MI361 Ch8 Review Questions
Under what circumstances would you use a VLAN backbone?
A VLAN backbone is useful when you want to put computers that are in different geographic locations in the same subnet. In addition, VLANs make it much simpler to manage the broadcast traffic and provide a better opportunity to prioritize traffic on the network.
What is a module and why are modules important?
A module is any of certain types of network devices that can be plugged directly into a chassis switch. Since a chassis switch must be able to support simultaneous activities of all connected module, each switch has an internal capacity (in Mbps) which limits the maximum number of modules that can be accepted by the switch. Modules can be switches, hubs, or routers.
Explain how multiswitch VLANs work.
A multi-switch VLAN works the same way as a single switch VLAN, except that several switches are used to build the VLANs. The switches must be able to send packets among themselves in a way that identifies the VLAN to which the packet belongs. There are two approaches to this: packet encapsulation and modifying the Ethernet packet. - In the encapaulation approach a proprietary protocol encapsulates the packet. When a packet needs to go from one VLAN switch to another VLAN switch, the first switch puts a new VLAN packet around the outside of the Ethernet packet. The VLAN packet contains the VLAN information and is used to move the packet from switch to switch within the VLAN network.. When the packet arrives at the final destination switch, the VLAN packet is stripped off and the unchanged Ethernet packet inside is sent to the destination computer. - In the modification approach the Ethernet packet itself is to modified to carry the VLAN information. 16-bytes of VLAN information (according to emerging standard IEEE 802.1q) are added to the standard Ethernet (IEEE 802.3) packet. The additional VLAN information is used to move the packet from switch to switch within the VLAN network. The original Ethernet packet is restored from the modified packet at the final destination switch and then sent to the destination computer.
What are the key advantages and disadvantages of routed and switched backbones?
Advantages - Routed Backbone Clear segmentation of parts of the network connected to the backbone as each network has a subnet address and can be managed separately. - Switched Backbone Performance is improved. With the traditional backbone network, the backbone circuit was shared among many LANs; each had to take turns sending messages. With the collapsed backbone, each connection into the switch is a separate point-to-point circuit. The switch enables simultaneous access, so that several LANs can send messages to other LANs at the same time. Throughput is increased significantly, often by 200% to 600%, depending upon the number of attached LANs and the traffic pattern. Since there are far fewer networking devices in the network, this reduces costs and greatly simplifies network management. All the key backbone devices are in the same physical location, and all traffic must flow through the switch. If something goes wrong or if new cabling is needed, it can all be done in one place. Software reconfiguration replaces hardware reconfiguration. Disadvantages - Routed Backbone Slower performance as routing takes more time than bridging or switching. Management and/or software overhead costs due to need to establish subnet addressing and provide reconfiguration when computers are moved (or support dynamic addressing). - Switched Backbone Because data link layer addresses are used to move packets, there is more broadcast traffic flowing through the network and it is harder to isolate and separately manage the individually attached LANs. Layer 3 switches can use the network layer address, so future collapsed backbones built with layer 3 will not suffer from this problem. Collapsed backbones use more cable, and the cable must be run longer distances, which often means that fiber optic cables must be used. If the switch fails, so does the entire backbone network. If the reliability of the switch has the same reliability as the reliability of the routers, then there is less chance of an failure (because there are fewer devices to fail). For most organizations, the relatively minor disadvantages of cable requirements and impacts of potential switch failure are outweighed by the benefits offered by collapsed backbones.
What are the advantages and disadvantages of VLANs?
Advantages: VLANs are often faster and provide greater opportunities to manage the flow of traffic on the LAN and BN than do the traditional LAN and routed BN architecture. Allow the ability to prioritize traffic They allow computers in separate geographic locations to be placed on the same LAN. Disadvantages: However, VLANs are significantly more complex, so they usually are used only for large networks. Cost
What are the preferred architectures used in each part of the backbone?
Answers can vary depending on preferences, but suggestions include: Access - switched backbones Distribution layer - Virtual LAN Core layer - routed backbones
What is IEEE 802.1q?
IEEE 802.1q is an emerging standard that inserts 16-bytes of VLAN information into the normal IEEE 802.3 Ethernet packet. When a packet needs to go from one VLAN switch to another VLAN switch, the first switch replaces the incoming Ethernet packet with an 802.1q packet that contains all the information in the original 802.3 Ethernet packet, plus 16-bytes of VLAN information. The additional VLAN information is used to move the packet from switch to switch within the VLAN network. When the packet arrives at the final destination switch, the IEEE 802.1q packet is stripped off and replaced with a new Ethernet packet that is identical to the one with which it entered the VLAN.
How can you improve the performance of a BN?
Improving the performance of backbone networks is similar to improving LAN performance. First, find the bottleneck, and then solve it (or more accurately, move the bottleneck somewhere else). You can improve the performance of the network by improving the computers and other devices in the network, by upgrading the circuits between computers, and by changing the demand placed on the network. Network performance can be improved by upgrading the computers and other devices in the network, by using static rather than dynamic routing if there are few routes through the network, by reducing switch-to-switch traffic in networks without standard protocols, by using the same protocols in the backbone network as in the attached LANs, by encapsulating rather than translating between different protocols, and by increasing the memory in backbone devices. Performance can also be improved by adding additional circuits to increase capacity, by going to a switched network, and by increasing the circuits on high traffic circuits. In addition, performance can be enhanced by reducing demand or by restricting applications that use lots of network capacity, and by using switches that filter certain broadcast messages.
Explain how single-switch VLANs work.
In a single switch VLAN the VLAN operates only inside one switch. The computers on the VLAN are connected into the one switch and assigned by software into different VLANs. The network manager uses special software to assign the dozens or even hundreds of computers attached to the switch to different VLAN segments. The VLAN segments function in the same way as physical LAN segments; the computers in the same VLAN act as though they are connected to the same physical switch or hub.
How does a layer-2 switch differ from a router?
Layer 2 switches operate by using the data link layer address or MAC address to forward packets between network segments. They connect the same or different types of cable. Layer-2 switches (or workgroup switches) operate at the Data Link layer, and typically provide ports for a small set of 16 to 24 computers. Layer-2 switches enable all ports to be in use simultaneously by managing paired combinations of ports as separate point-to-point circuits. Layer-2 switches "learn" addresses; a layer-2 switch builds a forwarding table after it is first turned on. To learn addresses, a layer-2 switch retransmits to all ports (except to the one from which it was received) only for a packet with a destination address not already in the forwarding table. The resulting ACK from the destination computer (that recognized its address) is then used by the layer-2 switch to add the new port number and address to the forwarding table. Routers operate at the network layer. They connect two different TCP/IP subnets. Routers strip off the data link layer packet, process the network layer packet, and forward only those messages that need to go to other networks on the basis of their network layer address. In general, they perform more processing on each message than switches and therefore operate more slowly.
How does a layer-2 switch differ from a VLAN?
Layer 2 switches operate by using the data link layer address or MAC address to forward packets between network segments. They connect the same or different types of cable. Layer-2 switches (or workgroup switches) operate at the Data Link layer, and typically provide ports for a small set of 16 to 24 computers. Layer-2 switches enable all ports to be in use simultaneously by managing paired combinations of ports as separate point-to-point circuits. Layer-2 switches "learn" addresses; a layer-2 switch builds a forwarding table after it is first turned on. To learn addresses, a layer-2 switch retransmits to all ports (except to the one from which it was received) only for a packet with a destination address not already in the forwarding table. The resulting ACK from the destination computer (that recognized its address) is then used by the layer-2 switch to add the new port number and address to the forwarding table. VLAN switches work a little differently. When a VLAN switch receives a frame that is destined for another computer in the same subnet on the same VLAN switch, the switch acts as a traditional layer-2 switch: it forwards the frame unchanged to the correct computer. VLAN switches use Ethernets 802.1q's tagging to move frames from one switch to another. When a VLAN switch receives an Ethernet frame that needs to go to a computer on another VLAN switch, it changes the Ethernet frame by inserting the VLAN ID number and a priority code into the VLAN tag field.
Under what circumstances would you want to use a routed backbone?
Routed backbones are good for connecting different buildings on the same enterprise campus backbone network. The primary advantage of the routed backbone is that it clearly segments each part of the network connected to the backbone. Each segment has its own subnet addresses that can be managed by a different network manager. Broadcast messages stay within each subnet and do not move to other parts of the network.
Explain how routed backbones work.
Routed backbones move packets along the backbone based on their network layer address (i.e., layer 3 address). The most common form of routed backbone uses a bus topology (e.g., using Ethernet 100Base-T). Routed backbones can be used at the core or distribution layers. At the core layer routed backbones are sometimes called subnetted backbones or hierarchical backbones and are most commonly used to connect different buildings within the same campus network. At the distribution layer a routed backbone uses routers or layer 3 switches to connect a series of LANs (access layer) to a single shared media backbone network. Each of the LANs are a separate subnet. Message traffic stays within each subnet unless it specifically needs to leave the subnet to travel elsewhere on the network, in which case the network layer address (e.g., TCP/IP) is used to move the packet.
Why are broadcast messages important?
Some application software packages and network operating system modules written for use on LANs broadcast status messages to all computers on the LAN (but not necessarily all computers served by a BN). For example, broadcast messages inform users when printers are out of paper, or when the network manager is about to shut down the server. These types of messages require filtering in a backbone network if their broadcast scope should be restricted to a particular LAN or segment.
Explain how switched backbones work.
Switched backbone networks use a star topology with one device, usually a switch, at its center. The traditional backbone circuit and set of routers or bridges is replaced by one switch and a set of circuits to each LAN. The collapsed backbone has more cable, but fewer devices. There is no backbone cable. The "backbone" exists only in the switch, which is why this is called a collapsed backbone. The original collapsed backbone technology uses layer-2 switches and suffers some disadvantage due to the load of data link layer overhead message traffic and limitations on network segmentation. As this weakness has been recognized, collapsed backbone technology is adapting by evolving to the use of layer-3 switches to overcome these problems. The result is better performance and improved network management capabilities for switched backbone networks. Collapsed backbones are probably the most common type of backbone network used in the distribution layer (i.e., within a building). Most new building backbone networks designed today use collapsed backbones. They also are making their way into the core layer as the campus backbone, but routed backbones still remain common.
Under what circumstances would you use a switched backbone?
Switched backbones can be used in situations where the network administrators wants to spread the traffic around the network more efficiently. In addition, it also provides an architecture where network capacity is no longer tied to the physical location of the computers, as computers in
Compare and contrast rack-based and chassis-switch based switched backbones.
The *rack-based collapsed backbone* has the advantage of placing all network equipment in one place for easy maintenance and upgrade, but does require more cable. In most cases, the cost of the cable itself is only a small part of the overall cost to install the network, so the cost is greatly outweighed by the simplicity of maintenance and the flexibility it provides for future upgrades. The room containing the rack of equipment is sometimes called the main distribution facility (MDF) or central distribution facility (CDF). The cables from all computers and devices in the area served by the MDF (often hundreds of cables) are run into the MDF room. Once in the run they are connected into the various devices. The devices in the rack are connected among themselves using very short cables called patch cables. With rack-based equipment, it becomes simple to move computers from one LAN to another. This convenience is used to spread the traffic around the network more efficiently so that network capacity is no longer tied to the physical location of the computers. Computers in the same physical area can be connected into very different network segments conveniently in the MDF. A *chassis switch* enables users to plug modules directly into the switch. Each module is a certain type of network device. The key advantage of chassis switches is their flexibility. It becomes simple to add new modules with additional ports as the LAN grows, and to upgrade the switch to use new technologies. For example, if you want to add gigabit Ethernet or ATM you simply lay the cable and insert the appropriate module into the switch.
In Figure 8-5, would the network still work if we removed the routers in each building and just had one core router? What would be the advantages and disadvantages of doing this?
The network would still work, although the traffic on the network would be significantly increased due to the creation of one large LAN instead of three subnets. The advantages of this would be slightly lower costs due to only purchasing one router instead of four and less maintenance and management. Each of these advantages are minor, and certainly not work the additional traffic on the network.
Some experts are predicting that Ethernet will move into the WAN. What do you think?
The new Ethernet/IP packet networks have become dominant for high-traffic networks (2 Mbps to 1Gbps), even though SONET and ATM remain preferred for some requirements. Since WAN required a network with high network capacity, I believe that Ethernet will move into the WAN into the near future.
How does a router differ from a VLAN?
VLAN switches can create multiple subnets, so they act like routers, except the subnets are inside the switch, not between switches. Therefore, broadcast messages sent by computers in one VLAN segment are sent only to the computers on the same VLAN.
