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Level 6

Building Ethernet LANs with Switches


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forward switch
Ethernet switches receive Ethernet frames in one port and then __ or __ the frames out one (or more) other port.
collisions
The increase in traffic volumes resulted in an increased number of __, requiring more retransmissions and wasting more LAN capacity.
collision domains
Bridges separated devices into groups called __.
bandwidth
Bridges increased __ by giving each collision domain its own separate __, with one sender at a time per collision domain.
buffer queue
The bridge forwards frames between its two interfaces, and unlike a hub, a bridge will __ or __ the frame until the outgoing interface can send the frame.
forward Ethernet frames
Ultimately, the role of a LAN switch is to __.
table
To decide whether to forward a frame, a switch uses a dynamically built __ that lists MAC addresses and outgoing interfaces.
switching table bridging table Content Addressable Memory
A switch’s MAC address table is also called the __, or __, or even the __ table, in reference to the type of physical memory used to store the table.
forward-versus-filter
The forwarding choice by a switch was formerly called a __ decision, because the switch also chooses to not forward (to filter) frames, not sending the frame out some ports.
MAC addresses interfaces
The second main function of a switch is to learn the __ and __ to put into its address table.
source MAC address
Switches build the address table by listening to incoming frames and examining the __ in the frame.
Learning
__ always occurs with switches by looking at the source MAC address in the frame.
inactivity timer 0
Switches keep a timer for each entry in the MAC address table, called an __. The switch sets the timer to __ for new entries.
reset to 0
Each time the switch receives another frame with that same source MAC address, the timer is __.
upward longest time
The timer counts __, so the switch can tell which entries have gone the __ since receiving a frame from that device.
old runs out of space
The switch then removes entries from the CAM table when they become __. Or, if the switch ever __ for entries in the MAC address table.
flooding
when there is no matching entry in the table, switches forward the frame out all interfaces (except the incoming interface) using a process called __.
unknown unicast frames
Switches flood __ (frames whose destination MAC addresses are not yet in the address table).
copy all
Switches also forward LAN broadcast frames, because this process helps deliver a __ of the frame to __ devices in the LAN.
same interface
The switch does not forward the frame back out F0/1, because a switch never forwards a frame out the __ on which it arrived.
loop prevention
The third primary feature of LAN switches is __, as implemented by Spanning Tree Protocol.
STP
Without __, any flooded frames would loop for an indefinite period of time in Ethernet networks with physically redundant links.
STP one active path
To prevent looping frames, __ blocks some ports from forwarding frames so that only __ exists between any pair of LAN segments.
physical loop
Because none of the switches list Bob’s MAC address in their address tables, each switch floods the frame. A __ exists through the three switches.
blocking forwarding
STP causes each interface on a switch to settle into either a __ state or a __ state.
Blocking forwarding
__ means that the interface cannot forward or receive data frames, while __ means that the interface can send and receive data frames.
active logical path
If a correct subset of the interfaces is blocked, only a single currently __ exists between each pair of LANs.
internal processing variations
As soon as a Cisco switch decides to forward a frame, the switch can use a couple of different types of __.
destination MAC address
Because the __ 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.
Store-and-forward
The switch fully receives all bits in the frame (store) before forwarding the frame (forward). This allows the switch to check the FCS before forwarding the frame.
Cut-through
The switch forwards the frame as soon as it can. This reduces latency but does not allow the switch to discard frames that fail the FCS check.
Fragment-free
The switch forwards the frame after receiving the first 64 bytes of the frame, thereby avoiding forwarding frames that were errored because of a collision.
cut-through fragment-free
The __ and __ processing methods allow the switch to start forwarding the frame before the entire frame has been received.
store-and-forward
Today’s switches typically use __ processing, because the improved latency of the other two switching methods is negligible at these speeds.
microsegment
Switch ports connected to a single device __ the LAN, providing dedicated bandwidth to that single device.
full-duplex
Switch ports connected to a single device support __, in effect doubling the amount of bandwidth available to the device.
rate adaptation
Switches support __, which means that devices that use different Ethernet speeds can communicate through the switch (hubs cannot).
Ethernet data link header
Switches use Layer 2 logic, examining the __ to choose how to process frames.
interface
For learning MAC address table entries, each received frame, switches examine the source MAC address and note the __ from which the frame was received.
collision domain
A __ is a set of network interface cards 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.
broadcast domain
A __ 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.
share
For a single collision domain, the devices __ the available bandwidth.
doubling
With replacing the hub with a switch, the ability to use full-duplex on each link, effectively __ the capacity.
spend time processing
Broadcasts happen, which is good, but broadcasts do require all the hosts to __ each broadcast frame.
broadcast domains
However, a design that separated the 500 PCs into five groups of 100, separated from each other by a router, would create five __.
security
Using smaller broadcast domains can also improve __, because of limiting broadcasts and because of robust __ features in routers.
Hub Switch Router
Greater cabling distances are allowed
Switch Router
Creates multiple collision domains
Switch Router
Increases bandwidth
Router
Creates multiple broadcast domains
VLAN
A LAN consists of all devices in the same broadcast domain.
VLANs
Without __, a switch considers all interfaces on the switch, and the devices connected to those links, to be in the same broadcast domain.
VLAN 1
All connected devices are in the same LAN. (Cisco switches accomplish this by putting all interfaces in __ by default.)
multiple broadcast domains
The switch creates __ by putting some interfaces into one VLAN and other interfaces into other VLANs.
VLANs
Before __ existed, if a design specified ten separate broadcast domains, ten switches would be used—one for each broadcast domain.
campus LAN
The term __ refers to the LAN created to support larger buildings, or multiple buildings in somewhat close proximity to one another.
types of Ethernet cabling lengths
When planning and designing a campus LAN, the engineers must consider the __ available and the __ supported by each type.
speeds
The engineers of the campus LAN, also need to choose the __ required for each Ethernet segment.
access distribution core
Cisco uses three terms to describe the role of each switch in a campus design: __, __, and __.
Access switches
__ connect directly to end users, providing user device access to the LAN.
Access switches
__ normally send traffic to and from the end-user devices to which they are connected and sit at the edge of the LAN.
distribution switches
In larger campus LANs, __ provide a path through which the access switches can forward traffic to each other.
distribution switch
By design, each of the access switches connects to at least one __, relying on them to forward traffic to other parts of the LAN.
cabling needs
Using designs that connect a larger number of access switches to a small number of distribution switches reduces __ while still allowing all devices to send data to all other devices in the LAN.
core switches
The largest campus LANs often use __ to forward traffic between distribution switches.
Access
Provides a connection point for end-user devices. Does not forward frames between two other __ switches under normal circumstances.
Distribution
Provides an aggregation point for access switches, forwarding frames between switches, but not connecting directly to end-user devices.
Core
Aggregates distribution switches in very large campus LANs, providing very high forwarding rates.
EIA/TIA
The __ defines Ethernet cabling standards, including the cable’s quality.
LED laser
__based hardware, often used with multimode fiber for short distances, is much less expensive than __based hardware, often used with singlemode fiber which supports the longest distances.
same standard
Ethernet devices on the ends of a link must use the __ or they cannot correctly send data.
autonegotiation 802.3u
IEEE __ (IEEE standard __) 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.
fastest speed best duplex setting
In autonegotiation, each node states what it can do, and then each node picks the best options that both nodes support: the __ and the __.
access layer LAN switches
Many networks use autonegotiation every day, particularly between user devices and the __.
10 Mbps full
The switch port claims it can go as fast as 1000 Mbps, but PC1’s NIC claims a top speed of 10 Mbps. Both the PC and switch choose the best speed both support (__) and the best duplex (__).
both ends
Disabling autonegotiation is not always a bad idea, but generally you should either use it on __ of the link or disable it on __ of the link.
10 Mbps
When autonegotiation fails, sense the speed, or if that fails, use the IEEE default (slowest supported speed, often __).
half-duplex full-duplex
When autonegotiation fails, use the IEEE Duplex defaults: If speed = 10 or 100, use __; otherwise, use __.
10 Mbps half-duplex
Hubs do not react to autonegotiation messages, and they do not forward the messages. Devices connected to a hub must use the IEEE rules for choosing default settings, which often results in the devices using __ and __.