Download Chapter 5: STP * What is STP? How does STP work? * What would

Chapter 5: STP
* 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?
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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 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.
o 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 spanningtree vlan vlan-id priority value global configuration mode command.
o This command gives you more granular control over the bridge priority
value.
o 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 interpret the output shown below?
- Root?
- Priority?
- Cost?
- STP on or off?
- 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.
– 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 are the 3 RSTP link types?
 The link type provides a categorization for each port participating in RSTP.
 Edge ports.
 Non-edge ports
 They are categorized into 2 link types, point-to-point and shared
* What are the 5 RSTP port roles?
 Root - A forwarding port that has been elected for the spanning-tree topology
 Designated - A forwarding port for every LAN segment
 Alternate - An alternate path to the root bridge. This path is different than using
the root port.
 Backup - A backup/redundant path to a segment where another bridge port
already connects.
 Disabled - Not strictly part of STP, a network administrator can manually disable
a port
* Here are some more about RSTP.
• Due to the limited bit count, the priority is conveyed in discrete values in
increments of 4096 rather than in increments of 1.
• 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.
• The Cisco RSTP implementation maintains the PortFast keyword using the
spanning-tree portfast command for edge port configuration.
• RSTP (802.1w) supersedes STP (802.1D) while retaining backward compatibility.
• To configure a switch (RSTP and STP) to become the root bridge for a specified
VLAN, use the spanning-tree vlan vlan-ID root primary global configuration
command.
* 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.
* What are the criteria that switch used to select the root bridge in each STP?
* 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?