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34 Cards in this Set
- Front
- Back
Three classes of problems solved by STP
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1. Broadcast storms
2. Mac table instability 3. Multiple frame transmission |
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IEEE designation for STP
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802.1d
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STP - Three criteria to put a port in forwarding state
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1. Port is on root switch
2. Port is root-port (lowest cost path to root) 3. Port is designated port (port on switch with lowest cost back to root) |
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STP - The bridge ID is made up of…
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2-byte priority followed by 6-byte MAC address
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STP - Which switch is elected as root?
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The switch with lowest bridge id
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STP - What is a BPDU?
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Bridge Protocol Data Unit
(name for STP messages) |
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STP - What are the fields in a Hello BPDU?
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1. Root bridge id
2. Sender's bridge id 3. Cost to reach root. 4. Timers: Hello timer, MaxAge timer, Forward Delay timer |
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STP - What is a root port?
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For non-root switches, one and only one port has the lowest-cost path back to root.
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STP - What is a designated port?
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For each LAN segment, the switch port that advertises the lowest-cost Hello BPDU onto that segment. In case of tie, lower bridge ID wins.
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STP - What are the default port costs? (original/revised)
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10 Mbps - 100/100
100 Mbps - 10/19 1 Gbps - 1/4 10 Gbps - 1/2 |
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STP - How often does root switch send Hello BPDU
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Every 2 seconds by default
Note: only the root switch sends Hello BPDUs |
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STP - What is the default MaxAge timer and how is it used?
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10 times Hello, normally 20 seconds
Defines how long a switch should wait before trying to change STP topology |
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STP - What is the default forward delay timer?
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15 seconds. Number of seconds that a port stays in the listening state, then learning state.
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STP - What are the four possible port states and which are stable?
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Blocking (stable)
Listening Learning Forwarding (stable) |
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STP - What does the listening state do?
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Does not forward frames. Old, incorrect MAC table entries are timed-out.
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STP - What does the learning state do?
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Does not forward frames, but begins to fill in the MAC address table
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STP - By default, what is the total time for convergence?
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50 seconds
MaxAge (20 seconds) + Listening (15 seconds) + Learning (15 seconds) |
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STP - in what state(s) will a port forward frames?
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Only in the forwarding state.
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STP - What is PortFast?
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Allows a switch to place a port immediately into the forwarding state. Appropriate for connection to end-user devices.
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STP - What is BPDUGuard?
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Disables a port if BPDUs are received. Appropriate for access ports.
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STP - How does Cisco modify the bridge ID for per VLAN STP?
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First 4 bits are the priority (multiple of 4096)
Next 12 bits are the System ID extension (typically VLAN ID) 6 byte MAC address |
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STP - CLI commands to set bridge priority
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#spanning-tree vlan <id> root {primary | secondary}
#spanning-tree vlan <id> priority <value> |
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STP - CLI command to set interface cost
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#spanning-tree vlan <id> cost <value>
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STP - CLI command to set PortFast
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#spanning-tree portfast
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STP - CLI command to set BPDUGuard
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#spanning-tree bpduguard
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STP - CLI command to show spanning-tree info
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#show spanning-tree vlan <vlanid>
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STP - CLI command to show spanning-tree roots
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#show spanning-tree root
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STP - CLI command to debug spanning tree
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#debug spanning-tree events
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STP - CLI command to configure Rapid PVST
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#spanning-tree mode rapid-pvst
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STP - What is UplinkFast?
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For access-layer switches, use uplinkfast on ports that uplink to distribution routers for faster reconvergence (2-4 seconds)
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STP - (802.1D): Which switches will send Hellos once the network is stable?
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Only the root
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STP: Two commands that will show the STP status (forwarding or blocking) for an interface?
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#show spanning-tree
#show spanning-tree interface <fa0/1> |
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STP: How is the cost of a forwarded HELLO message calcuated?
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Cost of received hello plus cost of port on which it was RECEIVED (the return path).
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STP: Will a switch forward HELLO messages out its root port if they were received on some other port?
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No.
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