S3C2 – LAN Switching

Addressing LAN Problems

Congestion is Caused By

• Multitasking, • Faster operating systems, • More Web-based applications• Client-Server

– client/server applications allow administrators to centralize information, thus making it easy to maintain and protect.

• Point-to-point (host to host) connection is collision free

LANS Impacted By:

• The data frame broadcast delivery nature of Ethernet/802.3 LANs

• Carrier sense multiple access collision detect (CSMA/CD) access methods allowing only one station to transmit at a time

• Multimedia applications with higher bandwidth demand such as video and the Internet, coupled with the broadcast nature of Ethernet, can create network congestion.

Impact Issues Continued

• Normal latency as the frames travel across the Layer 1 medium and through Layer 1, 2, and 3 networking devices, and the latency added by the extension of Ethernet/802.3 LANs by adding repeaters

• Extending the distances of the Ethernet/802.3 LANs by using Layer 1 repeaters

Latency

• Latency is caused by:– the time it takes the source NIC to place voltage

pulses on the wire and the time it takes the receiving NIC to interpret these pulses. This is sometimes called NIC delay (it is typically around 1 microsecond for10BASE-T NICs).

– A byte takes a minimum of 800 ns to transmit

Latency Continued

– Second, there is the actual propagation delay as the signal takes time -- albeit a very short time -- to actually travel down the cable (it is typically about .556 microseconds per 100 m for Cat 5 UTP). The longer the cable, the more propagation delay; the slower the nominal velocity of propagation (NVP) of the cable, the more the propagation delay.

– Third, latency is added according to which networking devices -- whether they be Layer 1, 2, or 3 (and how they are configured) -- are added in the path between the two communicating computers. The actual transmission time (the duration of the host actually sending bits) must also be included in understanding timing on networks.

Half Duplex or Duplex

• Ethernet LANs are half-duplex technology• Full-duplex Ethernet allows the transmission of a packet

and the reception of a different packet at the same time – requires full duplex NIC card– This simultaneous transmission and reception requires the use of

two pairs of wires in the cable and a switched connection between each node. This connection is considered point-to-point and is collision free.

– Because both nodes can transmit and receive at the same time, there are no negotiations for bandwidth. Full-duplex Ethernet can use an existing shared medium as long as the medium meets minimum Ethernet standards. Requires 2 pairs of wires and switched connection

Benefit of Full Duplex

• Ethernet usually can only use 50%-60% of the 10-Mbps available bandwidth because of collisions and latency.

• Full-duplex Ethernet offers 100% of the bandwidth in both directions.

• This produces a potential 20-Mbps throughput- 10-Mbps TX and 10-Mbps RX.

• Remember – Transmit connects to Receive– Think SIMPLE LAN

Why Segment

• Isolates certain traffic

• Creates smaller collision domains

• Data are passed between segments using bridges, switches, or routers

• IMPORTANT– Decreasing size of collision domains increases

the number of collision domains

Bridges

• Create tables to match segments and MAC addresses

• Layer 3 protocol independent• Stores and then forwards based on MAC

address• Increases latency on network (10-30%)• Creates smaller collision domains but

increases number of collision domains

Routers

• Operate at Layer 3 using IP addresses• More manageable, greater functionality, multiple

paths• Smaller collision domains• Introduce latency

– Protocols that provide acknowledgements introduce 30-40% delays

– Protocols that provide minimal acknowledgements have 20%-30% loss in throughput

– So routers introduce 20-40% delay depending on protocol

Switches/Switched Ethernet

• Low latency and high frame-forwarding rates• Eliminates impact of collisions through micro-

segmentation• Works with existing 802.3 standards• Create dedicated network segments (point to

point) with full bandwidth – virtual circuits• Create collision free domains• Cost more than bridges or routers

Bridges vs Switches

• Both bridges and switches connect LAN segments, use a table of MAC addresses to determine the segment on which a datagram needs to be transmitted, and reduce traffic.

• Switches are more functional in today’s networks than bridges because they operate at much higher speeds than bridges and can support new functionality, such as virtual LANs (VLANs).

• Bridges typically switch using hardware; switches typically switch using software.

Layer 2 Switching

• With Layer 2 switching, frames are switched based on MAC address information.

• If the Layer 2 switch does not know where to send the frame, it broadcasts the frame out all its ports to the network to learn the correct destination

• the switch learns the location of the new address and adds the information to the switching table– MAC Address and ports

Virtual LANs

• dedicated paths between sending and receiving hosts within the switch are temporary. The switch’s power comes from the fact that it can rapidly make and break these 1 to 1 connections through its various ports, depending upon the data in its switching table.

How A Switch Learns Addresses

• Examines source address• Sends out all ports except incoming port when

address is unknown, multicast, or broadcast• Forwards when the destination is at a different

interface• Filters when the destination is on the same interface• Date stamps each address– discards after a certain

time period• Addresses stored in CAM – Content Addressable

Memory

Benefits of Switching

• Number of collisions reduced

• Simultaneous multiple communications

• High speed uplinks

• Improved network response

• Increased user productivity

Symmetric/Asymmetric Switching

• Symmetric switching provides switching between like bandwidths– Multiple simultaneous conversations increase

throughput

• Asymmetric provides switching between unlike bandwidths– Requires the switch to use memory buffering

Spanning Tree Protocol

• Switches forward broadcast frames• Prevents loops

– Loops can cause broadcast storms and exponentially proliferate fragments

• Allows redundant links• Prunes topology to a minimal spanning tree• Resilient to topology changes and device failures• Spanning Tree Frames are called bridge• protocol data units (BPDUs)• Spanning Tree enabled by default on catalyst switch

Spanning Tree States

• States are initially set and then modified by STP– Blocking– Listening– Learning– Forwarding– Disabled

• Server ports can be configured to immediately enter SPT forwarding mode

• You can determine the status, cost, and priority of ports and VLANs by using the show spantree command

• Listening and learning create latency

Switching Modes

• Store and Forward – Entire frame received before forwarding takes place –

causes more latency but error detection is high

• Cut Though (Read first 6 bytes)– Switch reads destination address before receiving entire

frame and it is forwarded – decreases latency but higher error rate

• Fast forward immediately forwards• Fragment Free filters out collision fragments

• Fragment Free (Read first 64 bytes)– Ensures frame is not a runt and probably not an error

Buffering

• Two methods– Port-based

• Packets stored in queues that are linked to incoming ports – packets forwarded when queue is clear

– Shared Memory buffering• Deposits all packets into common memory buffer shared by all

ports– Dynamic location assigns port areas

– Switch maintains a map of ports and clears when packet is switched

Virtual LAN

• Logical network independent of their members’ physical locations

• Administratively defined broadcast domain• Users reassigned to different VLAN using software• Can be grouped by function, department,

application• Creates a single broadcast domain that is not

restricted to physical segment• INSTANT LEVEL OF SECURITY