Chapter Overview Bridging Switching Routing

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

Connecting LANs with a Bridge

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.

Broadcasts

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.

Transparent Bridging Transparent bridging is a method for automatically compiling a bridge’s 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.

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

Translation Bridge

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

Switching

Replacing Routers with Switches

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.

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.

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.

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

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

Routing Process Example

Hops

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

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

Chapter Summary Bridging Switching Routing 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.