CCNA R&S-07-Building Ethernet LANs with Switches
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Transcript of CCNA R&S-07-Building Ethernet LANs with Switches
©2015 Amir Jafari – www.amir-Jafari.com
Routing and Switching 200-1207 - Building Ethernet LANs with Switches
Building Ethernet LANs with Switches
©2015 Amir Jafari – www.amir-Jafari.com
Agenda
LAN Switching Concepts
Design Choices in Ethernet LANs
LAN Switching Concepts
©2015 Amir Jafari – www.amir-Jafari.com
Historical Progression: Hubs, Bridges, and Switches
10BASE-T used a centralized cabling model similar to today’s Ethernet LANs, with each device connecting to the LAN using a UTP cable
Instead of a LAN switch, the early 10BASE-T networks used hubs, because LAN switches had not yet been created
10BASE-T (with a Hub)
LAN Switching Concepts
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Historical Progression: Hubs, Bridges, and Switches
With 10BASE-T using hubs: When hubs receive an electrical signal in one port , the hub repeats
the signal out all other ports
When two or more devices send at the same time, an electrical collision occurs, making both signals corrupt
As a result, devices must take turns by using carrier sense multiple access with collision detection (CSMA/CD) logic, so the devices share the (10-Mbps) bandwidth
Broadcasts sent by one device are heard by, and processed by, all other devices on the LAN
Unicast frames are heard by all other devices on the LAN
LAN Switching Concepts
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Historical Progression: Hubs, Bridges, and Switches
Ethernet transparent bridges helped solve this performance problem with 10BASE-T:
Bridges separated devices into groups called collision domains
Bridges reduced the number of collisions that occurred in the network, because frames inside one collision domain did not collide with frames in another collision domain
Bridges increased bandwidth by giving each collision domain its own separate bandwidth, with one sender at a time per collision domain
LAN Switching Concepts
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Historical Progression: Hubs, Bridges, and Switches
Bridge will buffer or queue the frame until the outgoing interface can send the frame
Adding the bridge in Figure really creates two separate 10BASE-T networks
Bridge Creates Two Collision Domains and Two Shared Ethernets
LAN Switching Concepts
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Historical Progression: Hubs, Bridges, and Switches
LAN switches perform the same basic core functions as bridges, but at much faster speeds and with many enhanced features
Like bridges, switches segment a LAN into separate collision domains, each with its own capacity.
Switch Creates Four Collision Domains and Four Ethernet Segments
LAN Switching Concepts
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Switching Logic
Unicast frames have a unicast address as a destination, these addresses represent a single device
broadcast frame has a destination MAC address of FFFF.FFFF.FFFF, this frame should be delivered to all devices on the LAN
LAN switches receive Ethernet frames and then make a switching decision: either forward the frame out some other port(s) or ignore the frame
LAN Switching Concepts
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Switching Logic
To accomplish this primary mission, transparent bridges perform three actions:
1. Deciding when to forward a frame or when to filter (not forward) a frame, based on the destination MAC address
2. Learning MAC addresses by examining the source MAC address of each frame received by the switch
3. Creating a (Layer 2) loop-free environment with other bridges by using Spanning Tree Protocol (STP)
LAN Switching Concepts
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The Forward-Versus-Filter Decision
To decide whether to forward a frame, a switch uses a dynamically built table that lists MAC addresses and outgoing interfaces
Switches compare the frame’s destination MAC address to this table to decide whether the switch should forward a frame or simply ignore it
If the destination address is a known unicast address , and the outgoing interface is the same as the interface in which the frame was received, the switch filters the frame, meaning that the switch simply ignores the frame and does not forward it
A switch’s MAC address table is also called the switching table, or bridging table, or even the Content Addressable Memory (CAM) table, in reference to the type of physical memory used to store the table
LAN Switching Concepts
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The Forward-Versus-Filter Decision
Sample Switch Forwarding and Filtering Decision
LAN Switching Concepts
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The Forward-Versus-Filter Decision
A switch’s MAC address table lists the location of each MAC relative to that one switch
In LANs with multiple switches, each switch makes an independent forwarding decision based on its own MAC address table. Together, they forward the frame so that it eventually arrives at the destination
The forwarding choice by a switch was formerly called a forward-versus-filter decision, because the switch also chooses to not forward (to filter) frames, not sending the frame out some ports.
LAN Switching Concepts
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The Forward-Versus-Filter Decision
Forwarding Decision with Two Switches
LAN Switching Concepts
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How Switches Learn MAC Addresses
Switches build the address table by listening to incoming frames and examining the source MAC address in the frame
If a frame enters the switch and the source MAC address is not in the MAC address table, the switch creates an entry in the table
That table entry lists the interface from which the frame arrived
LAN Switching Concepts
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How Switches Learn MAC Addresses
Switch Learning: Empty Table and Adding Two Entries
LAN Switching Concepts
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How Switches Learn MAC Addresses
Switches keep a timer for each entry in the MAC address table, called an inactivity timer
The switch sets the timer to 0 for new entries. Each time the switch receives another frame with that same source MAC address, the timer is reset to 0.
The timer counts upward, so the switch can tell which entries have gone the longest time since receiving a frame from that device.
The switch then removes entries from the table when they become old. Or, if the switch ever runs out of space for entries in the MAC address table, the switch can then remove table entries with the oldest (largest) inactivity timers
Aging time for all MAC addresses can be configured. The default is 300 seconds
LAN Switching Concepts
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Flooding Frames
Unknown unicast frames: frames whose destination MAC addresses are not yet in the address table
Switches flood unknown unicast frames
Flooding means that the switch forwards copies of the frame out all ports, except the port on which the frame was received
If the unknown device receives the frame and sends a reply, the reply frame’s source MAC address will allow the switch to build a correct MAC table entry for that device
Switches also forward LAN broadcast frames, because this process helps deliver a copy of the frame to all devices in the LAN
LAN Switching Concepts
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Avoiding Loops Using Spanning Tree Protocol
Without STP, any flooded frames would loop for an indefinite period of time in Ethernet networks with physically redundant links
To prevent looping frames, STP blocks some ports from forwarding frames so that only one active path exists between any pair of LAN segments
The result of STP is good: Frames do not loop infinitely, which makes the LAN usable
However, STP has negative features as well, including the fact that it takes some work to balance traffic across the redundant alternate links
LAN Switching Concepts
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Avoiding Loops Using Spanning Tree Protocol
Network with Redundant Links but Without STP: The Frame Loops Forever
LAN Switching Concepts
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Avoiding Loops Using Spanning Tree Protocol
To avoid Layer 2 loops, all switches need to use STP
STP causes each interface on a switch to settle into either a blocking state or a forwarding state.
Blocking means that the interface cannot forward or receive data frames, while forwarding means that the interface can send and receive data frames.
If a correct subset of the interfaces is blocked, only a single currently active logical path exists between each pair of LANs
STP behaves identically for a transparent bridge and a switch
LAN Switching Concepts
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Internal Processing on Cisco Switches
As soon as a Cisco switch decides to forward a frame, the switch can use a couple of different types of internal processing variations
Three types of these internal processing methods are supported in at least one type of Cisco switch:1. Store-and-forward 2. Cut-through3. Fragment-free
With store-and-forward, the switch must receive the entire frame before forwarding the first bit of the frame.
LAN Switching Concepts
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Internal Processing on Cisco Switches
Because the destination MAC address occurs very early in the Ethernet header, a switch can make a forwarding decision long before the switch has received all the bits in the frame.
The cut-through and fragment-free processing methods allow the switch to start forwarding the frame before the entire frame has been received, reducing time required to send the frame (the latency, or delay)
With cut-through processing, the switch starts sending the frame out the output port as soon as possible. Although this might reduce latency, it also propagates errors. Because the Frame Check Sequence (FCS) is in the Ethernet trailer, the switch cannot determine whether the frame had any errors before starting to forward the frame. So, the switch reduces the frame’s latency, but with the price of having forwarded some frames that contain errors.
LAN Switching Concepts
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Internal Processing on Cisco Switches
Fragment-free processing works similarly to cut-through, but it tries to reduce the number of errored frames that it forwards.
One interesting fact about Ethernet CSMA/CD logic is that collisions should be detected within the first 64 bytes of a frame
Fragment-free processing works like cut-through logic, but it waits to receive the first 64 bytes before forwarding a frame.
The frames experience less latency than with store-and-forward logic and slightly more latency than with cut-through, but frames that have errors as a result of collisions are not forwarded
today’s switches typically use store-and-forward processing
LAN Switching Concepts
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Internal Processing on Cisco Switches
Switch Internal Processing
LAN Switching Concepts
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LAN Switching Features
Switch ports connected to a single device, providing dedicated bandwidth to that single device
Switches allow multiple simultaneous conversations between devices on different ports
Switch ports connected to a single device support full-duplex, in effect doubling the amount of bandwidth available to the device
Switches support rate adaptation, which means that devices that use different Ethernet speeds can communicate through the switch (hubs cannot)
Design Choices in Ethernet LANs
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Collision Domains
The different parts of an Ethernet LAN can behave differently, in terms of function and performance
The term collision domain referred to an Ethernet concept of all ports whose transmitted frames would cause a collision with frames sent by other devices in the collision domain
Collision Domains
Design Choices in Ethernet LANs
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Collision Domains
Only the hub allows a CD to spread from one side of the device to the other
If PC3 and the LAN switch both enabled half-duplex, which uses CSMA/CD, they would consider their frames to collide if they were sent and received at the same time
A collision domain is a set of network interface cards (NIC) for which a frame sent by one NIC could result in a collision with a frame sent by any other NIC in the same collision domain
Design Choices in Ethernet LANs
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Broadcast Domains
Only routers separate the LAN into multiple broadcast domains.
LAN switches flood Ethernet broadcast frames, extending the scope of the broadcast domain.
Routers do not forward Ethernet broadcast frames, either ignoring the frames, or processing and then discarding some broadcast from some overhead protocols used by routers.
bridges act like switches with broadcasts, and hubs repeat the signal, again not stopping the broadcasts
Design Choices in Ethernet LANs
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Broadcast Domains
Broadcasts sent by a device in one broadcast domain are not forwarded to devices in
another broadcast domain
A broadcast domain is a set of NICs for which a broadcast frame sent by one NIC is received by all other NICs in the same broadcast domain
Broadcast Domains
Design Choices in Ethernet LANs
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The Impact of Collision and Broadcast Domains on LAN Design
For a single collision domain: The devices share the available bandwidth The devices might inefficiently use that bandwidth because of
the effects of collisions, particularly under higher utilization
When a host receives a broadcast, the host must process the received frame. This means that the NIC must interrupt the computer’s CPU, and
the CPU must spend time thinking about the received broadcast frame
Broadcasts do require all the hosts to spend time processing each broadcast frame
Using smaller broadcast domains can also improve security, because of limiting broadcasts and because of robust security features in routers
Design Choices in Ethernet LANs
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The Impact of Collision and Broadcast Domains on LAN Design
Benefits of Segmenting Ethernet Devices Using Hubs, Switches, and Routers
Design Choices in Ethernet LANs
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Virtual LANs (VLAN)
A LAN consists of all devices in the same broadcast domain.
With VLANs, a switch groups interfaces into different VLANs (broadcast domains) based on configuration, with each interface in a different VLAN
Essentially, the switch creates multiple broadcast domains by putting some interfaces into one VLAN and other interfaces into other VLANs
So, instead of all ports on a switch forming a single broadcast domain, the switch separates them into many, based on configuration
Without VLANs, a switch considers all interfaces on the switch, and the devices connected to those links, to be in the same broadcast domain
Design Choices in Ethernet LANs
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Virtual LANs (VLAN)
Sample Network with Two Broadcast Domains and No VLANs
Sample Network with Two VLANs Using One Switch
Design Choices in Ethernet LANs
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Campus Design Terminology
The term campus LAN refers to the LAN created to support larger buildings, or multiple buildings in somewhat close proximity to one another
Cisco uses three terms to describe the role of each switch in a campus design:1. Access 2. Distribution3. Core
The roles differ based on whether: The switch forwards traffic from user devices and the rest of the
LAN (access) The switch forwards traffic between other LAN switches
(distribution and core)
Using designs that connect a larger number of access switches to a small number of distribution switches reduces cabling
Design Choices in Ethernet LANs
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Campus Design Terminology
Campus LAN with Design Terminology Listed
Design Choices in Ethernet LANs
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Campus Design Terminology
Access switches: Connect directly to end users, providing user device access to the
LAN. Send traffic to and from the end-user devices to which they are
connected and sit at the edge of the LAN
Distribution switches: Provide a path through which the access switches can forward traffic
to each other. Each of the access switches connects to at least one distribution
switch, relying on distribution switches to forward traffic to other parts of the LAN
Most designs use at least two uplinks to two different distribution switches for redundancy
Core switches: The largest campus LANs often use core switches to forward traffic between distribution switches
Design Choices in Ethernet LANs
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Ethernet LAN Media and Cable Lengths
When designing a campus LAN, an engineer must consider the length of each cable run and then find the best type of Ethernet and cabling type
10BASE-T, 100BASE-T, and 1000BASE-T have the same 100-meter cable restriction, but they use slightly different cables
The EIA/TIA defines Ethernet cabling standards, including the cable’s quality
Each Ethernet standard that uses UTP cabling lists a cabling quality category as the minimum category that the standard supports: 10BASE-T allows for Category 3 (CAT3) cabling or better 100BASE-T calls for higher-quality CAT5 cabling 1000BASE-T requires even higher-quality CAT5e or CAT6
Design Choices in Ethernet LANs
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Ethernet LAN Media and Cable Lengths
Optical cables support a variety of much longer distances than the 100 meters supported by Ethernet on UTP cables
Optical cables experience much less interference from outside sources as compared to copper cables
The type of optical cabling can also impact the maximum distances per cable: Multimode fiber supports shorter distances, but it is generally
cheaper cabling and it works fine with less-expensive LEDs. Single-mode fiber supports the longest distances but is more
expensive. Often use laser-based hardware
Design Choices in Ethernet LANs
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Ethernet LAN Media and Cable Lengths
Ethernet Types, Media, and Segment Lengths (Per IEEE)
Design Choices in Ethernet LANs
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Autonegotiation
Ethernet devices on the ends of a link must use the same standard or they cannot correctly send data
IEEE autonegotiation (IEEE standard 802.3u) defines a protocol that lets the two UTP-based Ethernet nodes on a link negotiate so that they each choose to use the same speed and duplex settings.
The protocol messages flow outside the normal Ethernet electrical frequencies as out-of-band signals over the UTP cable
Each node states what it can do, and then each node picks the best options that both nodes support: The fastest speed and the best duplex setting, with full-duplex
being better than half-duplex
Design Choices in Ethernet LANs
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Autonegotiation
Many networks use autonegotiation every day, particularly between user devices and the access layer LAN switches
IEEE Autonegotiation Results with Both Nodes Working Correctly
Design Choices in Ethernet LANs
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Autonegotiation Results When Only One Node Uses Autonegotiation
Most Ethernet devices can disable autonegotiation, so it is just as important to know what happens when a node tries to use autonegotiation but the node gets no response
If autonegotiation enabled on both ends of the link, the nodes should pick the best speed and duplex. However, when enabled on only one end, many issues can arise: The link might not work at all, or it might just work poorly
IEEE autonegotiation defines some rules that nodes should use when autonegotiation fails:
Speed: Use your slowest supported speed (often 10 Mbps) Duplex: If your speed = 10 or 100, use half-duplex; otherwise, use
full-duplex Cisco switches can actually sense the speed used by other node,
even without IEEE autonegotiation
Design Choices in Ethernet LANs
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Autonegotiation Results When Only One Node Uses Autonegotiation
IEEE Autonegotiation Results with Autonegotiation Disabled on One Side
Design Choices in Ethernet LANs
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Autonegotiation Results When Only One Node Uses Autonegotiation
PC1 shows a classic and unfortunately common end result: a duplex mismatch
The two nodes can send data However, PC1, using full-duplex, does not attempt to use CSMA/CD logic and sends frames at any time.
Switch port F0/1, with halfduplex, does use CSMA/CD. As a result, switch port F0/1 will believe collisions occur on the link, even if none physically occur
The switch port will stop transmitting, back off, resend frames, and so on. As a result, the link is up, but it performs poorly
when both devices are attempting to transmit at the same time, the packet sent by the full-duplex end will be discarded and lost due to an assumed collision and the packet sent by the half duplex device will be delayed or lost due to a CRC error in the frame
Design Choices in Ethernet LANs
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Autonegotiation and LAN Hubs
Hubs do not react to autonegotiation messages, and they do not forward the messages.
As a result, devices connected to a hub must use the IEEE rules for choosing default settings, which often results in the devices using 10 Mbps and halfduplex
IEEE Autonegotiation with a LAN Hub
Building Ethernet LANs with Switches
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References
1) Cisco Systems, Inc, www.cisco.com/
2) Wendell Odom ,”Cisco CCENT/CCNA ICND1 100-101 Official Cert Guide”, Cisco Press, USA, 2013