2 - STP Concepts Flashcards
Spanning Tree Protocol
What is a broadcast storm?
When any kind of Ethernet frame loops around the network indefinitely.
What are the three types of problems that occur without STP?
- Broadcast storms
- MAC table instability
- Multiple frame transmission
What is the IEEE protocol for STP?
802.1D
What is root cost?
it is the Administrative Cost from that port to the root switch.
What is the root port?
It is the port closest to the root switch. Root ports are always in forwarding state.
What are the contents of the 8-byte bridge ID or BID
- Priority field = 2 bytes;
- System ID based on MAC address = 6 bytes
What does a STP Hello BPDU consist of?
- Root BID
- Sender’s BID
- Sender’s Root Cost
- Timer values set by the Root switch
In a Hello BPDU what are the timers?
- Hello timer
- MaxAge timer
- Forward Delay timer
What determines a superior Hello?
The Bridge ID.
- First the lowest Priority field
- Then lowest MAC address.
What is the process to elect a root bridge?
- All switches claim to be root at first
- Hello BPDUs are circulated and BIDs are examined
- Once a superior BPDU is encountered then that switch gives up the claim to be Root.
- The bridge with the LOWEST Bridge ID wins.
What is the Designated Port (DP) ?
- Only one per LAN segment.
- It is the port on that LAN segment that advertises the lowest cost to get to the root.
- DPs are always placed in Forwarding State.
- The opposite port on that segment goes into Blocking State.
- Every interface on the Root Switch is a DP for that segment.
What command is used to convert 1998 IEEE port costs to 2004 IEEE port costs?
spanning-tree pathcost method long
By default, how often does the Root Switch send a new Hello BPDU?
Once every 2 seconds.
What fields in a BPDU does a switch change before forwarding it on?
- Changes Root Cost to its own Root Cost
- Changes Sender Bridge ID to its own BID
What ports does a non-root switch forward BPDUs?
All its Designated Ports.
What tools are available to an engineer to change STP topology?
- Configure the Priority Field in the switch BID
- Configure the Port Costs of the switchports.
How does a switch know its path to the Root Switch is still working?
It continues to receive Hello BPDUs on its Root Port.
What are the 3 timers required for the convergence process?
- Hello every 2 sec
- MaxAge = 20 (10 times the Hello timer)
- Forward Delay = 15 sec
Which switch determines what the timers are set at?
Root Switch sends them in Hello BPDUs.
What is the MaxAge timer and what is its default
The amount of time a switch will go without hearing BPDUs before it initiates STP Topology change. Default is 20 sec (10 times the Hello timer).
What are two ways a switch realizes a link is down?
- Fails to receive Hello BPDUs within the MaxAge time OR
- Notices that the RP interface has gone down.
What 3 things does a switch do when its RP goes down?
It re-evaluates:
- Who Root is
- Which port its RP should be
- its role as DP on any other interfaces.
What do interfaces do when in Blocking state?
- Only STP messages and some overhead messages are processed.
- Does not forward frames.
- Does not learn MAC addresses.
- Does not process user messages.
How long and what happens during the Listening State?
- Default is 15 seconds.
- Does not forward frames.
- Removes stale MAC table entries because these MAC table entries could have been the cause for loops.
How long and what happens during the Learning State?
- Default is 15 seconds.
- Begins to learn the MAC addresses
In STP how long could it take for a port to transition from Blocking to Forwarding?
- 50 seconds.
- 20 seconds MaxAge waiting for Hello BPDUs
- 15 sec Listening
- 15 sec Learning
When determining a cost of an interface is the cost based on the maximum possible bandwidth of the port or the actual bandwidth the port is operating at?
It’s based on the bandwidth the port is actually operating at. Not the maximum possible.
