Module 5: STP Concepts

Redundancy

is an important part of the hierarchical design for eliminating single points of failure and preventing disruption of network services to users.

loop-free topology

Ethernet LANs require a _____________ with a single path between any two devices.

loop

A ______ in an Ethernet LAN can cause continued propagation of Ethernet frames until a link is disrupted and breaks the loop.

Spanning Tree Protocol (STP)

is a loop-prevention network protocol that allows for redundancy while creating a loop-free Layer 2 topology.

Spanning Tree Protocol (STP)

logically blocks physical loops in a Layer 2 network, preventing frames from circling the network forever.

Spanning Tree Protocol (STP)

compensates for a failure in the network by recalculating and opening up previously blocked ports.

Path redundancy

provides multiple network services by eliminating the possibility of a single point of failure.

Layer 2 loop

When multiple paths exist between two devices on an Ethernet network, and there is no spanning tree implementation on the switches, a __________ occurs.

Layer 2 loop

A __________ can result in MAC address table instability, link saturation, and high CPU utilization on switches and enddevices, resulting in the network becoming unusable.

TTL (Time to Live)

A router will decrement the ___________ in every IPv4 packet

IPv6 packet

A router will decrement the Hop Limit field in every _________

0

When these fields are decremented to ? , a router will drop the packet.

MAC database instability

When a loop occurs, the MAC address table on a switch will constantly change with the updates from the broadcast frames, which results in ? This can cause high CPU utilization, which makes the switch unable to forward frames.

unknown unicast frame

An _______________ is when the switch does not have the destination MAC address in its MAC address table and must forward the frame out all ports, except the ingress port.

broadcast storm

A _____________ is an abnormally high number of broadcasts overwhelming the network during a specific amount of time.

Broadcast storms

can disable a network within seconds by overwhelming switches and end devices.

Broadcast storms

can be caused by a hardware problem such as a faulty NIC or from a Layer 2 loop in the network.

true

tru or false?

Layer 2 multicasts are typically forwarded the same way as a broadcast by the switch.

true

tru or false?

IPv6 packets are never forwarded as a Layer 2 broadcast, ICMPv6 Neighbor Discovery uses Layer 2 multicasts.

Radia Perlman

STP is based on an algorithm invented by __________  while working for Digital Equipment Corporation

1985

STP is published in the ______ paper "An Algorithm for Distributed Computation of a Spanning Tree in an Extended LAN.”

Spanning Tree Algorithm (STA)

creates a loop-free topology by selecting a single root bridge where all other switches determine a single least-cost path.

blocking-state ports

STP prevents loops from occurring by configuring a loop-free path through the network using strategically placed ___________.

• Selecting a Root Bridge

• Block Redundant Paths

• Create a Loop-Free Topology

• Recalculate in case of Link Failure

How does the STA create a loop-free topology?

Root Bridge

This bridge (switch) is the reference point for the entire network to build a spanning tree around.

STP

ensures that there is only one logical path between all destinations on the network by intentionally blocking redundant paths that could cause a loop. When a port is blocked, user data is prevented from entering or leaving that port.

blocked port

A _____________ has the effect of making that link a non-forwarding link between the two switches. This creates a topology where each switch has only a single path to the root bridge, similar to branches on a tree that connect to the root of the tree.

1. Elect the root bridge.

2. Elect the root ports.

3. Elect designated ports.

4. Elect alternate (blocked) ports

STP builds a loop-free topology in a four-step process:

Bridge Protocol Data Units (BPDUs)

During STA and STP functions, switches use __________________ to share information about themselves and their connections.

Bridge Protocol Data Units (BPDUs)

are used to elect the root bridge, root ports, designated ports, and alternate ports.

bridge ID (BID)

Each BPDU contains a __________ that identifies which switch sent the BPDU. This is involved in making many of the STA decisions including root bridge and port roles

32768

Bridge Priority: The default priority value for all Cisco switches is the decimal value ________

0

A lower bridge priority is preferable. A bridge priority of __ takes precedence over all other bridge priorities.

decimal value

The extended system ID value is a ___________ added to the bridge priority value in the BID to identify the VLAN for this BPDU.

single switch

The STA designates a _________ as the root bridge and uses it as the reference point for all path calculations.

root bridge

Switches exchange BPDUs to build the loop-free topology beginning with selecting the ?

Root ID

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 ?

lowest BID

The switch with the _________ will become the root bridge.

internal root path cost

The path information, known as the __________________, is determined by the sum of all the individual port costs along the path from the switch to the root bridge

root port

After the root bridge has been determined, the STA algorithm is used to select the ?

true

tru or false?

If one end of a segment is a root port, the other end is a designated port.

designated ports

All ports attached to end devices are ?

alternate (or backup) port

If a port is not a root port or a designated port, then it becomes an ?

• Lowest sender BID

• Lowest sender port priority

• Lowest sender port ID

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

Lowest Sender BID

This topology has four switches with switch S1 as the root bridge. Port F0/1 on switch S3 and port F0/3 on switch S4 have been selected as root ports because they have the root path cost to the root bridge for their respective switches.

S3 and S4

The bridge IDs of _____ and _____, will be used to break the tie.

32769.5555.5555.5555

S3 has a BID of ?

32769.1111.1111.1111.

S4 has a BID of ?

Lowest Sender Port Priority

This topology has two switches which are connected with two equalcost paths between them. S1 is the root bridge, so both of its ports are designated ports.

S4

____ has two ports with equal-cost paths to the root bridge. Because both ports are connected to the same switch, the sender’s BID (S1) is equal. So the first step is a tie

128

the sender’s (S1) port priority. The default port priority is ____, so both ports on S1 have the same port priority.

Lowest Sender Port ID

The last tie-breaker is the lowest sender’s port ID. The decision is based on the sender’s port ID, not the receiver’s port ID.

• Hello Timer

• Forward Delay Timer

• Max Age Timer

STP convergence requires three timers, as follows:

Hello Timer

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

Forward Delay Timer

The __________ 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.

Max Age Timer

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

STP

facilitates the logical loop-free path throughout the broadcast domain.

spanning tree

The _____________  is determined through the information learned by the exchange of the BPDU frames between the interconnected switches

• Disabled

• Blocking

• Listening

• Learning

• Forwarding

STP has five ports states:

Rapid Spanning Tree Protocol (RSTP)

The latest IEEE documentation on spanning tree (IEEE-802-1D-2004) says, "STP has now been superseded by the ?

STP

The IEEE uses "____" to refer to the original implementation of spanning tree

RSTP

"______" to describe the version of spanning tree specified in IEEE-802.1D-2004.

STP

This is the original IEEE 802.1D version (802.1D-1998 and earlier) that provides a loop-free topology in a network with redundant links.

Common Spanning Tree (CST)

STP is Also called ____________, it assumes one spanning tree instance for the entire bridged network, regardless of the number of VLANs.

PVST+

Per-VLAN Spanning Tree (PVST+) is a Cisco enhancement of STP that provides a separate 802.1D spanning tree instance for each VLAN configured in the network. PVST+ supports PortFast, UplinkFast, BackboneFast, BPDU guard, BPDU filter, root guard, and loop guard.

802.1D2004

This is an updated version of the STP standard, incorporating IEEE 802.1w.

Rapid Spanning Tree Protocol (RSTP)

? or IEEE 802.1w is an evolution of STP that provides faster convergence than STP.

Rapid PVST+

This is a Cisco enhancement of RSTP that uses PVST+ and provides a separate instance of 802.1w per VLAN. Each separate instance supports PortFast, BPDU guard, BPDU filter, root guard, and loop guard.

Multiple Spanning Tree Protocol (MSTP)

is an IEEE standard inspired by the earlier Cisco proprietary Multiple Instance STP (MISTP) implementation. MSTP maps multiple VLANs into the same spanning tree instance

Multiple Spanning Tree (MST)

is the Cisco implementation of MSTP, which provides up to 16 instances of RSTP and combines many VLANs with the same physical and logical topology into a common RSTP instance. Each instance supports PortFast, BPDU guard, BPDU filter, root guard, and loop guard.

RSTP

_______ increases the speed of the recalculation of the spanning tree when the Layer 2 network topology changes. it can achieve much faster convergence in a properly configured network

backup port

The _________ is a backup to a shared medium, such as a hub. It is less common because hubs are now considered legacy devices.

listening and learning states

When a device is connected to a switch port or when a switch powers up, the switch port goes through both the ___________ and _________, each time waiting for the Forward Delay timer to expire.

PortFast

When a switch port is configured with ________, that port transitions from blocking to forwarding state immediately, avoiding the 30 second delay. You can use this on access ports to allow devices connected to these ports to access the network immediately

PortFast

_________ should only be used on access ports. If you enable this on a port connecting to another switch, you risk creating a spanning tree loop.

Layer 3 routing

allows for redundant paths and loops in the topology, without blocking ports.