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CCNP SWITCH 642-813 - Spanning Tree
Spanning Tree
69
Computer Networking
07/07/2010

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Cards

Term
What is the correct terminology for a Spanning-Tree Loop?
Definition
A Bridging Loop.
Term
What is the well-known multicast address that BPDUs are sent to?
Definition
01-80-c2-00-00-00
Term
What is a BPDU?
Definition
a Bridge Protocol Data Unit. It is a data message that is exchanged between switches to support the Spanning Tree Protocol.
Term
What are the two types of BPDUs, and define their use.
Definition
-Configuration BPDU, used for Spanning Tree computation.
-Topology Change Notification (TCN) BPDU, used to announce changes in the network topology.
Term
By default, how often are BPDUs sent?
Definition
Every 2 seconds.
Term
What is a root bridge?
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.
Term
What is the value range and default value for bridge priority?
Definition
0-65535, defaults to 32768(0x8000)
Term
Will a lower value or higher value make a switch the root bridge?
Definition
Lower Bridge Priority/MAC
Term
How many root ports can a single switch have?
Definition
Only ONE, that always points towards the root bridge!
Term
The cumulative cost of all the links leading to the root bridge is called the _______?
Definition
root path cost.
Term
What is the STP path cost of 10G, 1G, 622M, 155M, 100M, and 10M?
Definition
2, 4, 6, 14, 19, and 100
Term
When are new path costs from BPDUs computed on a switch?
Definition
when the BPDU comes IN the switch.
Term
What is a designated port in Spanning Tree?
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!
Term
What are the 4 tie-breaking STP decisions?
Definition
1. Lowest Bridge ID
2. Lowest root path cost to root bridge
3. Lowest sender bridge ID
4. Lowest sender port ID
Term
What are the 5 STP port states?
Definition
-Disabled
-Blocking
-Listening
-Learning
-Forwarding
Term
Explain the 'Disabled' port state in STP.
Definition
a port that is administratively shut down by an administrator or by the system because of a fault.
Term
Explain the 'Blocking' port state in STP.
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.
Term
Explain the 'Listening' port state in STP.
Definition
This port can only send and receive BPDUs. Because of this it is allowed to become a root or designated port.
Term
Explain the 'learning' port state in STP.
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.
Term
Explain the 'Forwarding' port state in STP.
Definition
After another forwarding delay from the learning state, the port is now a fully functioning port, and can send and receive all data.
Term
What is the default forward delay timer?
Definition
15 seconds.
Term
What are the 5 steps to manual STP computation?
Definition
-ID path costs on all links.
-ID a root bridge.
-Select root ports (1 per switch)
-Select designated ports
-ID blocking ports.
Term
What are the three STP timers?
Definition
Hello time, Forward Delay, and Max Age
Term
What is the STP hello timer?
Definition
the time between Configuration BPDUs sent by the root bridge. Default is 2 seconds. Range is 1-10
Term
What is the STP Forwarding Delay?
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
Term
What is the STP Max Age timer?
Definition
The time that a switch stores a BPDU before discarding it. Default is 20 seconds. Range is 6-40
Term
When does STP acknowledge that a topology change has occurred?
Definition
When a port goes up or down, usually any port going to the forwarding state, or a port going to the blocking state.
Term
TCN BPDUs are send out what port?
Definition
The switches root port.
Term
What configuration will cause a TCN BPDU to NOT be sent when a topology change occurs?
Definition
spanning-tree PortFast on the interface.
Term
What is the default bridge table aging timer?
Definition
300 seconds.
Term
When a link goes down in STP, what is the total down time of that device based off of? / calculated from?
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!)
Term
What are the three main types of STP?
Definition
CST (common spanning tree), PVST (Per-VLAN spanning tree), and PVST+
Term
Define STP CST.
Definition
Common Spanning Tree. Based on IEEE 802.1Q when all BPDUs are transmitted over the native VLAN with untagged frames.
Term
Define STP PVST.
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.
Term
Can BPDUs be shared by CST and PVST?
Definition
Never, as they require different trunking methods.
Term
define PVST+
Definition
PVST plus. Allows all devices to operate with CST, PVST, and PVST+. It operates over both ISL and 802.1Q
Term
What is a secondary root bridge used for and how do you configure one?
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.
Term
Where is the ideal placement of the pri/sec root bridge?
Definition
Near the center of the Layer 2 network on a powerful switch.
Term
In STP extended system-id, bridge priority can range from 0-61440 but only in multiples of _____?
Definition
4096
Term
What command set the bridge priority for STP?
Definition
(config)
spanning-tree vlan ## priority #####
Term
What does the 'spanning tree vlan # root primary' command do?
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.
Term
What does the 'spanning tree vlan # root secondary' command do?
Definition
It sets the bridge priority to 28672. This is less than the default 32768, but more than the 'primary' keyword of 24576
Term
Will an STP bridge priority value of 0 work?
Definition
Not with the automatic method. Only with the manual method.
Term
What command will allow you to see the spanning tree Port Cost value of an interface?
Definition
(priv)
show spanning-tree interface TYPE MOD/NUM cost
Term
What is contained in the STP Port ID?
Definition
8-bits for the Port Priority and 8-bits for the Port Number.
Term
What is the value range and default value of STP port priority?
Definition
0-255, and defaults to 128.
Term
What command sets the STP port priority on a switch port?
Definition
(if)
spanning-tree vlan ## port-priority ###
(lower is better!)
Term
What happens when you change a STP timer on the root bridge?
Definition
It propagates all three timer values throughout the network with the BPDUs.
Term
What is the range for network diameter in STP?
Definition
1-7, default is 7.
Term
How can you change the network diameter value in STP?
Definition
add 'diameter #' to the end of the command 'spanning-tree vlan # root primary'
Term
When should you use STP PortFast?
Definition
Only from access layer switches to workstations.
Term
When should you use STP UplinkFast?
Definition
Only from access layer switches to distro layer switches when there are dual uplinks.
Term
When should you use STP BackboneFast?
Definition
Enables faster convergence on core layer switches after changes occur.
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?
Definition
An extra 20 seconds. This brings the total negation time to 50 seconds.
Term
What happens to a PortFast port that receives a BPDU?
Definition
It immediately enters the blocking state.
Term
What command enables all ports configured for Access mode to use PortFast?
Definition
(config)
spanning-tree portfast default
Term
What macro command enables PortFast, sets the port to access, and disabled PAgP?
Definition
(if)
switchport host
Term
Where should UplinkFast be used?
Definition
On leaf-node switches, or switches on the ends of spanning tree branches.
Term
What does the UplinkFast command do to a switch?
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.
Term
When is the UplinkFast command not allowed?
Definition
On the root bridge switch.
Term
What multicast address is sent to by UplinkFast when a new link comes up, and what is sent?
Definition
0100.0ccd.cdcd
-It sends all of the MAC addresses in the bridging table as the source address to update upstream devices.
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?
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
Term
In STP, a value of 0 on the max-update-rate parameter for UplinkFast means what?
Definition
It means that no multicast messages will be sent on fail-over.
Term
What is an indirect link failure in STP, and how does a switch detect them?
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.
Term
When are inferior BPDUs send in STP?
Definition
When a designated bridge loses connectivity to the root bridge, making it advertise itself as the new root.
Term
If a switch has BackboneFast and any blocked ports, what does it use to figure out alternate paths?
Definition
The Root Link Query (RLQ) protocol.
Term
If an RLQ reply is received on a root port, what does this mean? A non-root port?
Definition
-That the path to the root bridge is in tact and stable.
-Non-root: that a new path must be chosen.
Term
What does BackboneFast do to the Max-Age Timer on a switch?
Definition
It short-circuits the max age timer, and reduces the convergence delay from 50 to 30 seconds.
Term
If you use BackboneFast, what switches should it be used on?
Definition
It should be used on ALL switches on the network because of the RLQ request and reply process.