Networking Basics CCNA 1 Chapter 8

Ethernet Switch Operations Layer 2 Bridging and Switching Operations Earliest networking devices were repeaters and hubs Multiple LAN segments could be connected to make larger LANs, within 5-4-3 design rules As it became apparent that reducing size of collision domains was important, bridges were created Bridges are aware of Ethernet framing and Layer 2 MAC addressing (IEEE 802.3)

Ethernet Switch Operations Layer 2 Bridging and Switching Operations Bridges extend LAN distances, without some of the negative effects of repeaters and hubs Bridges were typically much more expensive than repeaters and hubs (were usually a PC running software to perform the bridging function) Bridges usually had only two interfaces, where hubs had multiple ports

Ethernet Switch Operations Layer 2 Bridging and Switching Operations Next major step in LAN devices was the LAN switch Does the same thing as a bridge Instead of using software, process could be done with a chip (sometimes called application-specific integrated circuits – ASICs) Switches have more interfaces than bridges, are smaller, and do the same work faster As switch prices fell, bridges disappeared from the market

Ethernet Switch Operations The Forwarding and Filtering Decision Repeaters and hubs simply react to the incoming signal make no decisions and require no programming logic Receive, regenerate and send signal out all ports except the one on which it was received Bridges implemented their logic in software

Ethernet Switch Operations The Forwarding and Filtering Decision Switches implement their logic in hardware Run much faster than bridges Cisco makes switches that can forward hundreds of millions of Ethernet frames per second

Ethernet Switch Operations The Forwarding and Filtering Decision Filtering and forwarding logic Examine incoming signal; interpret as 0s and 1s (OSI Layer 1 standards) Interpret the received bits based on Ethernet framing rules; find MAC destination address in frame (OSI Layer 2 standards, IEEE 802.3 MAC sublayer)

Ethernet Switch Operations The Forwarding and Filtering Decision Filtering and forwarding logic (continued) Examine table that maps MAC addresses with corresponding interfaces Find table entry that matches the destination MAC address of frame If frame came in on a different interface than the one listed on the table, process is called forwarding the frame If the frame came in on the same interface as the one it was received on, discard it (this is called filtering)

Ethernet Switch Operations The Forwarding and Filtering Decision The table a bridge or switch refers to may be called: Bridging table Switching table MAC address table Forwarding table Content Addressable Memory (CAM) table

Ethernet Switch Operations A Bridge Filtering Decision Based on the CAM

Ethernet Switch Operations A Bridge Forwarding Decision Based on the CAM

Ethernet Switch Operations Learning CAM Table Entries and Flooding Unknown Unicasts Switches and bridges learn entries in the CAM dynamically They use this logic: Examine the source MAC address of the frame and the interface on which it was received Add that source MAC address and corresponding interface to the table

Ethernet Switch Operations Learning CAM Table Entries: One Switch

Ethernet Switch Operations Learning CAM Table Entries: Two Switches

Ethernet Switch Operations Handling Unknown Unicasts Switches typically learn CAM entries for all working devices on the LAN as soon as those devices start sending data Sometimes a switch receives a frame that does not have a CAM entry – this is an unknown unicast frame The switch sends the unknown unicast frame out all ports, a process called flooding

Ethernet Switch Operations Forwarding Broadcasts and Multicasts Unicast frame has a destination MAC address of a single NIC or interface Broadcast frames are sent to a destination MAC address of FFFF.FFFF.FFFF.FFFF and are delivered to all devices on the LAN Multicast frames are sent to one of a range of MAC addresses

Ethernet Switch Operations Flooding Unknown Unicasts

Ethernet Switch Operations Forwarding Broadcasts and Multicasts Multicast addresses provide a way to send certain frames to a subset of devices Streaming video Some low-end switches flood multicasts like broadcasts Higher-end switches allow multicasting, making the process more efficient

Ethernet Switch Operations Different Forwarding Behavior for Multicasts

Ethernet Switch Operations The Cisco Switch CAM All switches and bridges use some table that lists the MAC address and port through which each MAC address can be reached Cisco calls this the CAM (Content Addressable Memory) The MAC address is input into the memory and CAM instantly outputs the table entry This process occurs quickly, every time, regardless of table size

Ethernet Switch Operations Switch Internal Processing The amount of time it takes for a frame to progress through a network from one device to another is called latency Some factors that affect latency cannot be improved, such as propagation delay (the amount of time it takes for electricity to go from one end of the network to another) Other types of delay vary with network conditions; frames may be waiting in a buffer (queuing delay)

Ethernet Switch Operations Switch Internal Processing – Factors that Impact Latency The finite speed that signals can travel (propagation delay) Circuit delays caused by electronics Software delays caused by software decisions being made Delays caused by frame contents and location of the frame switching decisions

Ethernet Switch Operations Store-and-Forward Switching Switch receives entire frame before forwarding it Advantages of store-and forward switching FCS field is at end of frame; frame can be checked for an error Can check for rare error in which the 802.3 Length field does not match the Data field length Can forward between ports running at different speeds (asymmetric switching) Disadvantage More latency than other switching types

Ethernet Switch Operations Cut-Through Switching Destination MAC address is located at beginning of Ethernet frame Advantage of cut-through switching Once destination MAC address is read, switch can begin forwarding frame Less latency than store-and-forward Disadvantages of cut-through switching Cannot check FCS; may forward frames with errors Forwards before some legitimate collisions have occurred Only works with symmetric switching

Ethernet Switch Operations Fragment-Free Switching Overcomes a problem that cut-through switching has: cut-through is too fast Collisions should occur while a frame’s first 64 bytes are being transmitted Cut-through switching often begins transmitting before 64 bytes are received Cut-through switching can forward collision fragments

Ethernet Switch Operations Fragment-Free Switching Fragment-free switching waits until it has received first 64 bytes to begin transmitting Ensures switch does not forward frames that have collided

Ethernet Switch Operations Cisco Enterprise Switch – Internal Processing Paths

Ethernet Switch Operations Spanning Tree Protocol Most LAN design include redundant physical paths A trunk is a link between two switches; sometimes called a backbone link Spanning tree protocol (STP) prevents switching loops from the logic used to forward unknown unicast and broadcast frames

Ethernet Switch Operations Typical Enterprise Campus Building Block Design, with Redundancy

Ethernet Switch Operations The Problem That STP Solves: Switching Loops

Ethernet Switch Operations The Problem That STP Solves: Switching Loops In previous slide, if PC1 sends a broadcast, it goes around LAN in both directions Each switch broadcast the frame(s) out every port (except the one on which it was received) This process continues for a long time, continuing until no other traffic can be sent over the LAN: a “broadcast storm”

Ethernet Switch Operations STP Protocol: STP Blocking STP makes some ports quit forwarding or receiving frames An interface that is not allowed to process traffic by STP is considered to be in an STP blocking state In the figure that follows, SW3’s port 1 is in a blocking state – it receives the broadcast frame but ignores it STP causes the LAN to use particular paths and leaves others idle and unused

Ethernet Switch Operations IEEE 802.1D STP Interface States

Ethernet Switch Operations IEEE 802.1D STP Interface States The forwarding and blocking states are the most common, because a working network interface stabilizes into one of these states Failed interfaces stabilize into a disabled state Listening and learning states are used to solve problems with CAM tables

Ethernet Switch Operations Stable STP Topology and Switch CAMs in a Three-Switch Network

Ethernet Switch Operations Changing the CAM with the Listening and Learning States The topology can fail when a trunk fails or when a new trunk comes up STP determines the topology by having switches send bridge protocol data units (BPDUs) to each other BPDUs and the Spanning Tree Algorithm (STA) are part of the IEEE 802.1D standard Information learned allows the switches to determine the topology and decide which interfaces should forward and which should block frames

Ethernet Switch Operations Changing the CAM with the Listening and Learning States The listening and learning states are used by STP when it needs to transition to a new topology An STP topology refers to the topology of the network when each interface is in one of three stable states STP remains in the stable topology until something happens A trunk goes down (perhaps cut) The network engineer shuts down a trunk A new switch is added An interface fails

Ethernet Switch Operations Changing the CAM with the Listening and Learning States Switches use listening and learning states as interim states when transitioning an interface for two reasons: For the switches’ CAM table entries to time out (during the listening state) For the switches to relearn the MAC addresses and (possibly different) interfaces used to reach the MAC addresses

Ethernet Switch Operations A New STP Topology After a Failure

LAN Design: Collision Domains and Broadcast Domains A collision domain is a set of LAN interfaces for which a frame sent out any two of these interfaces, at the same time, would cause a collision Hubs repeat signals out interfaces and do not consider CSMA/CD logic, so any frames sent simultaneously will collide The terms shared bandwidth and shared media refer to the fact that the devices in a hubbed network share the same media and bandwidth

LAN Design: Collision Domains and Broadcast Domains One Collision Domain with One 10BASE-T Hub

LAN Design: Collision Domains and Broadcast Domains Large/Long Collision Domains The 5-4-3 (or 5-4-3-2-1) Rule for 10BASE-T networks 5 segments of network media 4 repeaters or hubs at most 3 links at most, between two end-user devices If 5 segments exist between two end-user devices, 2 segments must not have any end-user devices connected to them It’s all 1 large collision domain

LAN Design: Collision Domains and Broadcast Domains One Collision Domain with Multiple 10BASE-T Hubs

LAN Design: Collision Domains and Broadcast Domains Large/Long Collision Domains The 5-4-3-2-1 rule for 10BASE-T restrictions are required due to the round-trip time of the collision domain Within one collision domain, all the devices share the 10 Mbps of bandwidth Within one collision domain, a (practically) simultaneous transmission of a frame by two or more PCs results in a collision

LAN Design: Collision Domains and Broadcast Domains Large/Long Collision Domains The more PCs in a collision domain, the less efficient it is The more frames, the more collisions The more collisions, the more time sent waiting to resend frames Once a LAN reaches about 30-40% of bandwidth utilization, the number of collisions increases dramatically

LAN Design: Collision Domains and Broadcast Domains High LAN Utilization Resulting in Much Higher Percentage of Collisions

LAN Design: Collision Domains and Broadcast Domains Large/Long Collision Domains Large collision domains should not be used for the following reasons: Shared bandwidth – as the size of the collision domain grows, each device has less available bandwidth Higher utilization – the more devices in a single collision domain, the better the chance of a collision and of driving the utilization rate higher

LAN Design: Collision Domains and Broadcast Domains Creating Many Small Collision Domains Significantly reduces the negative effects of a large collision domain Process of breaking a LAN into multiple collision domains is called segmentation Switches, bridges, and routers can segment LANs into multiple collision domains

LAN Design: Collision Domains and Broadcast Domains Two LANs with Many Small Collision Domains

LAN Design: Collision Domains and Broadcast Domains Creating Many Small Collision Domains Benefits of segmenting 10BASE-T LANs: Design rules (5-4-3-2-1) apply to each individual collision domain With smaller collision domains, reaching the point of utilization where performance is degraded is less likely Each domain gets its own bandwidth, so fewer devices are sharing the available bandwidth

LAN Design: Collision Domains and Broadcast Domains Creating Many Small Collision Domains When switches are used on the LAN, the terms switched LAN and switched bandwidth are used Each switch port connects to a separate collision domain Connecting a single end-user device to each switch port is a process called microsegmentation

LAN Design: Collision Domains and Broadcast Domains Creating Many Small Collision Domains Microsegments meet the requirements to allow full duplex Full duplex gives twice the bandwidth A 24 port 10BASE-T hub shares 10 Mbps of bandwidth among 24 ports A 24 port 10BASE-T switch gives each port 20 Mbps of bandwidth

LAN Design: Collision Domains and Broadcast Domains Main Benefits of Using Many Small Collision Domains Collision domain design rules are easier to achieve Smaller collision domains reduce the probability of LAN overutilization Each collision domain gets its own separate switched bandwidth With a collision domain consisting of only two interfaces/NICs, full duplex can be used

LAN Design: Collision Domains and Broadcast Domains How Switches and Bridges Prevent Collisions Switches reduce or prevent collisions by buffering or queuing frames Repeaters and hubs do not perform buffering Bridges, switches and routers follow CSMA/CD rules if not using full duplex

LAN Design: Collision Domains and Broadcast Domains Switch Buffering Example

LAN Design: Collision Domains and Broadcast Domains Layer 2 Broadcast Domains A broadcast domain is: The set of LAN interfaces (including NICs and network device interfaces) for which a broadcast frame sent by one device with be forwarded to all other interfaces in that same broadcast domain Bridges and switches forward broadcasts Routers do not forward broadcasts

LAN Design: Collision Domains and Broadcast Domains One Router Creating Two Broadcast Domains

LAN Design: Collision Domains and Broadcast Domains Performance Impact of Multicast and Broadcast Domains PC NICs see all frames on the LAN PC NICs can ignore unicast frames not for them PC NICs must send multicast and broadcast frames to their CPU for processing, which affects PC performance This is less of an issue today with fewer proprietary network protocols doing broadcasts and with more powerful processors

LAN Design: Collision Domains and Broadcast Domains NIC Giving Broadcasts and Multicasts to the CPU

LAN Design: Collision Domains and Broadcast Domains More Broadcasts, Less CPU Capacity for End-User Work

LAN Design: Collision Domains and Broadcast Domains The Impact of Broadcasts and Multicasts Today Biggest risk is in wasting CPU cycles from multicasts Switches flood multicasts just like broadcasts LAN engineers must enable multicast optimization tools in switches to prevent switches from flooding multicasts to every device in the LAN

LAN Design: Collision Domains and Broadcast Domains The Impact of Broadcasts and Multicasts Today Broadcasts such as RIP and ARP don’t cause problems in today’s networks, but did in the past when networks were slower ARP remembers the info it learns, so an individual PC might not send one ARP per minute RIP broadcasts may be sent by routers and UNIX workstations; now most UNIX workstations have it turned off by default so these are no longer an issue

LAN Design: Collision Domains and Broadcast Domains Identifying Networking Devices by OSI Layer Repeaters and hubs are Layer 1 devices Bridges and switches are Layer 2 devices Routers are Layer 3 devices

LAN Design: Collision Domains and Broadcast Domains Sample Network with Collision Domains and Broadcast Domains Shown

LAN Design: Collision Domains and Broadcast Domains Data Flow with Layer 1, Layer 2, and Layer 3 Devices

LAN Design: Collision Domains and Broadcast Domains The Ambiguous Term Segment Three main uses of the term segment LAN concepts – a segment is a collision domain LAN (physical) – in a LAN using a bus topology, a segment is a continuous electrical circuit, often connected to other segments with repeaters TCP – the process of taking a large piece of data and breaking it into smaller pieces; one of those pieces

Summary Bridges and switches work the same way regarding basic forwarding, learning, flooding and STP They build forwarding tables by examining the source MAC addresses of incoming frames They make filtering and forwarding decisions by looking at the destination MAC address of the frame and comparing it to the table They flood broadcast frames and also flood multicast frames, unless optimization features have been enabled

Summary Switches differ from bridges They have much more powerful hardware They use content addressable memory (CAM) to hold the switching table The CAM allows the switch to find a MAC address and its associated port very quickly every time Latency is the time that passes as a frame or packet is sent through the network Propagation delay is the time it takes for electrical or optical energy to pass over the cable, and contributes to latency

Summary Three internal switch processing options: Cut-through switching begins forwarding the frame as soon as the destination MAC address is read; does not check FCS to determine if frame is good; low latency Store-and-forward switching receives the entire frame; does error-checking; necessary for asymmetrical switching Fragment-free switching waits for the first 64 bytes to be received before beginning forwarding; enables it to detect normal collisions

Summary Switches and bridges use Spanning Tree Protocol (STP) to identify and block redundant paths through the network; gives a logical path with no loops A collision domain with a single device connected to a switch port is called a microsegment Microsegments use UTP cabling, allow the use of full duplex With no collisions possible, CSMA/CD can be disabled

Summary Placing a large number of PCs in a collision domain increases demand for bandwidth This increases possibility of collisions Breaking large collision domains into multiple smaller collision domains reduces the chance of collisions while adding bandwidth Separating LANs into more segments by using bridges and switches creates additional collision domains, one per bridge and switch port Broadcast domains are a set of devices in which if one device sends a broadcast, all other devices receive the broadcast; Layer 3 devices (routers) separate broadcast domains