each other, switches allow different
nodes (a network connection point, typically a
computer) of a network to communicate directly with one
another in a smooth and efficient manner.
Image courtesy Cisco Systems,
Inc. Illustration of a Cisco
Catalyst
switch
There are a lot of different types of switches and
networks. Switches that provide a separate connection for each
node in a company's internal network are called LAN
switches. Essentially, a LAN switch creates a series of
instant networks that contain only the two devices
communicating with each other at that particular moment. In
this edition of HowStuffWorks,
we will focus on Ethernet
networks that use LAN switches. You will learn what a LAN
switch is and how transparent bridging works, as well as about
VLANs, trunking and spanning trees.
Networking Basics Here are some of the
fundamental parts of a network:
Network - A network is a group of computers
connected together in a way that allows information to be
exchanged between the computers.
Node - A node is anything that is connected to
the network. While a node is typically a computer, it can
also be something like a printer
or CD-ROM
tower.
Segment - A segment is any portion of a network
that is separated, by a switch, bridge or router, from other
parts of the network.
Backbone - The backbone is the main cabling of a
network that all of the segments connect to. Typically, the
backbone is capable of carrying more information than the
individual segments. For example, each segment may have a
transfer rate of 10 Mbps (megabits
per second), while the backbone may operate at 100 Mbps.
Topology - Topology is the way that each node is
physically connected to the network. Common topologies
include:
Bus - Each node is
daisy-chained (connected one right after the other)
along the same backbone, similar to Christmas
lights. Information sent from a node travels along the
backbone until it reaches its destination node. Each end
of a bus network must be terminated with a resistor
to keep the signal that is sent by a node across the
network from bouncing back when it reaches the end of the
cable.
Bus network topology
Ring - Like a bus
network, rings have the nodes daisy-chained. The
difference is that the end of the network comes back
around to the first node, creating a complete circuit. In
a ring network, each node takes a turn sending and
receiving information through the use of a token.
The token, along with any data, is sent from the first
node to the second node, which extracts the data addressed
to it and adds any data it wishes to send. Then, the
second node passes the token and data to the third node,
and so on until it comes back around to the first node
again. Only the node with the token is allowed to send
data. All other nodes must wait for the token to come to
them.
Ring network topology
Star - In a star
network, each node is connected to a central device called
a hub. The hub takes a signal that comes from any
node and passes it along to all the other nodes in the
network. A hub does not perform any type of filtering or
routing of the data. It is simply a junction that joins
all the different nodes together.
Star network topology
Star bus - Probably the
most common network topology in use today, star bus
combines elements of the star and bus topologies to create
a versatile network environment. Nodes in particular areas
are connected to hubs (creating stars), and the hubs are
connected together along the network backbone (like a bus
network). Quite often, stars are nested within stars, as
seen in the example below:
A typical star bus network
Local Area Network (LAN) - A LAN is a network of
computers that are in the same general physical location,
usually within a building or a campus. If the computers are
far apart (such as across town or in different cities), then
a Wide Area Network (WAN) is typically used.
Network Interface Card (NIC) - Every computer
(and most other devices) is connected to a network through
an NIC. In most desktop computers, this is an Ethernet
card (normally 10 or 100 Mbps) that is plugged into a slot
on the computer's motherboard.
Media Access Control (MAC) address - This is the
physical address of any device -- such as the NIC in
a computer -- on the network. The MAC address has two parts,
each 3 bytes
long. The first 3 bytes identify the company that made the
NIC. The second 3 bytes are the serial number of the NIC
itself.
Unicast - A unicast is a transmission from one
node addressed specifically to another node.
Multicast - In a multicast, a node sends a packet
addressed to a special group address. Devices that are
interested in this group register to receive packets
addressed to the group. An example might be a Cisco
router sending out an update to all of the other Cisco
routers.
Broadcast - In a broadcast, a node sends out a
packet that is intended for transmission to all other nodes
on the network.
Adding Switches In the most basic type of
network found today, nodes are simply connected together using
hubs. As a network grows, there are some potential problems
with this configuration:
Scalability - In a hub network, limited shared
bandwidth makes it difficult to accommodate significant
growth without sacrificing performance. Applications today
need more bandwidth than ever before. Quite often, the
entire network must be redesigned periodically to
accommodate growth.
Latency - This is the amount of time that it
takes a packet
to get to its destination. Since each node in a hub-based
network has to wait for an opportunity to transmit in order
to avoid collisions, the latency can increase
significantly as you add more nodes. Or, if someone is
transmitting a large file across the network, then all of
the other nodes have to wait for an opportunity to send
their own packets. You have probably seen this before at
work -- you try to access a server or the Internet and
suddenly everything slows down to a crawl.
Network failure - In a typical network, one
device on a hub can cause problems for other devices
attached to the hub due to incorrect speed settings (100
Mbps on a 10-Mbps hub) or excessive broadcasts. Switches can
be configured to limit broadcast levels.
Collisions - Ethernet uses a process called
CSMA/CD (Carrier Sense Multiple Access with Collision
Detection) to communicate across the network. Under CSMA/CD,
a node will not send out a packet unless the network is
clear of traffic. If two nodes send out packets at the same
time, a collision occurs and the packets are lost. Then both
nodes wait a random amount of time and retransmit the
packets. Any part of the network where there is a
possibility that packets from two or more nodes will
interfere with each other is considered to be part of the
same collision domain. A network with a large number
of nodes on the same segment will often have a lot of
collisions and therefore a large collision domain.
While hubs provide an easy way to scale up and
shorten the distance that the packets must travel to get from
one node to another, they do not break up the actual network
into discrete segments. That is where switches come in.
Imagine that each vehicle is a packet of data
waiting for an opportunity to continue on its
trip.
Think of a hub as a four-way intersection where everyone
has to stop. If more than one car reaches the intersection at
the same time, they have to wait for their turn to proceed.
Now imagine what this would be like with a dozen or even a
hundred roads intersecting at a single point. The amount of
waiting and the potential for a collision increases
significantly. But wouldn't it be amazing if you could take an
exit ramp from any one of those roads to the road of your
choosing? That is exactly what a switch does for network
traffic. A switch is like a cloverleaf intersection -- each
car can take an exit ramp to get to its destination without
having to stop and wait for other traffic to go by.
A vital difference between a hub and a switch is that all
the nodes connected to a hub share the bandwidth among
themselves, while a device connected to a switch port has the
full bandwidth all to itself. For example, if 10 nodes
are communicating using a hub on a 10-Mbps network, then each
node may only get a portion of the 10 Mbps if other nodes on
the hub want to communicate as well. But with a switch, each
node could possibly communicate at the full 10 Mbps. Think
about our road analogy. If all of the traffic is coming to a
common intersection, then each car it has to share that
intersection with every other car. But a cloverleaf allows all
of the traffic to continue at full speed from one road to the
next.
In a fully switched network, switches replace all
the hubs of an Ethernet network with a dedicated segment for
every node. These segments connect to a switch, which supports
multiple dedicated segments (sometimes in the hundreds). Since
the only devices on each segment are the switch and the node,
the switch picks up every transmission before it reaches
another node. The switch then forwards the frame over the
appropriate segment. Since any segment contains only a single
node, the frame only reaches the intended recipient. This
allows many conversations to occur simultaneously on a
switched network.
Image courtesy Cisco Networks An example of a network using a
switch
Switching allows a network to maintain full-duplex
Ethernet. Before switching, Ethernet was half-duplex, which
means that data could be transmitted in only one direction at
a time. In a fully switched network, each node communicates
only with the switch, not directly with other nodes.
Information can travel from node to switch and from switch to
node simultaneously.
Fully switched networks employ either twisted-pair or
fiber-optic cabling, both of which use separate conductors for
sending and receiving data. In this type of environment,
Ethernet nodes can forgo the collision detection process and
transmit at will, since they are the only potential devices
that can access the medium. In other words, traffic flowing in
each direction has a lane to itself. This allows nodes to
transmit to the switch as the switch transmits to them -- it's
a collision-free environment. Transmitting in both directions
can effectively double the apparent speed of the network when
two nodes are exchanging information. If the speed of the
network is 10 Mbps, then each node can transmit simultaneously
at 10 Mbps.
A mixed network with two switches and three
hubs
Most networks are not fully switched because of the costs
incurred in replacing all of the hubs with switches. Instead,
a combination of switches and hubs are used to create an
efficient yet cost-effective network. For example, a company
may have hubs connecting the computers in each department and
then a switch connecting all of the department-level hubs.
Switching Technologies You can see that a
switch has the potential to radically change the way nodes
communicate with each other. But you may be wondering what
makes it different from a router.
Switches usually work at Layer 2
(Data or Datalink) of the OSI Reference
Model, using MAC addresses, while routers work at Layer 3
(Network) with Layer 3 addresses (IP, IPX or Appletalk,
depending on which Layer
3 protocols are being used). The algorithm
that switches use to decide how to forward packets is
different from the algorithms used by routers to forward
packets.
One of these differences in the algorithms between switches
and routers is how broadcasts are handled. On any
network, the concept of a broadcast packet is vital to the
operability of a network. Whenever a device needs to send out
information but doesn't know who it should send it to, it
sends out a broadcast. For example, every time a new computer
or other device comes on to the network, it sends out a
broadcast packet to announce its presence. The other nodes
(such as a domain server)
can add the computer to their browser list (kind of
like an address directory) and communicate directly with that
computer from that point on. Broadcasts are used any time a
device needs to make an announcement to the rest of the
network or is unsure of who the recipient of the information
should be.
The OSI Reference Model consists of seven
layers that build from the wire (Physical) to the
software
(Application).
A hub or a switch will pass along any broadcast packets
they receive to all the other segments in the broadcast
domain, but a router will not. Think about our four-way
intersection again: All of the traffic passed through the
intersection no matter where it was going. Now imagine that
this intersection is at an international border. To pass
through the intersection, you must provide the border guard
with the specific address that you are going to. If you don't
have a specific destination, then the guard will not let you
pass. A router works like this. Without the specific address
of another device, it will not let the data packet through.
This is a good thing for keeping networks separate from each
other, but not so good when you want to talk between different
parts of the same network. This is where switches come in.
LAN switches rely on packet-switching. The switch
establishes a connection between two segments just long enough
to send the current packet. Incoming packets (part of an
Ethernet frame) are saved to a temporary memory area
(buffer); the MAC address contained in the frame's header
is read and then compared to a list of addresses maintained in
the switch's lookup table. In an Ethernet-based LAN, an
Ethernet frame contains a normal packet as the payload
of the frame, with a special header that includes the MAC
address information for the source and destination of the
packet.
Packet-based switches use one of three methods for routing
traffic:
Cut-through
Store-and-forward
Fragment-free
Cut-through switches
read the MAC address as soon as a packet is detected by the
switch. After storing the 6 bytes that make up the address
information, they immediately begin sending the packet to the
destination node, even as the rest of the packet is coming
into the switch.
A switch using store-and-forward will save the
entire packet to the buffer and check it for CRC
errors or other problems before sending. If the packet has an
error, it is discarded. Otherwise, the switch looks up the MAC
address and sends the packet on to the destination node. Many
switches combine the two methods, using cut-through until a
certain error level is reached and then changing over to
store-and-forward. Very few switches are strictly cut-through,
since this provides no error correction.
A less common method is fragment-free. It works like
cut-through except that it stores the first 64 bytes of the
packet before sending it on. The reason for this is that most
errors, and all collisions, occur during the initial 64 bytes
of a packet.
LAN switches vary in their physical design. Currently,
there are three popular configurations in use:
Shared memory - This type of switch stores all
incoming packets in a common memory buffer shared by all the
switch ports (input/output connections), then sends
them out via the correct port for the destination node.
Matrix - This type of switch has an internal grid
with the input ports and the output ports crossing each
other. When a packet is detected on an input port, the MAC
address is compared to the lookup table to find the
appropriate output port. The switch then makes a connection
on the grid where these two ports intersect.
Bus architecture - Instead of a grid, an internal
transmission path (common bus) is shared by all of
the ports using TDMA.
A switch based on this configuration has a dedicated memory
buffer for each port, as well as an ASIC to
control the internal bus access.
Transparent Bridging Most Ethernet LAN
switches use a very cool system called transparent
bridging to create their address lookup tables.
Transparent bridging is a technology that allows a switch to
learn everything it needs to know about the location of nodes
on the network without the network administrator having to do
anything. Transparent bridging has five parts:
Learning
Flooding
Filtering
Forwarding
Aging
Here's how it works:
Click on the menu terms to learn more about how
transparent bridging works.
The switch is added to the network, and the various
segments are plugged into the switch's ports.
A computer (Node A) on the first segment (Segment A)
sends data to a computer (Node B) on another segment
(Segment C).
The switch gets the first packet of data from Node A. It
reads the MAC address and saves it to the lookup table for
Segment A. The switch now knows where to find Node A anytime
a packet is addressed to it. This process is called
learning.
Since the switch does not know where Node B is, it sends
the packet to all of the segments except the one that it
arrived on (Segment A). When a switch sends a packet out to
all segments to find a specific node, it is called
flooding.
Node B gets the packet and sends a packet back to Node A
in acknowledgement.
The packet from Node B arrives at the switch. Now the
switch can add the MAC address of Node B to the lookup table
for Segment C. Since the switch already knows the address of
Node A, it sends the packet directly to it. Because Node A
is on a different segment than Node B, the switch must
connect the two segments to send the packet. This is known
as forwarding.
The next packet from Node A to Node B arrives at the
switch. The switch now has the address of Node B, too, so it
forwards the packet directly to Node B.
Node C sends information to the switch for Node A. The
switch looks at the MAC address for Node C and adds it to
the lookup table for Segment A. The switch already has the
address for Node A and determines that both nodes are on the
same segment, so it does not need to connect Segment A to
another segment for the data to travel from Node C to Node
A. Therefore, the switch will ignore packets traveling
between nodes on the same segment. This is filtering.
Learning and flooding continue as the switch adds nodes
to the lookup tables. Most switches have plenty of memory
in a switch for maintaining the lookup tables; but to
optimize the use of this memory, they still remove older
information so that the switch doesn't waste time searching
through stale addresses. To do this, switches use a
technique called aging. Basically, when an entry is
added to the lookup table for a node, it is given a
timestamp. Each time a packet is received from a node, the
timestamp is updated. The switch has a user-configurable
timer that erases the entry after a certain amount of time
with no activity from that node. This frees up valuable
memory resources for other entries. As you can see,
transparent bridging is a great and essentially
maintenance-free way to add and manage all the information a
switch needs to do its job!
In our example, two
nodes share segment A, while the switch creates independent
segments for Node B and Node D. In an ideal LAN-switched
network, every node would have its own segment. This would
eliminate the possibility of collisions and also the need for
filtering.
Redundancy and Broadcast Storms When we
talked about bus and ring networks earlier, one issue was the
possibility of a single point of failure. In a star or
star-bus network, the point with the most potential for
bringing all or part of the network down is the switch or hub.
Look at the example below:
In this example, if either switch A or C fails, then the
nodes connected to that particular switch are affected, but
nodes at the other two switches can still communicate.
However, if switch B fails, then the entire network is brought
down. What if we add another segment to our network connecting
switches A and C?
In this case, even if one of the switches fails, the
network will continue. This provides redundancy,
effectively eliminating the single point of failure.
But now we have a new problem. In the last section, you
discovered how switches learn where the nodes are located.
With all of the switches now connected in a loop, a packet
from a node could quite possibly come to a switch from two
different segments. For example, imagine that Node B is
connected to Switch A, and needs to communicate with Node A on
Segment B. Switch A does not know who Node A is, so it floods
the packet.
The packet travels via Segment A or Segment C to the other
two switches (B and C). Switch B will add Node B to the lookup
table it maintains for Segment A, while Switch C will add it
to the lookup table for Segment C. If neither switch has
learned the address for Node A yet, they will flood Segment B
looking for Node A. Each switch will take the packet sent by
the other switch and flood it back out again immediately,
since they still don't know who Node A is. Switch A will
receive the packet from each segment and flood it back out on
the other segment. This causes a broadcast storm as the
packets are broadcast, received and rebroadcast by each
switch, resulting in potentially severe network congestion.
Which brings us to spanning trees...
Spanning Trees To prevent broadcast storms
and other unwanted side effects of looping, Digital
Equipment Corporation created the spanning-tree
protocol (STP), which has been standardized as the
802.1d specification by the Institute
of Electrical and Electronic Engineers (IEEE).
Essentially, a spanning tree uses the spanning-tree
algorithm (STA), which senses that the switch has more
than one way to communicate with a node, determines which way
is best and blocks out the other path(s). The cool thing is
that it keeps track of the other path(s), just in case the
primary path is unavailable.
Here's how STP works:
Each switch is assigned a group of IDs, one for the
switch itself and one for each port on the switch. The
switch's identifier, called the bridge ID (BID), is 8
bytes long and contains a bridge priority (2 bytes) along
with one of the switch's MAC addresses (6 bytes). Each
port ID is 16 bits long with two parts: a 6-bit
priority setting and a 10-bit port number.
A path cost value is given to each port. The cost
is typically based on a guideline established as part of
802.1d. According to the original specification, cost is
1,000 Mbps (1 gigabit per second) divided by the bandwidth
of the segment connected to the port. Therefore, a 10 Mbps
connection would have a cost of (1,000/10) 100.
To compensate for the speed of networks increasing beyond
the gigabit range, the standard cost has been slightly
modified. The new cost values are:
Bandwidth
STP Cost Value
4
Mbps
250
10
Mbps
100
16
Mbps
62
45
Mbps
39
100
Mbps
19
155
Mbps
14
622
Mbps
6
1
Gbps
4
10
Gbps
2
You should also note that the path cost can be an
arbitrary value assigned by the network administrator,
instead of one of the standard cost values.
Each switch begins a discovery process to choose which
network paths it should use for each segment. This
information is shared between all the switches by way of
special network frames called bridge protocol data
units (BPDU). The parts of a BPDU are:
Root BID - This is the BID of the current
root bridge.
Path cost to root bridge - This determines how
far away the root bridge is. For example, if the data has
to travel over three 100-Mbps segments to reach the root
bridge, then the cost is (19 + 19 + 0) 38. The segment
attached to the root bridge will normally have a path cost
of zero.
Sender BID - This is the BID of the switch that
sends the BPDU.
Port ID - This is the actual port on the switch
that the BPDU was sent from.
All of the switches are constantly sending BPDUs to each
other, trying to determine the best path between various
segments. When a switch receives a BPDU (from another
switch) that is better than the one it is broadcasting for
the same segment, it will stop broadcasting its BPDU out
that segment. Instead, it will store the other switch's BPDU
for reference and for broadcasting out to inferior
segments, such as those that are farther away from the
root bridge.
A root bridge is chosen based on the results of
the BPDU process between the switches. Initially, every
switch considers itself the root bridge. When a switch first
powers up on the network, it sends out a BPDU with its own
BID as the root BID. When the other switches receive the
BPDU, they compare the BID to the one they already have
stored as the root BID. If the new root BID has a lower
value, they replace the saved one. But if the saved root BID
is lower, a BPDU is sent to the new switch with this BID as
the root BID. When the new switch receives the BPDU, it
realizes that it is not the root bridge and replaces the
root BID in its table with the one it just received. The
result is that the switch that has the lowest BID is elected
by the other switches as the root bridge.
Based on the location of the root bridge, the other
switches determine which of their ports has the lowest path
cost to the root bridge. These ports are called root
ports, and each switch (other than the current root
bridge) must have one.
The switches determine who will have designated
ports. A designated port is the connection used to send
and receive packets on a specific segment. By having only
one designated port per segment, all looping issues are
resolved!
Designated ports are selected based on the lowest path
cost to the root bridge for a segment. Since the root bridge
will have a path cost of "0," any ports on it that are
connected to segments will become designated ports. For the
other switches, the path cost is compared for a given
segment. If one port is determined to have a lower path
cost, it becomes the designated port for that segment. If
two or more ports have the same path cost, then the switch
with the lowest BID is chosen.
Once the designated port for a network segment has been
chosen, any other ports that connect to that segment become
non-designated ports. They block network traffic from
taking that path so it can only access that segment through
the designated port.
Each switch has a table of
BPDUs that it continually updates. The network is now
configured as a single spanning tree, with the root bridge as
the trunk and all the other switches as branches. Each switch
communicates with the root bridge through the root ports, and
with each segment through the designated ports, thereby
maintaining a loop-free network. In the event that the root
bridge begins to fail or have network problems, STP allows the
other switches to immediately reconfigure the network with
another switch acting as root bridge. This amazing process
gives a company the ability to have a complex network that is
fault-tolerant and yet fairly easy to maintain.
Routers and Layer 3 Switching While most
switches operate at the Data layer (Layer 2) of the OSI Reference
Model, some incorporate features of a router and
operate at the Network layer (Layer 3) as well. In
fact, a Layer 3 switch is incredibly similar to a router.
Layer 3 switches actually work at the Network
layer.
When a router receives a packet, it looks at the Layer 3
source and destination addresses to determine the path the
packet should take. A standard switch relies on the MAC
addresses to determine the source and destination of a packet,
which is Layer 2 (Data) networking.
The fundamental difference between a router and a Layer 3
switch is that Layer 3 switches have optimized hardware to
pass data as fast as Layer 2 switches, yet they make decisions
on how to transmit traffic at Layer 3, just like a router.
Within the LAN environment, a Layer 3 switch is usually faster
than a router because it is built on switching hardware. In
fact, many of Cisco's Layer 3 switches are actually routers
that operate faster because they are built on "switching"
hardware with customized chips inside the box.
The pattern matching and caching on
Layer 3 switches is similar to the pattern matching and
caching on a router. Both use a routing protocol and routing
table to determine the best path. However, a Layer 3 switch
has the ability to reprogram the hardware dynamically
with the current Layer 3 routing information. This is what
allows for faster packet processing.
On current Layer 3 switches, the information received from
the routing protocols is used to update the hardware caching
tables.
VLANs As networks have grown in size and
complexity, many companies have turned to virtual local
area networks (VLANs) to provide some way of structuring
this growth logically. Basically, a VLAN is a collection of
nodes that are grouped together in a single broadcast
domain that is based on something other than physical
location.
You learned about broadcasts earlier, and how a router does
not pass along broadcasts. A broadcast domain is a network (or
portion of a network) that will receive a broadcast packet
from any node located within that network. In a typical
network, everything on the same side of the router
is all part of the same broadcast domain. A switch that you
have implemented VLANs on has multiple broadcast domains,
similar to a router. But you still need a router (or Layer
3 routing engine) to route from one VLAN to another -- the
switch can't do this by itself.
Here are some common reasons why a company might have
VLANs:
Security - Separating systems that have sensitive
data from the rest of the network decreases the chances that
people will gain access to information they are not
authorized to see.
Projects/Special applications - Managing a
project or working with a specialized application can be
simplified by the use of a VLAN that brings all of the
required nodes together.
Performance/Bandwidth - Careful monitoring of
network use allows the network administrator to create VLANs
that reduce the number of router hops and
increase the apparent bandwidth for network users.
Broadcasts/Traffic flow - Since a principle
element of a VLAN is the fact that it does not pass
broadcast traffic to nodes that are not part of the VLAN, it
automatically reduces broadcasts. Access lists
provide the network administrator with a way to control who
sees what network traffic. An access list is a table the
network administrator creates that lists which addresses
have access to that network.
Departments/Specific job types - Companies may
want VLANs set up for departments that are heavy network
users (such as multimedia or engineering), or a VLAN across
departments that is dedicated to specific types of employees
(such as managers or sales people).
You can create a
VLAN using most switches simply by logging into the switch via
Telnet
and entering the parameters for the VLAN (name, domain and
port assignments). After you have created the VLAN, any
network segments connected to the assigned ports will become
part of that VLAN.
While you can have more than one VLAN on a switch, they
cannot communicate directly with one another on that switch.
If they could, it would defeat the purpose of having a VLAN,
which is to isolate a part of the network. Communication
between VLANs requires the use of a router.
VLANs can span multiple switches, and you can have more
than one VLAN on each switch. For multiple VLANs on multiple
switches to be able to communicate via a single link between
the switches, you must use a process called trunking --
trunking is the technology that allows information from
multiple VLANs to be carried over a single link between
switches.
The VLAN trunking protocol (VTP) is the protocol
that switches use to communicate among themselves about VLAN
configuration.
In the image above, each switch has two VLANs. On the first
switch, VLAN A and VLAN B are sent through a single port
(trunked) to the router and through another port to the second
switch. VLAN C and VLAN D are trunked from the second switch
to the first switch, and through the first switch to the
router. This trunk can carry traffic from all four VLANs. The
trunk link from the first switch to the router can also carry
all four VLANs. In fact, this one connection to the router
allows the router to appear on all four VLANs, as if it had
four different physical ports connected to the switch.
The VLANs can communicate with each other via the trunking
connection between the two switches using the router. For
example, data from a computer on VLAN A that needs to get to a
computer on VLAN B (or VLAN C or VLAN D) must travel from the
switch to the router and back again to the switch. Because of
the transparent bridging algorithm and trunking, both PCs and
the router think that they are on the same physical segment!
As you can see, LAN switches are an amazing technology that
can really make a difference in the speed and quality of a
network.
For more information, check out the links on the next page.
