Bridges - Cisco RJ-45-to-AUX Brochure

Six types of hubs are found in the network:
Active hubs—Act as repeaters and eliminate attenuation by amplifying the signals they replicate to all
the attached ports.
Backbone hubs—Collect other hubs into a single collection point. This type of design is also known
as a multitiered design. In a typical setup, servers and other critical devices are on high−speed Fast
Ethernet or Gigabit uplinks. This setup creates a very fast connection to the servers that the
lower−speed networks can use to prevent the server or the path to the server from being a bottleneck
in the network.
Intelligent hubs—Contain logic circuits that shut down a port if the traffic indicates that malformed
frames are the rule rather than the exception.
Managed hubs—Have Application layer software installed so that they can be remotely managed.
Network management software is very popular in organizations that have staff responsible for a
network spread over multiple buildings.
Passive hubs—Aid in producing attenuation. They do not amplify the signals they replicate to all the
attached ports. These are the opposite of active hubs.
Stackable hubs—Have a cable to connect hubs that are in the same location without requiring the data
to pass through multiple hubs. This setup is commonly referred to as daisy chaining.
In all of these types of hub configurations, one crucial problem exists: All stations share the bandwidth, and
they all remain in the same collision domain. As a result, whenever two or more stations transmit
simultaneously on any hub, there is a strong likelihood that a collision will occur. These collisions lead to
congestion during high−traffic loads. As the number of stations increases, each station gets a smaller portion
of the LAN bandwidth. Hubs do not provide microsegmentation and leave only one collision domain.

Bridges

A bridge is a relatively simple device consisting of a pair of interfaces with some packet buffering and simple
logic. The bridge receives a packet on one interface, stores it in a buffer, and immediately queues it for
transmission by the other interface. The two cables each experience collisions, but collisions on one cable do
not cause collisions on the other. The cables are in separate collision domains.
Note Some bridges are capable of connecting dissimilar topologies.
The term bridging refers to a technology in which a device known as a bridge connects two or more LAN
segments. Bridges are OSI Data Link layer, or Layer 2, devices that were originally designed to connect two
network segments. Multiport bridges were introduced later to connect more than two network segments, and
they are still in use in many networks today. These devices analyze the frames as they come in and make
forwarding decisions based on information in the frames themselves.
To do its job effectively, a bridge provides three separate functions:
Filtering the frames that the bridge receives to determine if the frame should be forwarded
Forwarding the frames that need to be forwarded to the proper interface
Eliminating attenuation by amplifying received data signals
Bridges learn the location of the network stations without any intervention from a network administrator or
any manual configuration of the bridge software. This process is commonly referred to as self−learning.
When a bridge is turned on and begins to operate, it examines the MAC addresses located in the headers of
frames passed through the network. As the traffic passes through the bridge, the bridge builds a table of
known source addresses, assuming the port from which the bridge received the frame is the port to which the
device is a sending device is attached.
In this table, an entry exists that contains the MAC address of each node along with the bridge interface and
port on which it resides. If the bridge knows that the destination is on the same segment as the source, it drops
the packet because there is no need to transmit it. If the bridge knows that the destination is on another
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