Specifications

Chapter 1 Understanding Networking and IP Addressing
Introduction to Networking
Niagara Release 2.3
Niagara Networking & Connectivity Guide Revised: May 22, 2002
1–15
Bridges The function of a bridge is to connect separate networks together, as well as manage
traffic among segments of a LAN. Bridges examine the source and destination
addresses of data and they use this information to determine which transmissions
should be allowed to pass to another network segment. Traffic whose destination is
on the same segment as the sender is confined to that segment. Only inter-network
traffic is allowed to pass through the bridge. This is very useful for restricting traffic
flow across network segments. Bridges are often used to join two LANs that are
already fully expanded.
Bridges know which nodes are on which segments by referring to a filtering database.
Some bridges require that a system administrator manually enter address information
in this database, but most modern bridges are capable of learning where a node exists.
Learning bridges build their own routing tables by examining network traffic and
figuring out on which segment a node exists.
Advantages of bridges:
• Bridges can restrict the flow of unnecessary traffic across segments.
• Bridges can resynchronize signals.
• Bridges add reliability to a network.
• Bridged LANs minimize the impact of failures.
• Bridges can segment large LANs into smaller ones to reduce the potential for
a catastrophic event that impacts the entire network.
Disadvantages of bridges:
• Bridges are more complex than repeaters.
• Bridges are typically more expensive than repeaters.
• Since they examine all network traffic, bridges are generally much slower than
repeaters and might be a potential bottleneck.
Routers Routers do not simply forward data packets according to Ethernet address as bridges
do. Instead, they extract data from the packet and use protocol information in the
packet to move data through the network along the most expedient route to its
destination. Routers also divide networks logically instead of physically—an IP
router can divide a network up into separate subnets so that only traffic destined for
particular IP addresses can pass between segments. With all of this processing,
routers take more time to process packets than do bridges, but in more complex
networks, overall efficiency is improved.
Static routers require an administrator to manually configure a routing table stored in
the device that contains the current network topology, whereas dynamic routers
discover network topology much like learning bridges. Dynamic routers examine
information from other routers and make packet-by-packet decisions about how to
send data across the network.
Advantages of routers:
• Routers can choose the best path through a network when there are multiple
paths to a destination.