ENCOR 350-401 Chapter 2

How is a Root Port (RP) selected?

1. The interface associated to lowest path cost is more preferred. 2. The interface associated to the lowest system priority of the advertising switch is preferred next. 3. The interface associated to the lowest system MAC address of the advertising switch is preferred next. 4. When multiple links are associated to the same switch, the lowest port priority from the advertising switch is preferred. 5. When multiple links are associated to the same switch, the lower port number from the advertising switch is preferred.

What is a Root port (RP)?

A network port that connects to the root bridge or an upstream switch in the spanning-tree topology. There should be only one root port per VLAN on a switch.

What is a Root Port (RP)?

A network port that connects to the root switch or an upstream switch in the spanning-tree topology. There should be only one root port per VLAN on a switch.

What is an Alternate Port (AP)?

A network port that provides alternate connectivity toward the root switch through a different switch.

What is a Backup Port (BP)?

A network port that provides link redundancy toward the current root switch. The backup port cannot guarantee connectivity to the root bridge in the event that the upstream switch fails. A backup port exists only when multiple links connect between the same switches.

What is a Designated port (DP)?

A network port that receives and forwards BPDU frames to other switches. Designated ports provide connectivity to downstream devices and switches. There should be only one active designated port on a link.

What is a Designated Port (DP)?

A network port that receives and forwards frames to other switches. Designated ports provide connectivity to downstream devices and switches. There should be only one active designated port on a link.

What is a Blocking port (BP)?

A network that is not forwarding traffic because of STP calculations.

What is an Edge Port?

A port at the edge of the network where hosts connect to the Layer 2 topology with one interface and cannot form a loop. These ports directly correlate to ports that have the STP portfast feature enabled.

What is a Root Port?

A port that has the best path cost toward the root bridge. There can be only one root port on a switch.

When is a Root Port (RP) selected?

After the switches have identified the root bridge, they must determine their root port (RP). The root bridge continues to advertise configuration BPDUs out all of its ports. The switch compares the BPDU information to identify the RP.

What is a Point-to-Point Port?

Any port that connects to another Rapid Spanning-tree Protocol (RSTP) switch with full duplex. Full-duplex links do not permit more than two devices on a network segment, so determining whether a link is full duplex is the fastest way to check the feasibility of being connected to a switch.

What is step 1 of Building the RSTP Topology?

As the first two switches connect to each other, they verify that they are connected with a point-to-point link by checking the full-duplex status.

What are the two types of Bridge protocol data unit (BPDU) in STP?

Configuration BPDU Topology change notification (TCN) BPDU

What are the 802.1D Port States?

Disabled Blocking Listening Learning Forwarding Broken

What are the Port States used by IEEE 802.1W?

Discarding Learning Forwarding

What are the 802.1W port types?

Edge Port Root Port Point-to-Point Port

When does a switch ignore the Bridge Protocol Data Unit (BPDU)?

If the neighbor's configuration BPDU is inferior to its own BPDU, the switch ignores that BPDU.

When does the switch update it's Bridge Protocol Data Unit (BPDU)?

If the neighbor's configuration BPDU is preferred to its own BPDU, the switch updates its BPDUs to include the new root bridge identifier along with a new root path cost that correlates to the total path cost to reach the new root bridge. This process continues until all switches in a topology have identified the root bridge switch.

How is the Root Switch elected if the BPDU priority is the same?

If the priority is the same, then the switch prefers the BPDU with the lower system MAC.

In a stable Layer 2 Spanning-tree Protocol (STP) topology which direction do Bridge Protocol Data Units (BPDU's) flow?

In a stable Layer 2 topology, configuration BPDUs always flow from the root bridge toward the edge switches. However, changes in the topology (for example, switch failure, link failure, or links becoming active) have an impact on all the switches in the Layer 2 topology.

What is a side effect of flushing the MAC address table?

Increase in unknown unicast traffic which will impact devices because of CSMA/CD

What does PVST stand for?

Per VLAN Spanning Tree

PVST and PVST+ were combined with other enhancements to create what?

Rapid Spanning Tree Protocol (RSTP)

What is the IEEE 802.1W standard?

Rapid Spanning Tree Protocol (RTSP)

What are the 802.1W port roles?

Root Port (RP) Designated Port (DP) Alternate Port (AP) Backup Port (BP)

What are the three standard port types the 802.1D Spanning-tree Protocol (STP) defines?

Root port (RP) Designated port (DP) Blocking port (BP)

When a Switch receives a configuration BPDU with the Topology Change set what is the MAC address timer to?

The Forwarding Delay Timer (15 seconds)

Which port does a switch send out a Topology Change Notification (TCN) when a change is detected?

The Root Port (RP)

How long does the default 802.1D Spanning-tree Protocol (STP) take?

The entire 802.1D STP initialization time takes about 30 seconds for a port to enter the forwarding state using default timers.

What is Root Bridge Election?

The first step with STP is to identify the root bridge. As a switch initializes, it assumes that it is the root bridge and uses the local bridge identifier as the root bridge identifier.

What is step 6 of Building the Rapid Spanning-tree Protocol (RSTP) Topology?

The inferior switch (SW2) moves its Root Port (RP) to a forwarding state. The superior switch moves its Designated Port (DP) to a forwarding state, too.

What is step 4 of Building the RSTP Topology?

The inferior switch (SW2) recognizes that it is inferior and marks its local port (Gi1/0/1) as the RP. At that same time, it moves all non-edge ports to a discarding state. At this point in time, the switch has stopped all local switching for non-edge ports.

What is step 7 of Building the RSTP Topology?

The inferior switch (SW2) repeats the process for any downstream switches connected to it.

What is step 5 of Building the RSTP Topology?

The inferior switch (SW2) sends an agreement (configuration BPDU) to the root bridge (SW1), which signifies to the root bridge that synchronization is occurring on that switch.

What is a Root Bridge?

The root bridge is the most important switch in the Layer 2 topology. All ports are in a forwarding state. This switch is considered the top of the spanning tree for all path calculations by other switches. All ports on the root bridge are categorized as designated ports.

Describe Port Forwarding

The switch port forwards all network traffic and updates the MAC address table as expected. This is the final state for a switch port to forward network traffic.

Describe Port Discarding

The switch port is enabled, but the port is not forwarding any traffic to ensure that a loop is not created. This state combines the traditional STP states disabled, blocking, and listening.

Describe Port Learning

The switch port modifies the MAC address table with any network traffic it receives. The switch still does not forward any other network traffic besides BPDUs.

Who sends a Topology Change Notification? (TCN)

The switch that detects a link status change

What is Step 3 of Building the RSTP Topology?

There can be only one DP (Designated Port) per segment, so each switch identifies whether it is the superior or inferior switch, using the same logic as in 802.1D for the system identifier (that is, the lowest priority and then the lowest MAC address).

What is step 2 of Building the RSTP Topology?

They establish a handshake with each other to advertise a proposal (in configuration BPDUs) that their interface should be the DP for that port.

What is System ID extension in Spanning-tree Protocol (STP)?

This 12-bit value indicates the VLAN that the Bridge Protocol Data Unit (BPDU) correlates to. The system priority and system ID extension are combined as part of the switch's identification of the root bridge.

What is System priority in Spanning-tree Protocol (STP)?

This 4-bit value indicates the preference for a switch to be root bridge. The default value is 32,768.

What is Local bridge identifier in STP?

This is a combination of the local switch's bridge system MAC address, system ID extension, and system priority of the root bridge.

What is Root bridge identifier in STP?

This is a combination of the root bridge system MAC address, system ID extension, and system priority of the root bridge.

What is Forward delay in STP?

This is the amount of time that a port stays in a listening and learning state. The default value is 15 seconds, but the value can be changed to a value of 15 to 30 seconds with the command spanning-tree vlan vlan-id forward-time forward-time.

What is Root path cost in Spanning-tree Protocol (STP)?

This is the combined cost for a specific path toward the root switch.

What is the max age in STP?

This is the maximum length of time that passes before a bridge port saves its BPDU information. The default value is 20 seconds, but the value can be configured with the command spanning-tree vlan vlan-id max-age maxage. If a switch loses contact with the BPDU's source, it assumes that the BPDU information is still valid for the duration of the Max Age timer.

What is Hello time in STP?

This is the time that a BPDU is advertised out of a port. The default value is 2 seconds, but the value can be configured to 1 to 10 seconds with the command spanning-tree vlan vlan-id hello-time hello-time.

What is a Bridge protocol data unit (BPDU) in STP?

This network packet is used for network switches to identify a hierarchy and notify of changes in the topology. A BPDU uses the destination MAC address 01:80:c2:00:00:00.

What is Topology change notification (TCN) BPDU?

This type of BPDU is used to communicate changes in the Layer 2 topology to other switches.

What is a Configuration BPDU?

This type of BPDU is used to identify the root bridge, root ports, designated ports, and blocking ports. The configuration BPDU consists of the following fields: STP type, root path cost, root bridge identifier, local bridge identifier, max age, hello time, and forward delay.

How many Port States does IEEE 802.1W have?

Three

What is a (TCN) in STP?

Topology Change Notification

Explain Building the Rapid Spanning-tree Topology (RSTP) Topology

With RSTP, switches exchange handshakes with other RSTP switches to transition through the following STP states faster. When two switches first connect, they establish a bidirectional handshake across the shared link to identify the root bridge. This is straightforward for an environment with only two switches; however, large environments require greater care to avoid creating a forwarding loop. RSTP uses a synchronization process to add a switch to the RSTP topology without introducing a forwarding loop. The synchronization process starts when two switches are first connected.

During the Topology Change from a Topology Change Notification (TCN) does the switch maintain it's active MAC address list?

Yes all currently active MAC addresses remain in the table

What is the CLI command to view Spanning Tree on VLAN 32? (CLI)

sw1#show spanning-tree vlan 32 detail