Switching Services
Unlike bridges that use software to create and
manage a filter table, switches use application specific integrated circuits
(ASICs) to build and maintain their filter tables. But it’s still okay to think
of a layer 2 switch as a multiport bridge because their basic reason for being
is the same: to break up collision domains. Layer 2 switches and bridges are
faster than routers because they don’t take up time looking at the Network
layer header information. Instead, they look at the frame’s hardware addresses
before deciding to either forward the frame or drop it.
Switches create private dedicated collision domains
and provide independent bandwidth on each port, unlike hubs. Fig. 18 shows five
hosts connected to a switch-all running 100Mbps half duplex to the server.
Unlike a hub, each host has 100Mbps dedicated communication to the server.
Fig-18
Layer 2 switching provides the
following:
Hardware-based
bridging (ASIC)
Wire speed
Low
latency
Low cost
What makes
layer 2 switching so efficient is that no modification to the data packet takes
place. The device only reads the frame encapsulating the packet, which makes
the switching process considerably faster and less error-prone than routing
processes are. And if you use layer 2 switching for both workgroup connectivity
and network segmentation (breaking up collision domains), you can create a
flatter network design with more network segments than you can with traditional
routed networks. Plus, layer 2 switching increases bandwidth for each user
because, again, each connection (interface) into the switch is its own
collision domain. This feature makes it possible for you to connect multiple devices
to each interface.
Limitations
of Layer 2 Switching
Since we
commonly stick layer 2 switching into the same category as bridged networks, we
also tend to think it has the same hang-ups and issues that bridged networks
do. Keep in mind that bridges are good and helpful things if we design the
network correctly, keeping their features as well as their limitations in mind.
And to design well with bridges, the two most important considerations are:
We
absolutely must break up the collision domains correctly.
The right
way to create a functional bridged network is to make sure that its users spend
80 percent of their time on the local segment.
Bridged
networks break up collision domains, but network is still one large broadcast
domain. Neither layer 2 switches nor bridges break up broadcast domains by
default- something that not only limits your network’s size and growth
potential, but can also reduce its overall performance. Broadcasts and
multicasts, along with the slow convergence time of spanning trees, can give
you some major grief as your network grows. These are the big reasons why layer
2 switches and bridges cannot completely replace routers (layer 3 devices) in
the internetwork.
LAN
switches forward Ethernet frames-they just have to decide when to forward them
and when not to. Most switch logic relates somehow to the source and
destination MAC addresses inside the Ethernet frame headers of the frames sent
through the LAN. Switch logic is also dependent on the type of MAC addresses
used. So, a brief review of Ethernet addresses can help shed some light on how
LAN switches work.
The IEEE
defines three general categories of MAC addresses on Ethernet:
Unicast
addresses-A MAC address that identifies a single LAN interface card. Today,
most cards use the MAC address that is burned into the card.
Broadcast
addresses-The most often used IEEE group MAC address, the broadcast address,
has a value of FFFF.FFFF.FFFF (in hexadecimal notation). The broadcast address
implies that all devices on the LAN should receive and process a frame sent to
the broadcast address.
Multicast
addresses-Frames sent to unicast addresses are destined for a single device;
frames sent to a broadcast address are sent to all devices on the LAN. Frames
sent to multicast addresses are meant for all devices that care to receive the
frame, meaning that all devices might receive the frame, none, or some number
in between. Some applications need to communicate with multiple other devices.
By sending one frame, all the devices that care about receiving the data sent
by that application can process the data, and the rest can ignore it.
With these
reminders of the three types of Ethernet MAC addresses, you can appreciate the
logic used by a LAN switch. A switch listens for frames that enter all its interfaces.
After receiving a frame, about a switch decides whether to forward a frame and,
if so, out which port(s). Switches basically perform three tasks:
Learning-The
switch learns MAC addresses by examining the source MAC address of each frame
the bridge receives. By learning, the switch can make good forwarding choices
in the future.
Forwarding
or filtering-The switch decides when to forward a frame or when to filter (not
forward) it based on the destination MAC address. The switch looks at the
previously learned MAC addresses in an address table to decide where to forward
the frames.
Loop
prevention-The switch creates a loop-free environment with other bridges by
using Spanning Tree Protocol (STP). Having physically redundant links helps LAN
availability, and STP prevents the switch logic from letting frames loop around
the network indefinitely, congesting the LAN.
The third task, loop prevention, is
performed using STP, which is covered in depth in the next chapter.
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