Chapter Overview

1
Chapter Overview

• Bridging
• Switching
• Routing

2
What Is a Bridge?

• A data-link layer device that connects networks
  and filters packets
• Propagates only packets destined for the other
  side of the bridge
• Can reduce network traffic and collisions
• Can lessen delays

3
Connecting LANs with a Bridge
4
Bridges and Collisions

• A collision domain is a network (or part of a
  network) that is constructed so that a collision
  occurs when two computers transmit packets at
  precisely the same time.
• Adding a hub to a local area network (LAN)
  increases its size but maintains a single
  collision domain because hubs relay signals
  immediately at the physical layer, without
  filtering them.
• Hubs do not relay packets immediately they wait
  until the entire packet is received.
• Because the bridge delays propagation, computers
  on opposite sides of the bridge transmitting at
  the same time do not cause a collision.
• Adding a bridge to a LAN splits it into two
  collision domains, resulting in fewer collisions
  and increased efficiency.

5
Broadcasts
6
Bridges and Broadcasts

• Bridges propagate all broadcast packets without
  filtering them.
• The network segments on either side of a bridge
  are in the same broadcast domain.

7
Transparent Bridging

• Transparent bridging is a method for
  automatically compiling a bridges address
  tables.
• Bridges maintain an address table for each
  network segment.
• Bridges read the source and destination addresses
  of packets and compare them to the tables to
  determine whether to propagate them.
• When a bridge lacks the information needed to
  filter a packet, it propagates the packet by
  default.

8
Bridge Types

• Local bridge. Connects network segments of the
  same type and at the same location
• Translation bridge. Connects network segments at
  the same location that use different media or
  different protocols
• Remote bridge. Connects network segments at
  remote locations, using a wide area network (WAN)
  link

9
Translation Bridge
10
Switch Characteristics

• Data-link layer device
• Replaces bridges and sometimes routers
• Similar in appearance to a hub
• Forwards incoming traffic out to the destination
  port only
• Converts a shared network medium to a dedicated
  one
• Offers advantages such as reduced network
  congestion and the use of full network bandwidth
• Can operate in full-duplex mode

11
Switching
12
Replacing Routers with Switches
13
Virtual LANs

• A virtual LAN (VLAN) is a subnet that exists
  inside a switch.
• Broadcasts sent by a computer go only to the
  other computers in the VLAN.
• Communications within a VLAN are switched.
• Communications between VLANs can be routed or
  switched.

14
Layer 3 Switching

• Layer 3 switching minimizes the amount of routing
  between VLANs because routing occurs only when
  absolutely necessary.
• A router establishes a connection between systems
  and then switches take over.

15
Switch Types

• A cut-through switch begins to forward packets
  immediately.
• A store-and-forward switch waits until the entire
  packet arrives before forwarding it.

16
Router Characteristics

• Connect networks together to form an internetwork
• Are network layer devices
• Can connect LANs running different data-link
  layer protocols

17
Router Functions

• Forward packets to the destination network
• Strip the data-link layer frames from incoming
  packets and repackage the data into new frames
• Fragment packets when necessary

18
Routing Process Example
19
Hops
20
Routing Tables

• Routing tables are essential to the router
  operation and functions.
• There are two methods of creating routing tables
  
• Static routing. Manual creation of routing table
  entries by an administrator
• Dynamic routing. Automatic creation of routing
  table entries by using a specialized routing
  protocol

21
Router Types

• Routers can be stand-alone hardware devices or
  software running on a computer.
• Hardware routers. Range from expensive
  rack-mounted devices to small stand-alone units
• Software routers. Include Microsoft Windows 2000
  Internet Connection Sharing (ICS), and Routing
  and Remote Access

22
Chapter Summary

• Bridging
• Bridges selectively relay packets between network
  segments, depending on their data-link layer
  destination addresses.
• Bridges maintain a single broadcast domain and
  create separate collision domains.
• Switching
• Switches improve on the function of bridges by
  forwarding packets only to their destination
  systems.
• Switches reduce the collisions on a network and
  increase the bandwidth available to each
  computer.
• Routing
• Routers are used to connect networks together at
  the network layer of the OSI reference model.
• Routers strip away the data-link layer frame of
  incoming packets and build a new frame using the
  data-link layer protocol of the outgoing network.
  
• Routing tables can be created manually by a
  network administrator or automatically by a
  routing protocol.