Chapter 5: STP

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* What is STP? How does STP work?

* What would be the worst case scenario that could happen to a redundant path switched network with the STP is disabled?

–When multiple paths exist between two devices on the network and STP has been disabled on those switches, a Layer 2 loop can occur. If STP is enabled on these switches, which is the default, a Layer 2 loop would not occur.

–Broadcast frames are forwarded out all switch ports, except the originating port. This ensures that all devices in the broadcast domain are able to receive the frame. If there is more than one path for the frame to be forwarded out, it can result in an endless loop.

–Ethernet frames do not have a time to live (TTL) like IP packets traversing routers. As a result, if they are not terminated properly on a switched network, they continue to bounce from switch to switch endlessly or until a link is disrupted and breaks the loop.

* What are the switch/port roles that will be elected at the end of each STP process?

- Root bridge? How many will get elected as the root bridge in each STP?

- Root port? How many will get elected as the root port in each STP?

- Designated port? How many will get elected as the designated port in each STP?

* What are the criteria that switch used to select the root bridge in each STP?

  • Initially, each switch identifies itself as the root bridge. The priority is the initial deciding factor when choosing a root bridge.
  • If the priority of all the switches was the same, the MAC address would be the deciding factor.

* What are the criteria that switch used to select the root port in each STP?

* What are the criteria that switch used to select the designated port in each STP?

- The way to answer the above 3 questions is in such if there is a tie in each of the variable, what do they use as the tie breaker?

* What is BPDU? What is the purpose of BPDU? How does BPDU information is used by the switches?

–After a switch boots, it sends out BPDU frames containing the switch BID and the root ID every 2 seconds.

–BID is made up of a priority value, an extended system ID, and the MAC address of the switch.

–As the switches forward their BPDU frames, adjacent switches read the root ID information from the BPDU frame.

–Eventually, the switch with the lowest BID ends up being identified as the root bridge for the spanning-tree instance.

* What is the default priority value for the bridge priority? How man way that can be used to overwrite the priority value?

  • The switches in the network have the default 32768 priority value defined
  • To ensure the switch has the lowest priority value, use the spanning-tree vlan vlan-id root primary in global configuration.
  • The priority for the switch is set to the predefined value of 24576 or to the next 4096 increment value below the lowest bridge priority detected on the network.
  • Another method for configuring the bridge priority value is using the spanning-tree vlan vlan-id priority value global configuration mode command.
  • This command gives you more granular control over the bridge priority value.
  • The priority value is configured in increments of 4096 between 0 and 65536.

* What is “portfast”?

  • PortFast is a Cisco technology. When a switch port configured with PortFast

–The port is configured as an access port,

–The port transitions from blocking to forwarding state immediately, bypassing listening and learning states.

–Using a feature called BPDU guard, if an interface configured with PortFast receives a BPDU frame, spanning tree can put the port into the blocking state.

  • You can use PortFast on access ports, which are connected to a single workstation or to a server, to allow those devices to connect to the network immediately rather than waiting for spanning tree to converge.

–Without PortFast, a PC can send a DHCP request before the port is in forwarding state, denying the host from getting a usable IP address and other information.

* Can you interpreted the output shown below:

- Check priority?

- Check cost?

- Checek STP on or off?

- Check timer?

* What are the 5 STP states? Can you list all 5 of them?

* Can you describe the process of MAC address learning and data forwarding in each of the 5 states?

* What are the timer settings in the STP?

* What is RSTP? What are the differences between STP and RSTP?

–Rapid spanning tree protocol (RSTP) - First introduced in 1982 as an evolution of 802.1D  802.1W

–It provides faster spanning-tree convergence than 802.1D.

–If a port is configured to be an alternate or a backup port it can immediately change to a forwarding state without waiting for the network to converge.

–With RSTP, the role of a port is separated from the state of a port.

•For example, a designated port could be in the discarding state temporarily, even though its final state is to be forwarding.

–RSTP must determine the port role.

•Root ports: do not use the link type parameter.

•Root ports are able to make a rapid transition to the forwarding state as soon as the port is in sync.

•Alternate and backup ports: do not use the link type parameter in most cases.

•Designated ports: make the most use of the link type parameter.

* What is the modification in RSTP over the STP in order for it to reach faster transition to the forwarding state?

- Link types?

- Port states?

- Port roles?

* What are the differences between STP and RSTP?

- Frame format?

- Root port?

- Backup Port?

- Alternate port?

- Edge port?

- What happen when an edge port detect a BPDU frame?

- What is the process of command “portfast” in either the STP and RSTP?

- Does “portfast” command still support in RSTP?

- Does RSTP backward compatible with STP?

* What are the characteristics of RSTP edge port?

  • An RSTP edge port is a switch port that is never intended to be connected to another switch device. It immediately transitions to the forwarding state when enabled.
  • The edge port concept is well known to Cisco spanning-tree users, because it corresponds to the PortFast feature in which all ports directly connected to end stations anticipate that no switch device is connected to them.
  • Unlike PortFast, an RSTP edge port that receives a BPDU loses its edge port status immediately and becomes a normal spanning-tree port.

* What is PVST+? Compare to the original STP, the PVST+ is modified to accommodate what type of information?

  • PVST+ requires that a separate instance of spanning tree run for each VLAN.

–To support PVST+, the 8-byte BID field is modified to carry a VLAN ID (VID).

* Make sure you can identify: root bridge, root port, designated port and none designated port.