1. NT2640 Unit 9 Activity 1 Handout

2. 11/16/2018
STP Overview
When a host transmits a broadcast Ethernet frame, a switch will forward the broadcast frame out to every port. In a situation with multiple switches are connected that form a loop, the second switch will also forward the frame out every port. This continue and continue as Ethernet does not have a TTL mechanism that IP has to stop packets from existing forever. This behavior is call a broadcast storm.
The switches continue to propagate broadcast traffic, and very quickly become so busy processing the broadcast frames that no other traffic has a chance to be forwarded to its destination.
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3. 11/16/2018
STP Overview(cont’d)
A loop avoidance solution was needed for Ethernet networks that would contain redundant paths.
Hence Spanning Tree Protocol – 802.1d
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4. 11/16/2018
STP Port States
The goal of STP is to maintain a loop-free network topology. A loop-free topology is achieved by the switches listening and detecting a loop and transitions one or more ports into blocking state automatically.
STP Port States:
Blocking - A port that would cause a switching loop if no action were to be taken. No data is sent or received. Port may go into forwarding mode if the other links in use were to fail and the spanning tree algorithm determines the port may transition to the forwarding state. BPDU data is still received in blocking state.
Listening - The switch receives and processes BPDUs and awaits possible new information that would cause it to return to the blocking state.
Learning - Port does not yet forward frames but it does learn source addresses from frames received and adds them to the MAC address table
Forwarding - Normal port state for sending and receiving data. STP still processes received BPDUs for indication that is should transition the port to blocking state.
Disabled - Administratively disabled, not strictly part of STP
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5. STP Port States (cont’d)
11/16/2018
STP Port States (cont’d)
STP Port States:
All ports transition thru Blocking -> Forwarding
Default timers of 15 seconds for listening and learning
State
Forwards Data Frames?
Learn Source MACs of incoming Frames
Transitory or End State
Blocking No
End
Listening
Transitory
Learning
Yes
Forwarding
Disabled
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6. STP Convergence Process
11/16/2018
STP Convergence Process
Switches exchange bridge protocol data units (BPDUs)
Root bridge is elected
Port role and state are determined
Tree is fully converged
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7. STP Root Bridge Election
11/16/2018
STP Root Bridge Election
Root bridge is elected based on information contained within the BPDU frames
Criteria used for election:
Lowest Bridge ID
Bridge ID is Composed of Priority + System ID (Bridge Address)
Lower Priority Always Wins
Equal Priority, Lower Bridge Address Wins
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8. ** Numeric Lowest Wins**
11/16/2018
STP Port Roles
802.1d STP Roles:
Root port
The one port on each switch with the least cost path back to the root bridge
Designated Port
When multiple switch connect to a segment this is the switch that will forward the least cost BPDU hello into the segment
When the cost is tie, the following criteria is followed:
Bridge ID
Port Priority
Port Number
** Numeric Lowest Wins**
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9. STP Port Roles Lowest cost path to root bridge determines port role
11/16/2018
STP Port Roles
Lowest cost path to root bridge determines port role
Port role determines ending port state:
All ports on root bridge assume designated port role and forwarding state
Root ports on switches are placed in forwarding state and the root bridge has no root ports
Designated ports on non-root switches are placed into forwarding state
All other ports are put into blocking state
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10. STP Link Costs Shortest path is based on cumulative link costs
11/16/2018
STP Link Costs
Shortest path is based on cumulative link costs
Link costs are based on the speed of the link
Link Speed
Original IEEE Cost
Revised IEEE Cost
10 Mbps
100
100 Mbps 10 19
1 Gbps 1 4
10 Gbps 2
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11. 11/16/2018
STP Examples - 1
What switch will become the root bridge and what are the port states on all the ports?
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12. 11/16/2018
STP Examples - 2
What switch will become the root bridge and what are the port states on all the ports?
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13. 11/16/2018
STP Examples - 3
What switch will become the root bridge and what are the port states on all the ports?
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