Which layer of the OSI model does a bridge use to make decisions about forwarding data packets?

campus network (pg. 224)

a collection of two or more interconnected LANs, either within a building or housed externally in multiple buildings.

bridge (pg. 225)

A layer 2 device in the OSI model, meaning that it uses the MAC address information to make decisions regarding forwarding data packets. Only the data that needs to be sent across the bridge to the adjacent network segment is forwarded.

Bridge Example:

An example of using a bridge to segment two Ethernet LANs is shown in Figure 5-1. The picture shows that LAN A connects to port 1 of the bridge and LAN B connects to port 2 on the bridge, creating two segments, as shown. There are four computers in LAN A and three computers in LAN B. Bridges monitor all data traffic in each of the LAN segments connected to its ports.

bridging table (pg. 225)

List of MAC addresses and port locations for hosts
connected to the bridge ports Section 5

association (pg. 226)

Indicates that the destination MAC address for a host is connected to one of the ports on the bridge. If an association is found, the data is forwarded to that port.

For example, assume that computer I sends a message to
computer 5 (see Figure 5-1). The bridge detects an association between the destination MAC address for computer 5 and port 2. The bridge then forwards the data from computer 1 to computer 5 in LAN B via port 2.The capability of a bridge to forward data packets only when there is an association is used to isolate data traffic in each segment.

broadcast pg. 226

means the message is being sent to all computers on the network; therefore, all broadcasts in a LAN will be forwarded to all hosts connected within the bridged LANs. For example, the broadcast associated with an ARP will appear on all hosts.

ARP pg. 226

ARP stands for Address Resolution Protocol, which is a protocol used to map an IP address to its MAC address. In the address resolution protocol, a broadcast is sent to all hosts in a LAN connected to the bridge.

ARP Example:

This is graphically shown in Figure 5-2. The bridge forwards all broadcasts; therefore, an ARP request broadcasting the message "Who has this IP address?" is sent to all hosts on the LAN. The data packets associated with ARP requests are small, but it requires computer time to process each request.

broadcast storm (pg. 226)

Excessive amounts of broadcasts being forwarded by the bridge

network slowdown (pg.226)

Excessive amounts of broadcasts being forwarded by the bridge can lead to a broadcast storm, resulting in degraded network performance,

ARP cache (pg. 227)

all networking devices (for example, computers) contain
an ARP cache, a temporary storage of MAC addresses recently contacted.

ARP cache notes: (pg. 228)

The ARP cache holds the MAC address of a host, and this
enables the message to be sent directly to the destination MAC address without the computer having to issue an ARP request for a MAC address. The ARP cache contents on a Windows computer can be viewed using the arp -a command while in the command prompt, as shown here:
Windows Mac OS X
C:\arp -a jmac:~mymac$ arp -a

The following message is generated if all the ARP entries have expired:
c: \arp -a
No ARP Entries Found

ARP cache Example:

Following list outlines typical steps of a communication process between computer 1 and computer 2.

1. Computer 1 checks its ARP cache to determine if it already has the MAC address of computer 2. If it does, it will skip to the final step; otherwise, it proceeds to the next step.
2. Computer 1 generates an ARP request message for computer 2 with its own MAC and IP information included.
3. Computer 1 then broadcasts the ARP request message on its local network.
4. Every local network device processes the ARP request message. Those computers that are not computer 2 will discard the message.
5. Only a match, which is computer 2, generates an ARP reply message and updates its ARP cache with computer 1 MAC and IP information.
6. Computer 2 sends an ARP reply message directly to computer 1.
7. Computer 1 receives the ARP reply message and updates its ARP cache with the MAC and IP of computer 2.

ARP table (pg. 227)

Another name for the ARP cache

transparent bridge (pg. 228)

The name for the type of bridge used to interconnect two LAN s running the same type of protocol (for example, Ethernet)

translation bridge (pg. 228)

Bridges are also used to interconnect two LANs that are operating two different networking protocols. For example, LAN A could be an Ethernet LAN and LAN B could be a token ring.

Translation Bridge Example:
(pg. 232)

An example is provided in Figure 5-3. The bridge allows data from one LAN to be transferred to another. Also the MAC addressing information is standardized so the same address information is used regardless of the protocol. A common application today using a bridge is interconnecting LANs using wireless technology. The use of wireless bridges in LAN s is a popular choice for interconnecting the LANs when the cost of physically connecting them is prohibitive.

Networking Bridge
(advantages & disadvantages)(pg. 229)

Table 5-3 Summary Of the Advantages and disadvantages Of a Bridge for Interconnecting LANs:
Advantages:
1. Easy to install
2. Does an excellent job of isolating the data traffic in two segments
3. Relatively inexpensive
4. Can be used to interconnect two LANs with different protocols and hardware Reduces collision domains (remember how the CSMA/CDprotocol works)

Disadvantages:
1. Works best in low-traffic areas
2. Forwards broadcasts and is susceptible to broadcast storms

layer 2 switch (pg. 231)

an improved network technology that addresses the issue
of providing direct data connections, minimizing data collisions, and maximizing the use of a LAN's bandwidth; in other words, that improves the efficiency of the data
transfer in the network.

Layer 2 Switch notes: (pg. 231)

1. The switch operates at layer 2 of the OSI model and therefore uses the MAC or Ethernet address for making decisions for forwarding data packets.
2. The switch monitors data traffic on its ports and collects MAC address information in the same way the bridge does to build a table of MAC addresses for the devices connected to its ports.
3.The switch has multiple ports similar to the hub and
can switch in a data connection from any port to any other port, similar to the bridge. This is why the switch is sometimes called a multiport bridge.
4.The switch minimizes traffic congestion and isolates data traffic in the LAN. Figure 5-4 provides an example of a switch being used in a LAN.

multiport bridge: (pg. 231)

Another name for a layer 2 switch (because it has multiple ports similar to the hub and can switch in a data connection from any port to any other port, similar to the bridge).

Switch interconnecting a network
(pg. 231)(Figure 5-5)

The LAN shown in Figure 5-5 contains 14 computers and 2 printers connected to 16 ports on the switch, configured in a star topology. lf the computer connected to port 1 is printing a file on the laser printer (port 12), the switch will set up a direct connection between ports 1 and 12. The computer at port 14 could also be communicating
with the computer at port 7, and the computer at port 6 could be printing a file on the color printer at port 16. The use of the switch enables simultaneous direct data connections for multiple pairs of hosts connected to the network. Each switch connection provides a link with minimal collisions and therefore maximum use of the
LAN's bandwidth. A link with minimal collisions is possible because only the two computers that established the link will be communicating over the channel. Recall
that in the star topology each host has a direct connection to the switch. Therefore, when the link is established between the two hosts, their link is isolated from any
other data traffic. However, the exception to this is when broadcast or multicast messages are sent in the LAN. In the case of a broadcast message, the message is sent to
all devices connected to the LAN. A multicast message is sent to a specific group of hosts on the network.

multicast (pg. 232)

A multicast message is sent to a specific group of hosts on the network.

Hub-Switch Comparison (fig 5-6) (pg. 232)

1.The objective of this experiment was to show that data traffic is isolated with a switch but not with a hub.
2.For this experiment, a LAN using a hub and a LAN using a switch were assembled.
3.Each LAN contains four computers connected in a star topology.
4.In this experiment, computer 1 pinged computer 3. Computer 2 was used to capture the LAN data traffic using a network protocol analyzer.
5.Remember, a hub is a multiport repeater, and all data traffic input to the hub is passed on to all hosts connected to its ports.

ping (pg. 233)

command is used to verify that a network connection exists between two computers. The command format for ping is:

ping [ip address] {for this example ping 10.10.10_3}

PING COMMAND REVIEW

1.a series of echo requests and echo replies are issued by the networking devices to test the time it takes for data to
pass through the link.
2.The protocol used by the ping command is the Internet Connection Message Protocol (ICMP).
3.The ping command is issued to an IP address; however, delivery of this command to the computer designated by the IP address requires that a MAC address be identified for final delivery.
4.The computer issuing the ping might not know the MAC
address of the computer holding the identified IP address (no entry in the ARP cache table); therefore, an ARP request is issued.
5.An ARP request is broadcast to all computers connected in the LAN.
6.The computer that holds the IP address replies with its
MAC address, and a direct line of communications is then established.
7.

The Switch Experimental Results (pg. 234)(fig 5-6(b))

1.The ping command was sent from computer 1 to computer 3, ping 10.10.10.3.
2.The ARP cache for computer 1 is empty; therefore, the MAC address for computer 3 is not known by computer 1. 3.An ARP request is issued by computer 1, and computer 3 replies.
4. The series of echo requests and echo replies follow; however, the data traffic captured by computer 2 (Figure 5-8), shows the ARP request asking who has the IP address 10.10.10.3.
5. This is the last of the data communications between computers 1 and 3 seen by computer 2.
6. A directline of communication between computers 1 and 3 is established by the switch that prevents computer 2 from seeing the data traffic from computers 1 and 3.
7. The only data traffic seen by computer 2 in this process was the broadcast of the ARP request.
8.The results of this experiment show that the use of the switch substantially reduces data traffic in the LAN, particularly unnecessary data traffic.
9.The experiment shows that the broadcast associated with an ARP request is seen by all computers but not the ARP replies in a LAN using a switch.
10.Remember, the switch uses MAC addresses to establish which computers are connected to its ports. The switch
then extracts the destination MAC address from the Ethernet data packets to determine to which port to switch the data.

managed switch (pg. 235)

simply a network switch that allows the network administrator to monitor, configure, and manage certain network features such as which computers are allowed to access the LAN via the switch.

Cisco Network Assistant (pg. 235)
(CNA) Configure> Device Properties> IP Addresses.

1.A management software tool from Cisco that simplifies
switch configuration and troubleshooting.
2.Provides for a centralized mode for completing various network administration tasks for switches, routers, and wireless networking equipment.

dynamic assignment (pg. 236)

means that the MAC address was assigned to a port when a host was connected.

static assignment (pg. 236)

indicates that the MAC address has been manually assigned to an interface, and the port assignment does not
expire.

Secure tab (pg. 236)

shows what switch ports have been secured.

secure addresses (pg. 236)

means that a MAC address has been assigned to a port, and the port will automatically disable itself if a device with a different MAC address connects to the secured
port.

aging time (pg. 237)

1. the length of time a MAC address remains assigned to a port.
2. if time expires MAC address will be removed.
3. if computer initiates new data activity, aging time counter restarts & MAC address remains assigned to port

Network Switch-benefits (Pg. 239)

1. The benefits of using a network switch are many in a modern computer network.
2. These benefits include less network congestion, faster data transfers, and excellent manageability.
3. It has been shown that a network switch can be used to replace the network hub, and the advantage is that data traffic within a LAN is isolated.

ping (pg. 233)

...

isolating the collision-domain (pg. 239)

1. Breaking the network into segments where a segment is
a portion of the network where the data traffic from
one part of the network is isolated from the other networking devices.
2. A direct benefit of isolating collision domains is that there will be an increase in the data transfer speed and
throughput.
3. This is due to the fact that the LAN bandwidth is not being shared and chances of data collisions are minimized. 4. As a result, the LAN will exhibit faster data transfers and latency within the LAN will be significantly reduced.
5. Reduced latency means that the data packets will arrive at the destination more quickly.

time-stamp (pg. 239)
((CAM))

establishes the time when the mapping of the MAC address to a switch port is established.

What happens if the destination MAC address is not stored in CAM? (pg. 239)

In this case, the packet is transmitted out all switch ports except for the port where the packet was received. This is called flooding.

flooding (pg. 239)

The term used to describe what happens when a switch
doesn't have the destination MAC address stored in CAM.

broadcast domain (pg. 239)

1. any network broadcast sent over the network will be seen by all networking devices in the same network. Broadcasts within a LAN will be passed by switches.
2. switches do not reduce the broadcast domain.

store-and- forward (pg. 239)

1. In this mode, the entire frame of data is received before
any decision is made regarding forwarding the data packet to its destination.

2. There is switch latency in this mode because the destination and source MAC addresses must be extracted from the packet, and the entire packet must be received
before it is sent to the destination.

3. An advantage of the store-and- forward mode is that the switch checks the data packet for errors before it is sent on to the destination.

4. A disadvantage is lengthy data packets will take a longer time before they exit the switch and are sent to the
destination.

switch latency (pg. 239)

the length of time a data packet takes from the time it enters a switch until it exits.

content addressable memory (CAM) (pg. 239)

1. CAM is a table of MAC address and port mapping used by the switch to identify connected networking devices.

2. The extracted MAC addresses are then used by the switch to map a direct communication between two network devices connected to its ports.

3. The MAC address and port information remain in CAM as long as the device connected to the switch port remains active.

cut-through (pg. 239-240)

1. In this mode, the data packet is forwarded to the destination as soon as the destination MAC address has been read.

2. This minimizes the switch latency; however, no error detection is provided by the switch.

Cut-through forms (pg. 240)

1. Fast-Forward: This mode offers the minimum switch latency. The received data packet is sent to the destination as soon as the destination MAC address is extracted.

2. Fragment-Free: In this mode, fragment collisions are filtered out by the switch. Fragment-collisions are collisions that occur within the first 64 bytes of
the data packet.
a) The collisions create packets smaller than 64 bytes, which are discarded.
b) Latency is measured from the time the first bit is received until it is transmitted.

3. Adaptive Cut-Through: This is a combination of the store-and-forward mode and cut-through.
a) The cut-through mode is used until an error threshold (errors in the data packets) has been exceeded.
b) The switch mode changes from cut-through to store-and-forward after the error threshold has been exceeded.

Adaptive Cut-Through: (pg. 240)

This is a combination of the store-and-forward mode and cut-through.

error threshold (pg. 240)

The point where the number of errors in the data packets
has reached a threshold and the switch changes from the
cut-through to the store-and-forward mode.

multilayer switch (MLS) (pg. 240)

1. Operates at layer 2 but functions at the higher
layers.
2. Multilayer switches can also work at the upper layers of the OSI model.
3. An example is a layer 3 switch.

layer 3 switch (pg. 240)
((MLS))

1. Layer 3 switches still work at layer 2 but additionally
work at the network layer (layer 3) of the OSI model and use IP addressing for making decisions to route a data packet in the best direction.
2. The major difference is that the packet switching in basic routers is handled by a programmed
microprocessor.
3. The layer 3 switch uses application-specific integrated circuits (ASICs) hardware to handle the packet switching. 4. The advantage of using hardware to handle the packet switching is a significant reduction in processing time (software versus hardware).
5. In fact, the processing time of layer 3 switches can be as fast as the input data rate.
6.

wire speed routing (pg. 240) (layer 3 switch)

where the data packets are processed as fast as they are arriving.

layer 4 switch

processes data packets at the transport layer of the OSI model.

network address (pg. 241)

1. Another name for the layer 3 address

2. The router is a layer 3 device in the OSI model, which means the router uses the network address (layer 3 addressing) to make routing decisions regarding forwarding data packets.

3. In the OSI model, the layer 3 or network layer responsibilities include handling of the network address. The network address is also called a logical address, rather than being a physical address such as the MAC address.

logical address (pg. 241)

describes the IP address location of the network and the address location of the host in the network.

physical address (pg. 241)

is the hardware or MAC address embedded into the
network interface card.

Routers

1. are used to interconnect LANs in a campus network.
2. Routers can be used to interconnect networks that use the same protocol (for example, Ethernet), or they can be used to interconnect LANs that are using different layer 2 technologies such as an Ethernet and token ring.
3. Routers also make it possible to interconnect to LANs around the country and the world and interconnect to many different networking protocols.
4. Routers have multiple port connections for connecting to the LANs, and by definition a router must have a minimum of three ports.
5. Routers use the information about the network segments to determine where to forward data packets.

routing table (pg. 249)

Keeps track of the routes to use for forwarding data to its
destination.

Routing table Example (pg. 249)(fig. 5-21)

1. RouterA used its routing table to determine a network
data path so computer AI's data could reach computer Dl in LAN D.
2. RouterA determines that a path to the network where computer Dl is located can be obtained via RouterA's FAO/2 FastEthernet port to the FAOI2 FastEthernet port on RouterC.
3. RouterC determines that computer Dl is on LAN D, which connects to RouterC's FAOIO FastEthernet port.
4. An ARP request is issued by RouterC to determine the MAC address of computer D 1.
5. The MAC address is then used for final delivery of the data to computer Dl.
6. If RouterA determines that the network path to RouterC is down, RouterA can route the data packet to RouterC through RouterB.
7. After RouterB receives the data packet from RouterA, it uses its routing tables to determine where to forward the data packet.
8. RouterB determines that the data needs to be sent to RouterC, and it uses the FAO/3 FastEthernet port to forward the data.

gateway (pg. 249)

1. is used to describe the address of the networking device that enables the hosts in a LAN to connect to networks and hosts outside the LAN.

For example, for all hosts in LAN A, the gateway address will be 10.10.10.250. This address is configured on the host computer. Any IP packets with a destination outside the
LAN will be sent to the gateway address.

network segment (pg. 249)

1. defines the networking link between two LANs. There is a segment associated with each connection of an internetworking device (for example, router-hub, router-switch, router-router).
2. Routers use the information about the network segments to determine where to forward data packets.

For example, the IP address for the network segment connecting LAN A to the router is lO.lO.20.0. All hosts connected to this segment must contain a 10.lO.20.x because a subnet mask of 255.255.255.0 is being used.

auto-negotiation

1. Protocol used by interconnected electronic devices to negotiate a link speed.
2.Auto-negotiation is established when an Ethernet link is established.
3. The link information is only sent one time, when the link is established.
4. The negotiated link configuration will remain until the link is broken or the interfaces are reconfigured

ADVANTAGES
1. Useful in LANsthat have multiple users with multiple connection capabilities.
2. The auto-negotiation feature can maximize the data links' throughput.

DISADVANTAGES
3. Not recommended for fixed data links such as the backbone in a network.
4. A failed negotiation on a functioning link can cause a link failure.

Auto-Negotiation Steps (pg. 251)

1. Each link partner shares or advertises its data link capabilities with the other link partner.
2. The two link partners then use the advertised capabilities to establish the fastest possible data link rate for both links.
3. In the example of the link partners shown in Figure
5-22, computer 1 advertises that its interface supports lOMbps.
4. The switch advertises that it supports both lOMbps and lOOMbps.
5. The network interfaces on each link partner are set for auto-negotiation; therefore, the IOMbps operating mode is selected.
6. This is the fastest data rate that can be used in this data link.

half-duplex (pg. 252)(FIG. 5-23)

The communications device can transmit or receive but
not at the same time.

full-duplex (pg. 252)(fig 5-23)

1. means that the communications device can transmit and receive at the same time.
2. An example of networking devices that can run full-duplex are computers connected to a switch.
3. The switch can be configured to run the full-duplex
mode. This also requires that each end station on the link must be configurable to run full-duplex mode.
4. In full-duplex operation (10/100Mbps), the media must have separate transmit and receive data paths. This is provided for in CAT6/5e/5 cable with pairs 1-2 (transmit) and pairs 3-6 (receive).
5. Full-duplex with gigabit and 10 gigabit data rates
require the use of all four wire pairs (1-2, 3-6,4-5, 7-8).
6. An important note is that the full-duplex mode in computer network links is only for point-to-point links.

fast link pulse (FLP) (pg. 251)

1. Carries the configuration information between each
end of a data link.
2. The data rate for the fast link pulses is lOMbps, the same as for 1OBASE-T.
3. The link pulses were designed to operate over the limited bandwidth supported by CAT3 cabling.
4. Therefore, even if a link is negotiated, there is no guarantee that the negotiated data rate will work over the link.

console input (pg. 242)

1. This input provides an RS-232 serial communications link into the router for initial router configuration.
2. A special cable, called a console cable, is used to connect the console input to the serial port on a computer. 3. The console cable can have RJ-45 plugs on each end and requires the use of an RJ-45 to DB9 adapter for connecting to the computer's COMl or COM2 serial port.
4. The console cable can also have an RJ-45 connector on one end and an integrated DB9 connector on the other end.

media converter (pg. 245)

1. Media converters are commonly used in computer networks to adapt layer 1 or physical layer technologies from one technology to another.
2. is used to convert the 15-pin AUI port to the 8-pin RJ-45 connector.

For example:
AUI to twisted pair (RJ-45) AUI to fiber
RJ-45 to fiber

enterprise network (pg. 247)

1. Term used to describe the network used by a large
company.
2. The use of the network or logical address on computers allows the information to be sent from a LAN to a destination without requiring that the computer know the MAC address of the destination computer.

USB Interface: (pg. 242)

The USB ports are used for storage and security support

Serial Interface (pg. 242)

1. CTRLR Tl land CTRLR Tl O.
2. This is a serial connection, and it has a built-in CSU/DSU.
3. This interface is used to provide a T1 connection to the communications carrier.
4. The serial interfaces are typically used to interconnect
LAN s that connect through a data communications carrier such as a telephone company (Telco).

• AL-alarm
• LP-loop
• CD-Carrier Detect

FastEthernet port (FAO/O, / FAO/I, FAOI2, ... ) (pg. 242)

1. Naming of the FastEthernet ports on the router.

FEOIO: Fast Ethernet (lOIlOOMbps) and FEOIl: Fast
Ethernet (lOIlOOMbps).

small interconnected LAN (Fig 5-21)(pg. 247-248)

1. This example shows four Ethernet LANs interconnected using three routers.
2. The LANs are configured in a star topology using switches at the center of the LAN.

Router A connects directly to the LAN A switch via FastEthernet port FAO/O. Router A also connects directly to Router B via the FastEthernet port FAo/l and
connects to Router C via FastEthernet port FA012.

• Router B connects directly to the LAN B switch via FastEthernet port FAO/O. Router B connects to the LAN C switch via FastEthernet port FAo/I. Router B connects directly to Router A via FastEthernet port FA012 and connects to Router C via FastEthernet port FAO/3.

• Router C connects directly to the LAN D switch via the FastEthernet port FAO/O. Connection to Router B is provided via Ethernet port FAO/I. Router C connects to Router A via FastEthernet port FA012.

Voice Interface Card (VIC2-4FXO): (pg. 243)

This interface shows four phone line connections. This router can be programmed as a small Private Branch
Exchange (PBX) for use in a small office.

WAN Interface Card (WIC2AM): (pg. 243)

This interface has two RJ-l1 jacks and two V90 analog internal modems. These modems can be used to handle both incoming and outgoing modem calls.

VIC-4FXSIDID: (pg. 243)

1. This interface is a four-port FXS and DID voice/fax interface card.
2. FXS is a Foreign Exchange Interface that connects directly to a standard telephone.
3. DID is Direct Inward Dialing and is a feature that enables callers to directly call an extension on a PBX.

serial port (SO/O,SO/I, SOI2, ... ) (pg. 248)

1. Naming of the serial ports on the router.

serial ports (pg. 243)

Provides a serial data communication link into and out of the router, using V35 serial interface cables.

DSU Port (pg. 244)

1. This Tl controller port connection is used to make the serial connection to Telco.
2. This module has a built-in CSU/DSU module.
3. There are five LEDs next to the RJ45 jack.
These LEDs are for the following:

*TD-Transmit Data
* LP-Loop
* RD-Receive Data
* CD-Carrier Detect
* AL-Alarm

Ethernet Port (pg. 244)

This connection provides a lO/lOOMbps Ethernet data link.

Analog Modem Ports (pg. 244)

This router has a 16-port analog network module.

router interface (pg. 242)(fig 5-14)

1. Often the router ports are called the router interface, the physical connection where the router connects to the network.
2. The arrows pointing in and out indicate that data enters and exits the routers through multiple ports.
3. The router ports are bidirectional, meaning that data can enter and exit the same router port.
4. The router interface provides a way to access the router for configuration either locally or remotely.
5. Interfaces are provided for making serial connections to the router and to other devices that require a serial communications link.
6. For example, interfaces to wide area networking devices require a serial interface.

auxiliary input (pg. 242)

This input is used to connect a dial-in modem into the router. The auxiliary port provides an alternative way to remotely log in to the router if the network is down. This port also uses an RJ-45 connection.

power on/off (pg. 243)

Turns on/off electrical power to the router.

AUI port (pg. 244)

This is a lOMbps Ethernet port. AUI stands for "attachment unitinterface."

What OSI layer is a bridge?

Where does bridge operation in OSI model?

What is Layer 2 bridging?

What do bridges and switches use to make data forwarding decisions?