Spanning Tree Protocol

Question 1

Loop protection

Answer 1

• Connect two switches to each other
  
  • They’ll send traffic back and forth forever
  • There’s no “counting” mechanism at the MAC layer
• This is an easy way to bring down a network
  
  • And somewhat difficult to troubleshoot
  • Relatively easy to resolve

• IEEE standard 802.1D to prevent loops in bridged
(switched) networks (1990)

Question 2

Switch operation

Answer 2

• Forwarding decisions made by MAC address
• Keeps a big table of MAC address that have been
seen
• All forwarding decisions are filtered through this list

• If the destination MAC is unknown, the frame is
flooded
• Sent to every switch port in the local subnet/VLAN
• Hopefully the destination station will respond

• Flooding is hopefully a temporary process
  
  • Directed traffic resumes when the MAC is seen

Question 3

STP port states

Answer 3

• Blocking - Not forwarding to prevent a loop
• Listening - Not forwarding and cleaning the MAC
table
• Learning - Not forwarding and adding to the MAC
table
• Forwarding - Data passes through and is fully
operational
• Disabled - Administrator has turned off the port

Question 4

RSTP (802.1w)

Answer 4

• Rapid Spanning Tree Protocol (802.1w)
  
  • A much-needed updated of STP
  • This is the latest standard
• Faster convergence
  
  • From 30 to 50 seconds to 6 seconds
• Backwards-compatible with 802.1D STP
  
  • You can mix both in your network
• Very similar process
  
  • An update, not a wholesale change