Youngstown State University Cisco Regional Academy Curriculum Review Semester 3, v.2.1.2 Chapter 2: LAN Switching

Chapter 2 Learning Objectives You will be able to: Identify various LAN Communication Problems. Define Full-Duplex Transmitting, Fast Ethernet Standard and LAN Segmentation Identify and describe Switching and VLANs Explain the operation of Spanning-Tree Protocol.

LAN Problems Factors That Impact Network Performance: Modern Operating Systems Windows, Unix and Mac Multitasking Increased O/S speed Network Intensive Applications Client/server applications World Wide Web access

Factors That Impact Network Performance Performance of Ethernet/802.3 LANs is degraded by: Data frame broadcast nature of 802.3 CSMA/CD access method; collisions, etc. Multimedia demand on bandwidth. Normal latency as frames travel the medium and through Layer 1, 2, and 3 devices. Latency added by using repeaters  to extend medium distances.

10-Base T Transmission Times Total time for transmission equals the number of bits being sent, multiplied by the bit time for a given technology. Bit time of each 10 Mbps Ethernet bit is 100 ns. 1 byte (8 bits) takes a minimum transmit time of 800 ns. A 64-byte frame takes 51,200 ns, or 51.2 microseconds.

Half & Full Duplex Ethernet Ethernet is half-duplex technology. Packets can be sent or received, but not at the same time. Full-duplex Ethernet allows sending and receiving packets at the same time! Requires using two pairs of wires in the UTP and a switched connection between each node. This connection is considered point-to-point and is collision free. 

Full Duplex Ethernet Full-duplex Ethernet offers 100% of the bandwidth in both directions. This produces a potential 20-Mbps throughput for 10-Mbps TX and 10-Mbps RX.

Full Duplex Ethernet Hardware requirements 10BASE-T, 100BASE-TX, or 100BASE-FX media. Full-duplex NICs. Full-duplex Ethernet switch. A direct connection is made between the transmit at one end of the circuit and the receive at the other end.

Segmenting LAN’s Why? Decrease network congestion within each segment. Reduce the size of collision domains. Increase the number of collision domains.

Segmenting LAN’s with Bridges Layer 2 devices that forward data frames according to MAC addresses. Learn and build tables of MAC addresses. Increases the latency in a network by 10%-30%.

Segmenting LAN’s with Routers Operate at Network Layer. Use network-layer protocol addresses to forward packets. Greater functionality than a bridge. But, 20% - 40% loss in throughput!

Segmenting LAN’s with Switches Segments a LAN into “microsegments,” or single host segments. Creates collision-free domains. Each node or segment is directly connected to one switch port. Behaves like there are only two nodes - the sending node and the receiving node.

Switch Operation and Functionality Switches perform two basic operations: Switch data frames A frame arrives on an input media and is transmitted to an output media. Maintain switching operations It builds and maintains switching tables of MAC addresses.

Benefits of Switching Number of collisions reduced. Simultaneous, multiple connections. High-speed uplinks. Improved network response. Increased user productivity.

Microsegmentation Switches improve bandwidth by separating collision domains and selectively forwarding traffic to the appropriate segments of a network.

How Switches Build Address Tables Addresses are learned dynamically. As a packet enters the switch port, the address is read, learned, and stored in content addressable memory (CAM). Addresses are associated with the port it entered. Addresses are time stamped. Aged or old addresses, are removed from the table.

Symmetric Switching A symmetric switch provides switched connections between ports with the same bandwidth. All 10-Mbps ports, Or, all 100-Mbps ports.

Asymmetric Switching Switching between ports of unlike Bandwidth. Memory buffering in an asymmetric switch is required. Allows traffic from the 100-Mbps port to be sent to a 10-Mbps port without causing too much congestion at the 10-Mbps port.

Switch Latency Latency depends on how the switch forwards the frames. The faster the switching mode, the smaller the latency in the switch. When using faster frame forwarding, the switch takes less time to check for errors. The tradeoff is more retransmissions due to errors.

Memory Buffering Technique to store and forward packets to the correct port or ports. Two methods for forwarding packets: port-based memory buffering shared memory buffering

Port-based Memory Buffering Packets are stored in queues linked to specific incoming ports. A packet is transmitted to the outgoing port only when all the packets ahead of it have been transmitted. A single packet can delay transmission of all packets in memory because of a busy destination port.

Shared Memory Buffering Deposits all packets into a common memory buffer shared by all the ports on the switch. Amount of memory allocated to a port is determined by how much is required by each port. Packets in the buffer are linked dynamically to the transmit port.

Two Switching Methods Store-and-forward Cut-Through Complete frame is received before forwarding. Cut-Through Switch checks the destination address as the data enters and immediately forwards the frame.

Cut-Through Switching Fast-forward switching; has lowest level of latency. It immediately forwards a packet after receiving the destination address. The packets may be relayed with errors!

Fragment-Free Option Reduces the number of packets forwarded with errors. Filters out collision fragments (the majority of packet errors) before forwarding begins. Waits and decides if a packet is not a collision fragment before forwarding. Anything greater than 64 bytes is a valid packet and is usually received without error.

Switch Broadcast Domains All switch ports are still part of one broadcast domain. Each segment still gets a broadcast from all other segmented LANs. Any broadcast reaches all hosts because it is sent out all switch ports!

VLAN’s A logical grouping of network devices or users that are not restricted to a physical switch segment by function, department, application, etc. Can be in different physical locations.

VLAN’s Creates a single broadcast domain not restricted to a physical segment treated like a subnet. Setup done in the switch by software. Not standardized. VLAN software is proprietary by manufacturer.

Spanning Tree Protocol Allows duplicate switched paths without the latency effects of loops in the network. “Looped” connection put in standby mode. It is activated in the event of a connection failure. Used on all Ethernet and Fast Ethernet VLANs.

Bridge Protocol Data Units Spanning-Tree Protocol “hello” packet sent out at configurable intervals. Exchanges information among bridges or switches in the network. Unique bridge ID of the root switch. The spanning tree path cost to the root switch. The bridge ID of the transmitting switch. Message age. Identity of the transmitting port. Hello, forward delay, and max-age protocol timers.

Spanning Tree Protocol States Blocking No frames forwarded, BPDUs heard. Listening No frames forwarded, listening for frames Learning No frames forwarded, learning addresses Forwarding Frames forwarded, learning addresses Disabled No frames forwarded, no BPDUs heard

Spanning Tree Protocol States The state for each VLAN port is set by the initial configuration. STP then determines whether the port forwards or blocks frames. Ports can be configured to immediately enter the forwarding mode when a connection is made where immediate access is required.

Summary, part 1 Increased LAN bandwidth needed due to more powerful PC’s and network-intensive applications. Greater congestion caused by client-server applications, multimedia, and Internet traffic. Segmenting a LAN with bridges, routers and switches is a solution to this problem. Switched Ethernet creates a point to point collision free connection. Switches use MAC addresses and work at high transfer rates.

Summary, part 2 Asymmetric LAN switching connects ports of unlike bandwidth, such as 10Mbps to 100Mbps. Symmetric ports have the same BW. Store and forward switching stores the complete frame before sending it out. Cut through sends the frame as soon as the address is read. VLAN is a group of hosts not restricted to a physical switch segment. Spanning Tree Protocol allows duplicate switch paths without suffering latency due to loops.

End Study Sheet: Semester 3 Chapter 2 v. 2.1.2   1)      Explain what happens, step by step, when a collision occurs on an Ethernet connection. 2)      Define and describe the reasons and effects of latency. 3)      Define the phrase “transmission time.” 4)      Describe the function of a repeater. 5)      What layer 1 items must be used to employ full-duplex connectivity. 6)      What affect does full-duplex connectivity have on available BW? 7)      How can a congested Ethernet LAN performance be improved? 8)      Describe how a bridge works. 9)      What effect does segmenting a LAN with a switch have on bandwidth, and the collision and broadcast domains. 10)  Describe how a switch handles an incoming frame. 11)  Describe how a switch works with MAC addresses. 12)  Define and describe a “Virtual Circuit” as it applies to switches. 13)  Define asymmetric and symmetric switching. 14)  What is the function of a switch memory buffer. 15)  Define and describe the following: Store-and-Forward, and Cut-Through. 16)  What is a VLAN and how are they grouped? 17)  Define and describe Spanning Tree Protocol.. 18)  List and describe the STP states.