Term
| What is the correct terminology for a Spanning-Tree Loop? |
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Definition
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Term
| What is the well-known multicast address that BPDUs are sent to? |
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Definition
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Term
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Definition
| 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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Term
| What are the two types of BPDUs, and define their use. |
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Definition
-Configuration BPDU, used for Spanning Tree computation.
-Topology Change Notification (TCN) BPDU, used to announce changes in the network topology. |
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Term
| By default, how often are BPDUs sent? |
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Definition
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Term
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Definition
| 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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Term
| What is the value range and default value for bridge priority? |
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Definition
| 0-65535, defaults to 32768(0x8000) |
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Term
| Will a lower value or higher value make a switch the root bridge? |
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Definition
| Lower Bridge Priority/MAC |
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Term
| How many root ports can a single switch have? |
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Definition
| Only ONE, that always points towards the root bridge! |
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Term
| The cumulative cost of all the links leading to the root bridge is called the _______? |
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Definition
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Term
| What is the STP path cost of 10G, 1G, 622M, 155M, 100M, and 10M? |
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Definition
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Term
| When are new path costs from BPDUs computed on a switch? |
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Definition
| when the BPDU comes IN the switch. |
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Term
| What is a designated port in Spanning Tree? |
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Definition
| 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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Term
| What are the 4 tie-breaking STP decisions? |
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Definition
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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Term
| What are the 5 STP port states? |
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Definition
-Disabled
-Blocking
-Listening
-Learning
-Forwarding |
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Term
| Explain the 'Disabled' port state in STP. |
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Definition
| a port that is administratively shut down by an administrator or by the system because of a fault. |
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Term
| Explain the 'Blocking' port state in STP. |
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Definition
| 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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Term
| Explain the 'Listening' port state in STP. |
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Definition
| 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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Term
| Explain the 'learning' port state in STP. |
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Definition
| 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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Term
| Explain the 'Forwarding' port state in STP. |
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Definition
| 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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Term
| What is the default forward delay timer? |
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Definition
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Term
| What are the 5 steps to manual STP computation? |
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Definition
-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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Term
| What are the three STP timers? |
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Definition
| Hello time, Forward Delay, and Max Age |
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Term
| What is the STP hello timer? |
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Definition
| the time between Configuration BPDUs sent by the root bridge. Default is 2 seconds. Range is 1-10 |
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Term
| What is the STP Forwarding Delay? |
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Definition
| 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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Term
| What is the STP Max Age timer? |
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Definition
| The time that a switch stores a BPDU before discarding it. Default is 20 seconds. Range is 6-40 |
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Term
| When does STP acknowledge that a topology change has occurred? |
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Definition
| 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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Term
| TCN BPDUs are send out what port? |
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Definition
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Term
| What configuration will cause a TCN BPDU to NOT be sent when a topology change occurs? |
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Definition
| spanning-tree PortFast on the interface. |
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Term
| What is the default bridge table aging timer? |
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Definition
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Term
| 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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Definition
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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Term
| What are the three main types of STP? |
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Definition
| CST (common spanning tree), PVST (Per-VLAN spanning tree), and PVST+ |
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Term
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Definition
| Common Spanning Tree. Based on IEEE 802.1Q when all BPDUs are transmitted over the native VLAN with untagged frames. |
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Term
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Definition
| 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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Term
| Can BPDUs be shared by CST and PVST? |
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Definition
| Never, as they require different trunking methods. |
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Term
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Definition
| PVST plus. Allows all devices to operate with CST, PVST, and PVST+. It operates over both ISL and 802.1Q |
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Term
| What is a secondary root bridge used for and how do you configure one? |
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Definition
| 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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Term
| Where is the ideal placement of the pri/sec root bridge? |
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Definition
| Near the center of the Layer 2 network on a powerful switch. |
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Term
| In STP extended system-id, bridge priority can range from 0-61440 but only in multiples of _____? |
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Definition
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Term
| What command set the bridge priority for STP? |
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Definition
(config)
spanning-tree vlan ## priority ##### |
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Term
| What does the 'spanning tree vlan # root primary' command do? |
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Definition
| 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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Term
| What does the 'spanning tree vlan # root secondary' command do? |
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Definition
| 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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Term
| Will an STP bridge priority value of 0 work? |
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Definition
| Not with the automatic method. Only with the manual method. |
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Term
| What command will allow you to see the spanning tree Port Cost value of an interface? |
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Definition
(priv)
show spanning-tree interface TYPE MOD/NUM cost |
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Term
| What is contained in the STP Port ID? |
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Definition
| 8-bits for the Port Priority and 8-bits for the Port Number. |
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Term
| What is the value range and default value of STP port priority? |
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Definition
| 0-255, and defaults to 128. |
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Term
| What command sets the STP port priority on a switch port? |
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Definition
(if)
spanning-tree vlan ## port-priority ###
(lower is better!) |
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Term
| What happens when you change a STP timer on the root bridge? |
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Definition
| It propagates all three timer values throughout the network with the BPDUs. |
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Term
| What is the range for network diameter in STP? |
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Definition
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Term
| How can you change the network diameter value in STP? |
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Definition
| add 'diameter #' to the end of the command 'spanning-tree vlan # root primary' |
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Term
| When should you use STP PortFast? |
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Definition
| Only from access layer switches to workstations. |
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Term
| When should you use STP UplinkFast? |
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Definition
| Only from access layer switches to distro layer switches when there are dual uplinks. |
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Term
| When should you use STP BackboneFast? |
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Definition
| Enables faster convergence on core layer switches after changes occur. |
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Term
| How much extra delay time is added to the default spanning tree process of 30 seconds when a PAgP Etherchannel link in configured for STP? |
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Definition
| An extra 20 seconds. This brings the total negation time to 50 seconds. |
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Term
| What happens to a PortFast port that receives a BPDU? |
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Definition
| It immediately enters the blocking state. |
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Term
| What command enables all ports configured for Access mode to use PortFast? |
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Definition
(config)
spanning-tree portfast default |
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Term
| What macro command enables PortFast, sets the port to access, and disabled PAgP? |
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Definition
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Term
| Where should UplinkFast be used? |
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Definition
| On leaf-node switches, or switches on the ends of spanning tree branches. |
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Term
| What does the UplinkFast command do to a switch? |
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Definition
It keeps one functioning root port and one or more potential root ports in blocking mode. When that primary root port fails, another blocked potential root port immediately comes up.
It also changes the bridge priority to 49152 and increases all path costs by 3000. |
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Term
| When is the UplinkFast command not allowed? |
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Definition
| On the root bridge switch. |
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Term
| What multicast address is sent to by UplinkFast when a new link comes up, and what is sent? |
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Definition
0100.0ccd.cdcd
-It sends all of the MAC addresses in the bridging table as the source address to update upstream devices. |
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Term
| What is the command to enable UplinkFast on a switch and set the max-update-rate? Also what is the ranges and purpose of this command? |
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Definition
(config)
spanning-tree uplinkfast max-update-rate #####
This limits how often the device will send multicast messages on fail-over, and the range is 0-65535 |
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Term
| In STP, a value of 0 on the max-update-rate parameter for UplinkFast means what? |
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Definition
| It means that no multicast messages will be sent on fail-over. |
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Term
| What is an indirect link failure in STP, and how does a switch detect them? |
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Definition
| 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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Term
| When are inferior BPDUs send in STP? |
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Definition
| When a designated bridge loses connectivity to the root bridge, making it advertise itself as the new root. |
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Term
| If a switch has BackboneFast and any blocked ports, what does it use to figure out alternate paths? |
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Definition
| The Root Link Query (RLQ) protocol. |
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Term
| If an RLQ reply is received on a root port, what does this mean? A non-root port? |
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Definition
-That the path to the root bridge is in tact and stable.
-Non-root: that a new path must be chosen. |
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Term
| What does BackboneFast do to the Max-Age Timer on a switch? |
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Definition
| It short-circuits the max age timer, and reduces the convergence delay from 50 to 30 seconds. |
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Term
| If you use BackboneFast, what switches should it be used on? |
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Definition
| It should be used on ALL switches on the network because of the RLQ request and reply process. |
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