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What are the three switch functions?
- address learning
- forwared/filter decisions
- loop avoidance
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What command do you use to show you the foward/filter table used on the LAN switch?
show mac address-table
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What is the main purpose of the Spanning Tree Protocol in a switched LAN?
The main purpose of STP is to to prevent switching loops in a network with redundant switched paths.
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What command do you use to determine who the root bridge is?
show spanning-tree
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Switches use______________to build and maintain their filter tables.
application specific integrated circuits (ASICs)
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What makes Layer 2 switches and bridges faster than routers?`
they don't take up time looking at the Network Layer header information, instead they look at the frame's hardware addresses before deciding to forward, flood or drop the frame
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What does Layer 2 switching provide?
- hardware-based bridging (ASIC)
- wire speed
- low latency
- low cost
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What makes layer 2 switching so efficient?
no modification to the data packet takes place
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What makes switching process faster and less error-prone than routing process?
the device only reads the frame encapsulating the packet
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Layer 2 switching increases bandwidth for each user because?
each connection into the switch is its own collision domain
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What are two important things to consider when designing layer 2 switching?
- must break up the collision domains correctly
- make sure users spend 80% of their time on the local segment
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What is the process of Address Learning?
remembers the source hardware address of each frame received on an interface, and they enter it into a MAC database called the forward/filter table
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What is the process of forward/filter decisions?
when a frame is received on an interface, the switch looks at the destination hardware address and finds the exit interface in the MAC database. the frame is only forwarded out the specified destination port.
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What is loop avoidance?
when multiple connections between switches are created for redundancy purposes, network loops can occur. Spanning tree protocol is used to stop network loops while still permitting redundancy
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This layer 2 protocol is used to maintain a loop-free switched network.
STP - Spanning Tree Protocol
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What commands would you use to stop MAC addresses from dynamically appearing in the MAC forward/filter database?
switchport port-security
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What commands do you enter to setup Port Security?
- conf t
- int "specify interface"
- swithchport port-security maximum "#"
- switchport port-security violation shutdown
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What does the "sticky" command do?
provides static MAC address security without having to type in everyone's MAC address on the network
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The bridge with the best bridge ID?
this is known as the root bridge
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This is how STP keeps track of all the switches in the network?
bridge ID - determined by a combo of the bridge priority and the base MAC address.
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Which bridge becomes the root bridge in the network?
the bridge with the lowest bridge ID
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What is the default bridge priority on all Cisco switches?
32,768
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These are bridges that are NOT the root bridge and exchange BPDUs with all bridges and update the STP topology database
nonroot bridges
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A Spanning Tree Protocol initializing packets that is sent at definable intervals for the purpose of exchanging information among bridges in networks
BPDU - Bridge Protocol Data Unit
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This determines the best path when multiple links are used between two switches and none of the links is a root port?
Port cost
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How is port cost determined?
by the bandwidth of a link
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This port is always the link that is directly connected to the root bridge, or the shortest path to the root bridge.
Root port
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This port is marked as a forwarding port and determined as having the best (lowest) cost.
designated port
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This port has higher cost, not forwarding ports and put in blocking mode
nondesignated port
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This port forwards frames
forwarding port
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The port that will not forward frames in order to prevent loops.
a blocked port
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How is port cost determined if more than one link connects to the root bridge?
this is determined by checking the bandwidth of each link
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What happens if multiple links have the same cost?
the bridge with the lower advertising bridge ID is use.
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What is the job of STP?
To find all links in the network and shut down any redundant ones, preventing network loops from occurring.
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What is the first step in STP Operations process?
Electing a root bridge that will forward all ports and act as a point of reference for all other devices in the STP domain
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How does STP select its root bridge?
combine priority of each bridge with its MAC address
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What occurs if two switches or bridges happen to have the same priority value?
The MAC address becomes the tiebreaker for figuring out which one has the lowest (best) ID.
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How long is the bridge ID and what does it include?
bridge ID is 8 bytes long and includes priority and the MAC address of the device
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What is the rate that BPDUs are sent out all active ports on a bridge/switch?
every two seconds
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How do you force a bridge/switch to become a root bridge?
You change the bridge ID by lowering it's priority
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To prevent a switching loop, how are frames handled on a bridge/switch when a port is blocked?
it won't forward/transmit frames out the block port, but it will receive them, including BPDUs
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What command do you use to set the priority on a switch and use any value from 0 through?
spanning-tree vlan 1 priority "#"
value can be 0 thru 61440
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What command do you use to view status of STP?
show spanning-tree
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How many different states can STP transition thru running on a bridge or switch and what are they?
- 5 different states
- Blocking
- Listening
- Learning
- Fowarding
- Disabled
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This STP port state won't forward frames; it just listens to BPDUs. The purpose of this state is to prevent the use of looped paths. All ports are in this state by default when the switch is powered up.
Blocking state
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This STP port state sends and receives all data frames on the bridged port. If the port is still a designated or root port at the end of the learning state, it enters the fowarding state.
Forwarding State
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This STP port state listens to DPDUs to make sure no loops occur on the network before passing data frames. A port in this state prepares to forward data frames without populating the MAC address table.
Listening state
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This STP port status does not participate in the frame forwarding or STP. A port in this state is virtually nonoeprational.
Disabled state
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This STP port state listens to BPDUs and learns all paths in the switched network. It populates the MAC address table but doesn't forward data frames. Forward delay means the time it takes to transition a port from listening to this STP port state mode, which is set to 15 seconds by default and can be seen in the show spanning-tree output.
Learning state
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_______ occurs on bridges and switches have transitioned to either forwarding or blocking modes.
Convergence
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What is happening when STP is converging?
all host data stops transmitting
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Why is convergence important?
because it ensures that all devices have the same database
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What is the cost rate to go from blocking to forwarding mode?
50 seconds
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You would use this command if you have a server or other devices connected into your switch that you aren't sure won't create a switching loop if STP is disabled?
spanning-tree portfast
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Using this command means the port won't spend the usual 50 seconds to come up and into forwarding mode while STP is converging?
spanning-tree portfast
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What preceding command allows you to set multiple ports on a switch into portfast mode?
interface range and can be used in conjunction with any command
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This Cisco-specific feature improves the convergence time of STP in case of a link failure and designed to run in a switched environment when the switch has at least one alternate/backup root port.
spanning-tree uplinkfast
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This Cisco-proprietary STP extension is used for speeding up convergence when a link that's not directly connected to the switch fails?
spanning-tree backbonefast
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When does Cisco recommend to use spanning tree uplink fast?
This should only be enabled for switches with blocked ports and, typically, at the Access layer.
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When does Cisco recommend using and enabling BackboneFast?
This should only be enabled on all Catalyst switches to allow for detection of indirect link failures allowing
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What is the Cisco versions of EtherChannel and the IEEE version?
Cisco's version is called Port Aggregation Protocol (PAgP) and the IEEE 802.3ad standard called Link Aggregation Control Protocol (LACP)
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To "fix" holes and liabilities the IEEE 802.1d standard presented, Cisco created this protocol to work with all brands of swtiches.
Rapid Spanning Tree Protocol (RSTP) 802.1w
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