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eBook Category: Technology/Science/General Nonfiction
eBook Description: This reference guide will aide you in studying for Cisco Exam 640-507 to become a Cisco Certified Network Associate as well as being a handy reference tool that you can always have close at hand.

eBook Publisher: PDABookstore.com, Published: Dallas, Texas, 2002
Fictionwise Release Date: August 2002

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LAN Design: Ethernet- When we talk about a LAN, Ethernet is the most popular physical layer LAN technology today. It is standard is defined by the Institute for Electrical and Electronic Engineers as IEEE Standard 802.3. According to IEEE, information for configuring an Ethernet as well as specifying how elements in an Ethernet network interact with one another is clearly defined in 802.3. For half-duplex- Ethernet 10BaseT topologies, data transmissions occur in one direction at a time, leading to frequent collisions and data retransmission. In contrast, full-duplex devices use separate circuits for transmitting and receiving data. As a result, collisions are largely avoided. Collisions are when two nodes are trying to send data at the same time. On an ethernet network, the node will stop sending when it detects a collision, and will wait for a random amount of time before attempting to resend. Also, with full-duplex transmissions the available bandwidth is effectively doubled, as we are using both directions simultaneously. REMEMBER: to enjoy full-duplex transmission, we need a switch port, not a hub. Fast Ethernet--For networks that need higher transmission speeds, there is the Fast Ethernet standard called IEEE 802.3u that raises the Ethernet speed limit to 100 Mbps! Of course, we need new cabling to support this high speed. In 10BaseT network we use Cat3 cable, but in 100BaseT network we need Cat 5 cables. The three types of Fast Ethernet standards are 100BASE-TX for use with level 5 UTP cable, 100BASE-FX for use with fiber-optic cable, and 100BASE-T4 which utilizes an extra two wires for use with level 3 UTP cable. Gigabit Ethernet--Gigabit Ethernet is an emerging technology that will provide transmission speeds of 1000mbps. It is defined by the IEEE standard The 1000BASE-X (IEEE 802.3z). Just like all other 802.3 transmission types, it uses Ethernet frame format, full-duplex and media access control technology. Token Ring--Token Ring is an older standard that isn't very widely used anymore as most have migrated to some form of Ethernet or other advanced technology. Ring topologies can have transmission rates of either 4 or 16mbps. Token passing is the access method used by token ring networks, whereby, a packet called a token is passed around the network. A computer that wishes to transmit must wait until it can take control of the token, allowing only one computer to transmit at a time. This method of communication aims to prevent collisions. Token Ring networks use multistation access units (MSAUs) instead of hubs on an Ethernet network. LAN Switching Switching--examines MAC address. Same as multiport bridge Three Switch Functions: 1. Address learning 2. Forward/filter decision 3. Loop avoidance Address Learning: maintains MAC address table used to track the location of devices connected to the switch. Forward/filter decision: when a frame arrives with a known destination address, it is forwarded only on the specific port connected to that station. Broadcast and Multicast frames: may be of interest to all stations. The switch normally floods to all ports other than the origination port. A switch never learns a broadcast or multicast address because broadcast and multicast addresses never appear as the source address of a frame. All nodes on an Ethernet network can transmit at the same time, so the more nodes you have the greater the possibility of collisions happening. This can slow the network down. Redundant Topology--eliminates single points of failure. Causes broadcast storms, multiple frame copies, and MAC address table instability problems. Multiple Frame Copies--when a new switch is added, the other switches may not have learned its correct MAC address. The host may send a unicast frame to the new switch. The frame is sent through several paths at the same time. The new switch will receive several copies of the frame. This causes MAC Database Instability. MAC Database Instability--results when multiple copies of a frame arrive on different ports of a switch. Multiple Loop Problems--complex topology can cause multiple loops to occur. Layer 2 has no mechanism to stop the loop. This is the main reason for Spanning--Tree Protocol. Spanning-Tree Protocol (STP) IEEE 802.1d.--developed to prevent routing loops. STA (Spanning-Tree Algorithm) is implemented by STP to calculate a loop-free network topology. In most switches, BPDUs (Configuration Bridge Protocol Data Unit), are sent and received by all switches, and processed to determine the spanning-tree topology. (STP is on by default). A port is in either a forwarding or blocking state. Forwarding ports provide the lowest cost path to the root bridge. All ports start in the blocking state to prevent bridge loops. The port stays in a blocked state if the spanning tree determines that there is another path to the root bridge that has a better cost. Blocking ports can still receive BPDUs. Spanning-Tree operation--Selects one root bridge. All the ports are designated ports (forwarding). For non-root bridge, there will be one root port. This offers the lowest cost path from non-root bridge to the root bridge. On each segment, there is one designated part. This port also has the lowest cost to the root bridge. Time to Convergence--the time for all the switches and bridges ports transition to either the forwarding or blocking state. When network topology changes, switches and bridges must re-compute the Spanning-Tree Protocol, which disrupts traffic. Bridging Compared to LAN Switching--Bridging: primarily software based. One spanning-tree instance per bridge. Usually up to 16 ports per bridge. LAN Switching: primarily hardware based. Many spanning-tree instances per switch. More ports per switch, (up to 100). Faster than a Bridge. Transmitting Frames through a Switch: Store-and-Forward--copies entire frame into buffer, checks for CRC errors. Higher latency. Cut-Through: reads only the destination address into buffer, and forwards immediately. Low latency. Fragment free--(modified cut-through). Switch will read into the first 64 bytes before forwarding the frame. Collisions will usually occur within the first 64 bytes. (default for 1900 series). Full-Duplex Ethernet--can provide double the bandwidth of traditional Ethernet, but requires a single workstation on a single switch port, and NIC must support it. Collision free because there are separate send and receive wires, and only one workstation is on the segment. Half-Duplex must provide for collision detection, therefore can only use 50% of bandwidth available. It sends and receives on the same set of wires. LAN Segmentation: breaking up the collision domains by decreasing the number of workstations per segment. Fast Ethernet (100bt)--provides 10 times the bandwidth of older 10bastT Ethernet. Must have Cat5 cable, no longer than 100 meters, and Fast Ethernet NIC's and Hubs/Switches. Bridges--examines MAC address, and forwards frames unless the address was local. Forwards to all other segments it is attached to. Forwards multicast packets, so broadcast storms can occur. Routers--examines network address, and forwards using the best available route to destination network. Can have multiple active paths. Virtual LAN's--sets different ports on a switch to be part of different sub-networks. Some benefits: simplify moves, adds, changes; reduce administrative costs; have better control of broadcasts; tighten security; and distribute load. Relocate the server into a secured location. Network Devices: In a typical LAN, there are various types of network devices available as outlined below. Hub- Repeat signals received on each port by broadcasting to all the other connected ports. Repeaters--Used to connect two or more Ethernet segments of any media type, and to provide signal amplification for a segment to be extended. In a network that uses repeater, all members are contending for transmission of data onto a single network. We like to call this single network a collision domain. Effectively, every user can only enjoy a percentage of the available bandwidth. Ethernet is subject to the "5-4-3" rule regarding repeater placement, meaning we can only have five segments connected using four repeaters with only three segments capable of accommodating hosts. Bridge- A layer 2 device used to connect different networks types or networks of the same type. It maps the Ethernet addresses of the nodes residing on each segment and allows only the necessary traffic to pass through the bridge. Packet destined to the same segment is dropped. This "store-and-forward" mechanism inspects the whole Ethernet packet before making a decision. Unfortunately, it cannot filter out broadcast traffic. Also, it introduces a 20 to 30 percent latency when processing the frame. Only 2 networks can be linked with a bridge. Switch--Can link up four, six, eight or even more networks. Cut-through switches run faster because when a packet comes in, it forwards it right after looking at the destination address only. A store-and-forward switch inspects the entire packet before forwarding. Most switches cannot stop broadcast traffic. Switches are layer 2 devices. Routers- Can filter out network traffic also. However, they filter based on the protocol addresses defined in OSI layer 3(the network layer), not based on the Ethernet packet addresses. Note that protocols must be routable in order to pass through the routers. A router can determine the most efficient path for a packet to take and send packets around failed segments. Brouter- Has the best features of both routers and bridges in that it can be configured to pass the unroutable protocols by imitating a bridge, while not passing broadcast storms by acting as a router for other protocols. Gateway- Often used as a connection to a mainframe or the internet. Gateways enable communications between different protocols, data types and environments. This is achieved via protocol conversion, whereby the gateway strips the protocol stack off of the packet and adds the appropriate stack for the other side. Gateways operate at all layers of the OSI model without making any forwarding decisions. The goal of LAN segmentation is to effectively reduce traffic and collisions by segmenting the network. In a LAN segmentation plan, we do not consider the use of gateways and hubs at all and the focus turns to device such as switches and routers.