CRT 5.2 STP Operations

The port on the segment (with two switches) that has the lowest path cost to the root bridge is the: - Designated port - Blocked port or non-designated port - Root port - Routed Port

- Designated port

Which of the following ports will forward Ethernet frames? (Choose two.) - Designated port - Blocked port or non-dedicated port - Root port

- Designated port - Root port

How often does a switch send a BPDU? - Every 2 seconds - Every 15 seconds - Every 20 seconds - Only when there is a change in the topology

- Every 2 seconds

The root bridge will be the switch with the: - Lowest bridge ID - Highest bridge ID - Lowest port priority - Highest port priority

- Lowest bridge ID

The sum of individual port costs along the path from the switch to the root bridge is known as the: - Least cost path - Shortest path cost - Best path cost - Root path cost

- Root path cost

The port closest to the root bridge in terms of least overall cost (best path) to the root bridge is the: - Designated port - Blocked port or non-designated port - Root port - Routed Port

- Root port

By default (without any configuration on a switch), what will determine which switch is the root bridge? - The bridge priority - The extended system ID - The MAC address of the switch - The bridge ID

- The MAC address of the switch

STP has five ports states

1) Blocking- The port is an alternate port and does not participate in frame forwarding. The port receives BPDU frames to determine the location and root ID of the root bridge. BPDU frames also determine which port roles each switch port should assume in the final active STP topology. With a Max Age timer of 20 seconds, a switch port that has not received an expected BPDU from a neighbor switch will go into the blocking state. 2) Listening- After the blocking state, a port will move to the listening state. The port receives BPDUs to determine the path to the root. The switch port also transmits its own BPDU frames and informs adjacent switches that the switch port is preparing to participate in the active topology. 3) Learning- A switch port transitions to the learning state after the listening state. During the learning state, the switch port receives and processes BPDUs and prepares to participate in frame forwarding. It also begins to populate the MAC address table. However, in the learning state, user frames are not forwarded to the destination. 4) Forwarding- In the forwarding state, a switch port is considered part of the active topology. The switch port forwards user traffic and sends and receives BPDU frames. 5) Disabled- A switch port in the disabled state does not participate in spanning tree and does not forward frames. The disabled state is set when the switch port is administratively disabled.

The BID includes 3 fields

1) Bridge Priority- 4 bits 2) Extended System ID- 12 bits 3) MAC Address- 48 bits

Using the STA, STP builds a loop-free topology in a four-step process:

1) Elect the root bridge. 2) Elect the root ports. 3) Elect designated ports. 4) Elect alternate (blocked) ports.

STP convergence requires three timers, as follows:

1) Hello Timer 2) Forward Delay Timer 3) Max Age Timer The default times can be changed on the root bridge, which dictates the value of these timers for the STP domain.

When a switch has multiple equal-cost paths to the root bridge, the switch will determine a port using the following criteria:

1) Lowest sender BID 2) Lowest sender port priority 3) Lowest sender port ID

Elect the Root Bridge

An election process determines which switch becomes the root bridge. All switches in the broadcast domain participate in the election process. After a switch boots, it begins to send out BPDU frames every two seconds. These BPDU frames contain the BID of the sending switch and the BID of the root bridge, known as the Root ID. The switch with the lowest BID will become the root bridge. At first, all switches declare themselves as the root bridge with their own BID set as the Root ID. Eventually, the switches learn through the exchange of BPDUs which switch has the lowest BID and will agree on one root bridge.

Bridge Protocol Data Unit (BPDU)

During STA and STP functions, switches use Bridge Protocol Data Units (BPDUs) to share information about themselves and their connections. BPDUs are used to elect the root bridge, root ports, designated ports, and alternate ports.

Elect Designated Ports

Every segment between two switches will have one designated port. The designated port on the segment (with two switches) that has the LOWEST internal root path cost to the root bridge. In other words, the designated port has the best path to receive traffic leading to the root bridge.

Elect Alternate (Blocked) Ports

If a port is not a root port or a designated port, then it becomes an alternate (or backup) port. Alternate ports and backup ports are in discarding or blocking state to prevent loops.

Per-VLAN Spanning Tree (PVST)

In Per-VLAN Spanning Tree (PVST) versions of STP, there is a root bridge elected for each spanning tree instance. This makes it possible to have different root bridges for different sets of VLANs. STP operates a separate instance of STP for each individual VLAN. If all ports on all switches are members of VLAN 1, then there is only one spanning tree instance.

Bridge Priority

The default priority value for all Cisco switches is the decimal value 32768. The range is 0 to 61440 in increments of 4096. A lower bridge priority is preferable. A bridge priority of 0 takes precedence over all other bridge priorities.

Forward Delay Timer

The forward delay is the time that is spent in the listening and learning state. The default is 15 seconds but can be modified to between 4 and 30 seconds.

Hello Timer

The hello time is the interval between BPDUs. The default is 2 seconds but can be modified to between 1 and 10 seconds.

Max Age Timer

The max age is the maximum length of time that a switch waits before attempting to change the STP topology. The default is 20 seconds but be modified to between 6 and 40 seconds.

Because the default priority is 32768, it is possible for two or more switches to have the same priority. In this scenario, where the priorities are the same, the switch with the _____ MAC address will become the root bridge.

lowest

Elect the Root Ports

After the root bridge has been determined, the STA algorithm is used to select the root port. Every non-root switch will select one root port. The root port is the port closest to the root bridge in terms of overall cost (best path) to the root bridge. This overall cost is known as the internal root path cost.

Bridge ID (BID)

Each BPDU contains a bridge ID (BID) that identifies which switch sent the BPDU. The BID is involved in making many of the STA decisions including root bridge and port roles. The BID contains a priority value, an extended system ID, and the MAC address of the switch. The lowest BID value is determined by the combination of these three fields.

Internal root path cost

When the root bridge has been elected for a given spanning tree instance, the STA starts the process of determining the best paths to the root bridge from all destinations in the broadcast domain. The path information, known as the internal root path cost, is determined by the sum of all the individual port costs along the path from the switch to the root bridge. The BPDU includes the root path cost. This is the cost of the path from the sending switch to the root bridge. When a switch receives the BPDU, it adds the ingress port cost of the segment to determine its internal root path cost.