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42 Cards in this Set
- Front
- Back
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How many root ports can a single switch have?
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Only ONE, that always points towards the root bridge!
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What are the 4 tie-breaking STP decisions?
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1. Lowest Bridge ID
2. Lowest root path cost to root bridge 3. Lowest sender bridge ID 4. Lowest sender port ID |
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What is the correct terminology for a Spanning-Tree Loop?
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A Bridging Loop
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Explain the 'Disabled' port state in STP.
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Explain the 'Disabled' port state in STP.
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The cumulative cost of all the links leading to the root bridge is called the _______?
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root path cost.
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What is the STP Max Age timer?
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The time that a switch stores a BPDU before discarding it. Default is 20 seconds. Range is 6-40
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By default, how often are BPDUs sent?
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Every 2 seconds.
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Explain the 'learning' port state in STP
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After the 'forward delay' from the listening state, it becomes a learning state. This port can still send and receive BPDUs, but can also now learn new MAC addresses.
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What does the 'spanning tree vlan # root primary' command do?
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It makes the switch attempt to become the root bridge. If the current root bridge priority is more than 24576, it sets its priority to 24576. If the current root bridge is less than that, it sets its priority to 4096 less than the current root
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define PVST+
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PVST plus. Allows all devices to operate with CST, PVST, and PVST+. It operates over both ISL and 802.1Q
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Will a lower value or higher value make a switch the root bridge?
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Lower Bridge Priority/MAC
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When should you use STP PortFast?
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Only from access layer switches to workstations.
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What is an indirect link failure in STP, and how does a switch detect them?
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It occurs when a link that is not directly connected to a switch fails. It is detected whenever a switch starts receiving inferior BPDUs from its designated bridge on the root or blocked ports.
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Where is the ideal placement of the pri/sec root bridge?
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Near the center of the Layer 2 network on a powerful switch.
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What is the default bridge table aging timer?
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300 seconds
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In STP extended system-id, bridge priority can range from 0-61440 but only in multiples of _____?
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4096
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Explain the 'Listening' port state in STP.
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This port can only send and receive BPDUs. Because of this it is allowed to become a root or designated port.
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When are new path costs from BPDUs computed on a switch?
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when the BPDU comes IN the switch.
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What is contained in the STP Port ID?
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8-bits for the Port Priority and 8-bits for the Port Number.
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What is a designated port in Spanning Tree?
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A port where only BDPUs with HIGHER root path costs are received. This means the that the other switch attached to that port is using that port as a root port! This port should stay online
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What happens to a PortFast port that receives a BPDU?
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It immediately enters the blocking state
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What is the STP hello timer?
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the time between Configuration BPDUs sent by the root bridge. Default is 2 seconds. Range is 1-10
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What does the 'spanning tree vlan # root secondary' command do?
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It sets the bridge priority to 28672. This is less than the default 32768, but more than the 'primary' keyword of 24576
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What is a secondary root bridge used for and how do you configure one?
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It is used in case the primary root bridge goes down you can engineer its replacement. Simply make another switch have a value lower than the default but higher than the primary root.
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What is the value range and default value for bridge priority?
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0-65535, defaults to 32768(0x8000)
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What is the STP path cost of 10G, 1G, 622M, 155M, 100M, and 10M?
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2, 4, 6, 14, 19, and 100
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What is a BPDU?
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a Bridge Protocol Data Unit. It is a data message that is exchanged between switches to support the Spanning Tree Protocol.
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What command sets the STP port priority on a switch port?
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(if)
spanning-tree vlan ## port-priority ### (lower is better!) |
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What are the 5 steps to manual STP computation?
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-ID path costs on all links.
-ID a root bridge. -Select root ports (1 per switch) -Select designated ports -ID blocking ports |
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What happens when you change a STP timer on the root bridge?
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It propagates all three timer values throughout the network with the BPDUs.
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When a link goes down in STP, what is the total down time of that device based off of? / calculated from?
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1. The max-age timer (20 seconds)
2. The Hello timer (2 seconds) 3. The forwarding delay for both listening and learning (15 seconds x2!) |
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Define STP PVST
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Per-VLAN Spanning Tree. Cisco Proprietary, and operates a separate instance of STP on each VLAN. Requires ISL encapsulation between switches. PVST and CST cannot easily coexist
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What is a root bridge?
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The root bridge is the bridge with the lowest Bridge Priority value, or the lowest MAC is all values are equal. It has no disabled(blocking) ports
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What command will allow you to see the spanning tree Port Cost value of an interface?
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(priv)
show spanning-tree interface TYPE MOD/NUM cost |
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What are the 5 STP port states?
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-Disabled
-Blocking -Listening -Learning -Forwarding |
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What command set the bridge priority for STP?
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(config)
spanning-tree vlan ## priority ##### |
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What is the default forward delay timer?
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15 seconds.
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Explain the 'Forwarding' port state in STP.
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After another forwarding delay from the learning state, the port is now a fully functioning port, and can send and receive all data
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Explain the 'Blocking' port state in STP
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Ports start in this state. This port cannot send or receive data and cannot add MAC addresses to its table. It can only receive BPDUs
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What is the STP Forwarding Delay?
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The time that a switch port must spend in the listening and learning states before advancing to the forwarding state. Default is 15 seconds. Range is 4-30
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When does STP acknowledge that a topology change has occurred?
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When a port goes up or down, usually any port going to the forwarding state, or a port going to the blocking state.
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What are the three STP timers?
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Hello time, Forward Delay, and Max Age
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