Layer-2 Switching
contain the following:
• Hardware addresses for hosts
• The port each hardware address is associated with
Using this information, Layer-2 devices will make intelligent forwarding
decisions based on the frame (or data-link) headers. A frame can then be
forwarded out only the appropriate destination port, instead of all ports.
Layer-2 forwarding was originally referred to as bridging. Bridging is a
largely deprecated term (mostly for marketing purposes), and Layer-2
forwarding is now commonly referred to as switching.
There are some subtle technological differences between bridging and
switching. Switches usually have a higher port-density, and can perform
forwarding decisions at wire speed, due to specialized hardware circuits
called ASICs (Application-Specific Integrated Circuits). Otherwise,
bridges and switches are nearly identical in function.
Ethernet switches build MAC address tables through a dynamic learning
process. A switch behaves much like a hub when first powered on. The
switch will flood every frame, including unicasts, out every port but the
originating port.
The switch will then build the MAC-address table by examining the source
MAC address of each frame. Consider the following diagram:
When ComputerA sends a frame to
ComputerB, the switch will add ComputerA’s
MAC address to its table, associating it with
port fa0/10. However, the switch will not
learn ComputerB’s MAC address until
ComputerB sends a frame to ComputerA, or
to another device connected to the switch.
Switches always learn from the source
MAC address in a frame.
While hubs were limited to half-duplex communication, switches can
operate in full-duplex. Each individual port on a switch belongs to its own
collision domain. Thus, switches create more collision domains, which
results in fewer collisions.
Like hubs though, switches belong to only one broadcast domain. A Layer-
2 switch will forward both broadcasts and multicasts out every port but the
originating port. Only Layer-3 devices separate broadcast domains.
Because of this, Layer-2 switches are poorly suited for large, scalable
networks. The Layer-2 header provides no mechanism to differentiate one
network from another, only one host from another.
This poses significant difficulties. If only hardware addressing existed, all
devices would technically be on the same network. Modern internetworks
like the Internet could not exist, as it would be impossible to separate my
network from your network.
Imagine if the entire Internet existed purely as a Layer-2 switched
environment. Switches, as a rule, will forward a broadcast out every port.
Even with a conservative estimate of a billion devices on the Internet, the
resulting broadcast storms would be devastating. The Internet would simply
collapse.
Both hubs and switches are susceptible to switching loops, which result in
destructive broadcast storms. Switches utilize the Spanning Tree Protocol
(STP) to maintain a loop-free environment. STP is covered in great detail in
another guide.
Remember, there are three things that switches do that hubs do not:
• Hardware address learning
• Intelligent forwarding of frames
• Loop avoidance
Hubs are almost entirely deprecated – there is no advantage to using a hub
over a switch. At one time, switches were more expensive and introduced
more latency (due to processing overhead) than hubs, but this is no longer
the case.
Layer-2 Forwarding Methods
contain the following:
• Hardware addresses for hosts
• The port each hardware address is associated with
Using this information, Layer-2 devices will make intelligent forwarding
decisions based on the frame (or data-link) headers. A frame can then be
forwarded out only the appropriate destination port, instead of all ports.
Layer-2 forwarding was originally referred to as bridging. Bridging is a
largely deprecated term (mostly for marketing purposes), and Layer-2
forwarding is now commonly referred to as switching.
There are some subtle technological differences between bridging and
switching. Switches usually have a higher port-density, and can perform
forwarding decisions at wire speed, due to specialized hardware circuits
called ASICs (Application-Specific Integrated Circuits). Otherwise,
bridges and switches are nearly identical in function.
Ethernet switches build MAC address tables through a dynamic learning
process. A switch behaves much like a hub when first powered on. The
switch will flood every frame, including unicasts, out every port but the
originating port.
The switch will then build the MAC-address table by examining the source
MAC address of each frame. Consider the following diagram:
ComputerB, the switch will add ComputerA’s
MAC address to its table, associating it with
port fa0/10. However, the switch will not
learn ComputerB’s MAC address until
ComputerB sends a frame to ComputerA, or
to another device connected to the switch.
Switches always learn from the source
MAC address in a frame.
A switch is in a perpetual state of learning. However, as the MAC address
table becomes populated, the flooding of frames will decrease, allowing the
switch to perform more efficient forwarding decisions.
table becomes populated, the flooding of frames will decrease, allowing the
switch to perform more efficient forwarding decisions.
While hubs were limited to half-duplex communication, switches can
operate in full-duplex. Each individual port on a switch belongs to its own
collision domain. Thus, switches create more collision domains, which
results in fewer collisions.
Like hubs though, switches belong to only one broadcast domain. A Layer-
2 switch will forward both broadcasts and multicasts out every port but the
originating port. Only Layer-3 devices separate broadcast domains.
Because of this, Layer-2 switches are poorly suited for large, scalable
networks. The Layer-2 header provides no mechanism to differentiate one
network from another, only one host from another.
This poses significant difficulties. If only hardware addressing existed, all
devices would technically be on the same network. Modern internetworks
like the Internet could not exist, as it would be impossible to separate my
network from your network.
Imagine if the entire Internet existed purely as a Layer-2 switched
environment. Switches, as a rule, will forward a broadcast out every port.
Even with a conservative estimate of a billion devices on the Internet, the
resulting broadcast storms would be devastating. The Internet would simply
collapse.
Both hubs and switches are susceptible to switching loops, which result in
destructive broadcast storms. Switches utilize the Spanning Tree Protocol
(STP) to maintain a loop-free environment. STP is covered in great detail in
another guide.
Remember, there are three things that switches do that hubs do not:
• Hardware address learning
• Intelligent forwarding of frames
• Loop avoidance
Hubs are almost entirely deprecated – there is no advantage to using a hub
over a switch. At one time, switches were more expensive and introduced
more latency (due to processing overhead) than hubs, but this is no longer
the case.
Layer-2 Forwarding Methods
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