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Flashcards

1.6 Configure and verify spanning tree

Dustin Sanders Networking CWNA English 152 cards

Cards in this deck (152)

  1. 1What kind of protocol is STP?layer 2 management protocol
  2. 2what are the 4 port roles within STP?Root—A forwarding port elected for the spanning-tree topology
    Designated—A forwarding port elected for every switched LAN segment
    Alternate—A blocked port providing an alternate path to the root bridge in the spanning tree
    Backup—A blocked port in a loopback configuration
  3. 3how many root ports does the root switch have?none. Root ports are only on non-root ports because they are the port that GOES to the root.
  4. 4which ports on the root switch are designated ports?all of them
  5. 5what is a designated switch?A switch with at least 1 port that is designated as the path to the root.
  6. 6what are BPDUs?bridge protocol data units, sent at regular intervals. Used to tell other switches about sender switch's ports/mac address.
    THESE ARE NOT FWD'd. Each switch makes their own and sends them 1 hop to a neighbor.
  7. 7Where does STP get the info to elect a root?from the BPDUs
  8. 8what does STP use to determine what ports are put into fwding and which ones are put into blocking?port priority and path cost
  9. 9what messages are sent regularly to see if a link is still alive?keepalive
    (NOT SENT ON SFP interfaces)
  10. 10Where does a BPDU's SOURCE MAC address come from?The unique MAC address of the PORT (NOT SWITCH!) it was sent from. It may still contain the switch's mac, but it's important that the PORT is noted
  11. 11what is the STP Multicast address?01-80-c2-00-00-00
  12. 12what are the 2 types of BPDUs?Configuration BPDU: used for spanning-tree computation
    Topology Change Notification (TCN)
    BPDU: used to announce changes in the network topology
  13. 13how often are BPDUs sent out all switch ports?every 2 seconds
  14. 14remember:Notice that several key fields in the BPDU are related to bridge identification, path costs, and timer values. These all work together so that the network of switches can converge on a common spanning-tree topology and select the same reference points within the network.
  15. 15Why is it important to have a root bridge?To agree on a loop-free topology, a common frame of reference must exist to use as a guide. This reference point is called the root bridge (switch)
  16. 16what is a bridge ID?a unique ID that identifies a switch to other switches
  17. 17what 2 values make up the Bridge ID of a Switch?Bridge Priority (2 bytes) Priority field can have a value of 0 to 65,535 and defaults to 32,768 (or 0x8000) switch
    MAC Address (6 bytes)
  18. 18what does a switch do when it first boots up?sends out BPDUs with root ID = to it's own ID and sender ID = to it's own ID.
    (sender ID just says who is sending BPDU msg)
  19. 19Who sends configuration BPDUs after a root is elected?ONLY THE ROOT. Other bridges forward these and just change sender ID to their ID before forwarding it
  20. 20What are BPDUs analyzed for?To see if a "better" root bridge is being announced.
  21. 21What determines if a root bridge is "better"A root bridge is considered better if the root bridge ID value is lower than another.
    Think of the root bridge ID as being broken into Bridge Priority and MAC Address fields. If two bridge priority values are equal, the lower MAC address makes the bridge ID better.
  22. 22What happens when a switch hears of a better root bridge?it replaces its own root bridge ID with the root bridge ID announced in the BPDU. The switch then is required to recommend or advertise the new root bridge ID in its own BPDU messages.
  23. 23When are root bridge elections going on?Forever. They're always being checked every 2 seconds to see if something changed.
  24. 24What is PVST?Per Vlan Spanning Tree
    (run instances of STP on a VLAN because a VLAN is like it's own logical LAN)
  25. 25How many bridge IDs must a switch have for vlans?1 for each vlan (well, kinda.)
    Each VLAN is it's own logical bridge within that VLAN network, so each VLAN has to get its own bridge ID for the purposes of PVST in that vlan)
  26. 26how many bytes make up the bridge ID of a a vlan bridge ID?8 bytes.
    2 for switch priority
    6 for switch MAC
  27. 27What bridge ID to switches in a stack use?All use the same as the master for a given instance of STP
  28. 28What are the 5 interface states of STP?Blocking—The interface does not participate in frame forwarding.
    Listening—The first transitional state after the blocking state when the spanning tree decides that the interface should participate in frame forwarding.
    Learning—The interface prepares to participate in frame forwarding.
    Forwarding—The interface forwards frames.
    Disabled—The interface is not participating in spanning tree because of a shutdown port, no link on the port, or no spanning-tree instance running on the port.
  29. 29what is the process with which an interfaces moves though the interface states?Initialization -> Blocking
    Blocking -> Listening or Disabled
    Listening -> Learning or Disabled
    Learning -> Forwarding or Disabled
    Forwarding -> Disabled
  30. 30Does blocking state participate in frame forwarding?no
  31. 31What functions does a blocking state port perform?Discards frames received on the interface
    Discards frames switched from another interface for forwarding
    Does not learn addresses
    Receives BPDUs
  32. 32Can listening states send or recieve data frames?no. Only BDPUs
  33. 33what functions does a port in the listening state perform?Discards frames received on the interface
    Discards frames switched from another interface for forwarding
    Does not learn addresses
    Receives BPDUs
  34. 34what functions does a port in the learning state perform?Discards frames received on the interface
    Discards frames switched from another interface for forwarding
    Learns addresses
    Receives BPDUs
  35. 35what functions does a port in the FWDing state perform?Receives and forwards frames received on the interface
    Forwards frames switched from another interface
    Learns addresses
    Receives BPDUs
  36. 36What functions does a port in a disabled state perform?Discards frames received on the interface
    Discards frames switched from another interface for forwarding
    Does not learn addresses
    Does not receive BPDUs
    (disabled by either Admin or fault)
  37. 37What happens after a root bridge is elected?switches must elect root ports that represent the best way to get from their switch to the root
  38. 38Where does the root port point?towards the root bridge, or the lowest path to the root bridge (through another switch)
  39. 39what is the root path cost?a cumulative cost of all the links leading to the root bridge.
  40. 40how is path cost represented?1-byte values
    10gbps = 2
    1gbps = 4
    622 mbps = 6
    155mbps = 14
  41. 41How is root path cost determined?Root sends BPDU with root path cost = 0
    Next switch gets it, adds cost of it's port
    Switch then sends out new cost to next SW
    (!! The path cost is updated when it comes IN, not when it goes OUT!!)
  42. 42What does a switch know if it only gets higher cost BPDUs on another port?That it's port is the designated port
  43. 43how are STP ties resolved?Lowest root bridge ID
    Lowest root path cost to root bridge
    Lowest sender bridge ID
    Lowest sender port ID
  44. 44how can you see STP on an interface?SW1# show spanning interface
  45. 45Why is it crucial for STP timers to work?It takes time for BPDU updates to flow across a network. If some switches start converging before all have gotten the message, that causes issues
  46. 46what 3 timers does STP use to make sure the network properly converges?Hello timer – 2 Seconds (sent by root)
    Forward Delay timer – 15 Seconds: Time interval that a switch port spends in both the Listening and Learning states.
    Max Age timer – 20 Seconds: Time interval that a switch stores a BPDU before discarding it.
  47. 47what is the reference model network size that STP timers are based on?A network that's 7 switches "wide"
  48. 48how many different multicast addresses does 802.1d specify to be used by different bridging protocols?17 multicast addresses, ranging from 0x00180C2000000 to 0x0180C2000010
  49. 49What kind of BPDU does a switch send to announce a new network topology?TCN BPDU. 4 bytes
  50. 50do portfast ports send TCN BPDUs?no
  51. 51Where does a switch send it's TCN BPDU?out it's root port to tell the root about the change
  52. 52when a switch receives a notification of a topology change, what does it do to it's bridge table aging time?shortens from 300 seconds to 15
  53. 53What is a Direct Topology Change?A direct topology change is one that can be detected on a switch interface. If a trunk link suddenly goes down, the switch on each end of the link can immediately detect a link failure. The absence of that link changes the topology, so other switches should be notified.
  54. 54for what environment was STP originally developed?for single LAN (VLAN) environments.
    It needs tweaking to work in a multi-VLAN environment
  55. 55What is Common Spanning Tree (CST) ?802.1Q standard specifies how VLANs are to be trunked between switches. It also specifies only a single instance of STP that encompasses all VLANs.
    All CST BPDUs are transmitted over trunk links using the native VLAN with untagged frames.
  56. 56what is Per-VLAN Spanning Tree – PVST?(Cisco Proprietary)
    operates a separate instance of STP for each individual VLAN
    This allows the STP on each VLAN to be configured independently, offering better performance and tuning for specific conditions
  57. 57What encapsulation technology is required for the use of PVST?ISL
  58. 58what is Per-VLAN Spanning Tree Plus – PVST+ ?Allows devices to interoperate with both PVST and CST. PVST+ effectively supports three groups of STP operating in the same campus network:
    CST – 1 instance of STP, over the native VLAN (802.1Q based)
    PVST – 1 instance of STP per VLAN (Cisco ISL based)
    PVST+ – Provides interoperability between CST and PVST (both 802.1Q and ISL)
  59. 59What is the default stp mode used on all thernet port-based vlans?PVST+
    Provides Layer 2 load-balancing for the VLAN on which it runs. You can create different logical topologies by using the VLANs on your network to ensure that all of your links are used but that no link is oversubscribed.
    Each instance of PVST+ on a VLAN has a single root switch. This root switch propagates the spanning-tree information associated with that VLAN to all other switches in the network.
  60. 60what is Rapid PVST+ ?Its the same as PVST+ except that is uses a rapid convergence. To provide rapid convergence, the rapid PVST+ immediately deletes dynamically learned MAC address entries on a per-port basis upon receiving a topology change.
  61. 61what is Multiple Spanning-Tree (MSTP) ?You can map multiple VLANs to the same spanning-tree instance, which reduces the number of spanning-tree instances required to support a large number of VLANs. The MSTP runs on top of the RSTP (based on IEEE 802.1w), which provides for rapid convergence of the spanning tree by eliminating the forward delay and by quickly transitioning root ports and designated ports to the forwarding state.
  62. 62where is MSTP most commonly found?in the backbone and distribution layers of a Layer 2 switched network.
  63. 63What is the problem with STP and 802.1Q trunks working together?802.1q vlan trunks require only 1 STP for all vlans BUT cisco switches connected by a trunk use 1 STP for EACH vlan on the trunk
    PVST+ is used to get around this
  64. 64What is the default STP mode?PVST+
  65. 65what is the default STP port priority?128
  66. 66What is the default switch priority?32768
  67. 67what are the default STP vlan port costs?1gig = 4
    100mbps = 19
    10mbps = 100
  68. 68Which switch becomes the root? The one with the highest bridge ID or the lowest?lowest
  69. 69How can you manually configure a switch to become the root for a specific vlan?DSW1(config)# spanning-tree vlan 1 root primary
    (this changes SW priority from 32768 to 24576. If another switch already has that priority, it knocks it down to 4094)
  70. 70how do you change the network diameter for STP calculations for a given vlan?DSW1(config)# spanning-tree vlan 1 root primary diameter ?
  71. 71what is the purpose of configuring port priority?If a loop occurs, spanning tree uses the port priority when selecting an interface to put into the forwarding state. You can assign lower priority values (lower is better) to interfaces that you want selected first and lower priority values (higher values) that you want selected last.
  72. 72what command must you use when editing the priority of a switch in a stack?"spanning-tree [vlan vlan-id] cost" command instead of the "spanning-tree [vlan vlan-id] port-priority" command
  73. 73if all interfaces have the same port priority, what breaks the tie?lowest interface # wins
  74. 74how can you configure switch priority for a vlan?SW1(config)# spanning-tree vlan 1 priority 32769
  75. 75how can you configure the port cost for all ports in a port channel?SW1(config)# interface port-channel 1
    SW1(config-if)# spanning-tree cost 16
  76. 76how can you configure port priority?Switch(config-if)# spanning-tree [vlan vlan-list] port-priority port-priority
  77. 77what values make up a Port ID?The port priority (8 bits totaling 0-255)
    The port number (8 bits totaling 0-255)
    Prio.Nbr
    128.102
  78. 78When modifying STP timers, where must you modify them?on the ROOT
    only the ROOT propagates the timer values
  79. 79what are the 3 stp timers and how do you change them?Switch(config)# spanning-tree [vlan vlan-id] hello-time seconds
    Switch(config)# spanning-tree [vlan vlan-id] forward-time seconds
    Switch(config)# spanning-tree [vlan vlan-id] max-age seconds
    (leaving out vlan changes it for EVERYTHING)
  80. 80what two factors are crucial for STP timers and getting the time right?hello timers and network diameter
  81. 81what macro command can you use to change the STP timers all in 1 go?Switch(config)# spanning-tree vlan vlan-list root {primary | secondary} [diameter diameter [hello-time hello-time]]
  82. 82How can you verity STP timers in use?Switch# show spanning-tree vlan 100
    look for:
    Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
  83. 83What are some additional methods that allow faster STP convergence?PortFast
    UplinkFast
    BackboneFast
  84. 84What is PortFastPortFast: Enables fast connectivity to be established on access layer switch ports to workstations that are booting.
  85. 85What is UplinkFastUplinkFast: Enables fast-uplink failover on an access layer switch when dual uplinks are connected into the distribution layer.
  86. 86What is BackboneFastBackboneFast: Enables fast convergence in the network backbone or core layer switches after a spanning-tree topology change occurs.
  87. 87How do additonal methods to allow fast convergence like port/uplink/backbone fast work?Instead of modifying timer values, these methods work by controlling convergence on specifically located ports within the network hierarchy.
  88. 88How does portfast work in a nutshell?when a port is connected (like to a user computer) it auto trusts the port and puts it in FWDing state, but keeps checking if a loop is formed. If a loop is formed, it then blocks the port before a storm developed.
  89. 89do TCN BPDUs get sent for ports that are in portfast mode?nope. So if a port goes up or down, it doesn't trigger recalculations every time someone disconnects their laptop.
  90. 90how can you verify the portfast status of an interface?SW1# show spanning-tree interface e1/0 portfast
  91. 91how does uplink fast work in a nutshell?if a switch (like an ASW) has 2 ports that uplink to a root switch (or a DSW) it puts one in blocking state and one in FWD unless the FWDing one fails, at which point the blocked one immediately becomes the FWDing one.
  92. 92how do you enable uplink fast?Switch(config)# spanning-tree uplinkfast [max-update-rate pkts-per-second]
  93. 93can uplink fast be configured on the root bridge?UplinkFast works by keeping track of possible paths to the root bridge. Therefore, the command is not allowed on the root bridge switch.
  94. 94how can you check the current status of uplink fast?Switch# show spanning-tree uplinkfast
  95. 95how does backbone fast work in a nutshell?BackboneFast works by having a switch actively determine whether alternative paths exist to the root bridge, in case the switch detects an indirect link failure.
    BackboneFast operates by short-circuiting the Max Age timer when needed.
  96. 96how do you configure backbone fastSwitch(config)# spanning-tree backbonefast
  97. 97how do you verify backbonefastSwitch# show spanning-tree backbonefast
  98. 98What is RLQ and what uses it?Root Link Query, used by backbone fast to see whether upstream switches have stable connections to the root bridge.
  99. 99What is Root GuardRoot Guard is a means to control where candidate root bridges can be connected and found on a network. Root Guard designates that a port can only forward or relay BPDUs, not receive BPDUs. Root Guard prevents the port from ever becoming a root port where BPDUs normally would be received from the root bridge.
  100. 100how do you configure root guard on an interface?Switch(config-if)# spanning-tree guard root
  101. 101how can you see ports that root guard has put into an inconsistent state?Switch# show spanning-tree inconsistentports
  102. 102what is BPDU Guard?immediately err-disables a port if it gets a BPDU
    (AUTOMATICALLY ENABLED WHEN USING PORTFAST!!!!!!)
  103. 103how do you enable BPDU guard as defaultSwitch(config)# spanning-tree portfast bpduguard default
  104. 104What two features help detect or prevent the unexpected loss of BPDUs?Loop Guard
    Unidirectional Link Detection (UDLD)
  105. 105What is Loop Guard?loop guard keeps track of BPDU activity on nondesignated ports. If a NDport gets a BPDU, it behaves normally. If it stops getting BPDUs, it is put into a loop-inconsistent state
  106. 106is loop guard enabled by default?nope. Have to do this to enable:
    Switch(config)# spanning-tree loopguard default
  107. 107What is UDLD?(cisco only)
    UDLD sends frames across a link asking for the other side to send it back. If it doesn't, then the link is 1 directional.
    default 15 seconds
  108. 108what are the two modes of UDLD?normal (just makes an note of 1 dir. port)
    aggressive (hello msg every 8 secs, shuts down 1 dir. port)
  109. 109can UDLD be enabled globally or per interface?both.
    Switch(config)# udld {enable | aggressive | message time seconds}
    Switch(config-if)# udld {enable | aggressive | disable}
  110. 110how do you recover an err-disabled switch that UDLD disabled?Switch# udld reset
  111. 111what ports are BPDUs sent on?BPDUs are sent on all switch ports, even ports where PortFast is enabled. BPDUs also can be received and processed if they are sent by neighbors.
  112. 112what is the difference between BPDU guard and BPDU filtering?BPDU Guard is used to detect inbound BPDUs on ports where BPDUs are not expected to be seen, then protect the STP stability by preventing those BPDUs from being processed. In contrast, BPDU filtering stops all BPDUs from being received or sent on a switch port, effectively disabling STP.
  113. 113Can STP be used in a single instance, or multiple?both
  114. 114Can RSTP be used in a single instance or multiple?both
  115. 115Is RSTP used as part of MST?yes
  116. 116What are port roles vs port statesa port can have a role (root/designated/blocking) and can have a state (disabled/blocking/listening/learning/forwarding)
    RSTP lets a port communicate with its neighbor based on role
  117. 117What are the 4 RSTP port roles?root
    designated
    alternate
    backup
  118. 118what are the 3 types of ports in RSTP?Edge, root, point to point
  119. 119what is an Edge port in RSTP?Edge: A port at the “edge” of the network, where only a single host connects. This has been identified by enabling the STP PortFast feature. The port cannot form a loop as it connects to one host, so it can be placed immediately in the Forwarding state. If a BPDU is received on an edge port, the port immediately loses its edge port status.
  120. 120what is a root port in RSTP?Root: The port that has the best cost to the root of the STP. Only one root port can be selected and active at any time, (alternative paths to the root can exist through other ports). If alternative paths are detected they are identified as alternative root ports and immediately can be placed in the Forwarding state when the existing root port fails.
  121. 121What is a point to point port in RSTP?Point-to-Point: Any port that connects to another switch and becomes a designated port. BPDUs are exchanged back and forth in the form of a proposal and an agreement. One switch proposes that its port becomes a designated port; if the other switch agrees, it replies with an agreement message.
  122. 122What must a switch do first in order to participate in RSTP?a switch must decide the state of each of its ports.
    Convergence begins with a switch sending a proposal message. The recipient must synchronize itself by isolating itself from the rest of the topology. All non-edge ports are blocked until a proposal message can be sent, causing the nearest neighbors to synchronize themselves. This creates a moving “wave” of synchronizing switches, which quickly can decide to start forwarding on their links only if their neighbors agree.
  123. 123When does RSTP detect a topology change?only when a non-edge port transitions to the Forwarding state.
    This might seem odd because a link failure is not used as a trigger.
  124. 124how can you configure an edge port with RSTP?Switch(config-if)# spanning-tree portfast
  125. 125what is the default STP mode on catalyst switches?PVST+
  126. 126how does PVST+ workone spanning tree instance is created and used for each active VLAN that is defined on the switch. Each STP instance behaves according to the traditional 802.1D STP rules.
  127. 127how can you improve the efficency of each STP instance?by configuring a switch to begin using RSTP instead. This means that each VLAN will have its own independent instance of RSTP running on the switch. This mode is known as Rapid PVST+ (RPVST+)
  128. 128how do you configure RPVST+Switch(config)# spanning-tree mode rapid-pvst
  129. 129why must you be careful when enabling RPVST+ in production?it will restart any running STP processes. This can cause temp outages as links move through the various STP states
  130. 130how do you revert back to PVST+Switch(config)# spanning-tree mode pvst
  131. 131how can you verify STP mode?look for the spanning-tree mode command under running config
  132. 132What are the two STP implementations?IEEE 802.1Q and PVST+, both based on the 802.1D STP
  133. 133what is the difference between 802.1q STP and PVST+?802.1q (CST) has only 1 instance of STP for the entire switch
    PVST+ has 1 STP instance for EVERY vlan.
  134. 134what's the trade off with PVST+the more VLANS running their own instance of PVST+, the less resources for other tasks like switching
  135. 135What is MST, and what was it designed to do?Multiple Spanning Tree (802.1s) designed to solve PVST+ tradeoff issue, allows admin to configure as many instances of STP as they need and not have to run it everywhere by default
  136. 136How does MSTP work?MSTP enables multiple VLANs to be mapped to the same spanning-tree instance, reducing the number of spanning-tree instances needed to support a large number of VLANs. The MSTP provides for multiple forwarding paths for data traffic and enables load-balancing.
  137. 137Where do you most often find MST?in the backbone and distribution layers of a Layer 2 switched network
  138. 138when MST is enabled, what else is automatically enabled?RSTP
  139. 139What is an MST region?where every switch in a region runs MST with compatible parameters
  140. 140Within an MST region, what attributes define the instace of MST that is running?Configuration Name (32 characters)
    Configuration Revision Number (0 to 65535)
    Instance-to-VLAN Mapping Table (4096 entries)
  141. 141If two switches have the same MST attributes, they belong to...the same MST region. If not, then they belong to different regions
  142. 142How are spanning-tree instances within MST like black boxes?ink of the entire enterprise network as having a single CST topology so that one instance of STP represents any and all VLANs and MST regions present.
    The CST maintains a common loop-free topology while integrating all forms of STP that might be in use. CST must regard each MST region as a single “black box” bridge because it has no idea what is inside the region, nor does it care. CST maintains a loop-free topology only with the links that connect the regions to each other and to standalone switches running 802.1Q CST.
  143. 143what is IST?Internal Spanning Tree (IST) instance runs to work out a loop-free topology between the links where CST meets the region boundary and all switches inside the region.
    Think of the IST instance as a locally significant CST, bounded by the edges of the region
  144. 144How many MST instances are supported within a single region?16
  145. 145What kind of BPDUs are sent in an out of MST regions?only IST BPDUs
  146. 146How must MSTP be configured?Manually. There is no VTP for MST regions.
    Must be done on EVERY switch in a region
  147. 147How do you enable MST on a switch?Switch(config)# spanning-tree mode mst
  148. 148How do you enter MST configuration mode?Switch(config)# spanning-tree mst configuration
  149. 149How do you set the MST instance name and revision number?Switch(config-mst)# name instance1
    Switch(config-mst)# revision 1
    (you need to manually increment revision number. It's to help you keep track. Rev number must match on all switches.)
  150. 150how do you map vlans to an MST instance?Switch(config-mst)# instance 1 vlan-range 10-20
  151. 151how can you see pending MST changes?Switch(config-mst)# show pending
  152. 152How do you commit the changes to the active MST region config?Switch(config-mst)# exit

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CWNA Networking (CCNP Switch Exam Topics) Flashcards on 1.6 Configure and verify spanning tree, created by Dustin Sanders on 21/01/2017.
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  • Created by Dustin Sanders · Jan 21, 2017
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