CHAPTER 5 - STP Concepts

Spanning tree protocol

stp

Redundancy

is an important part of the hierarchical design for eliminating single points of failure and preventing disruption of network services to users. Such networks require the addition of physical paths, but logical redundancy must also be part of the design.

alternate physical paths

Having _________ ________ _____ for data to traverse the network makes it possible for users to access network resources, despite path disruption.

Layer 2 loops

redundant paths in a switched Ethernet network may cause both physical and logical_____ _ _____

Loop-free

Ethernet LANs require a ____-____ topology with a single path between any two devices. A loop in an Ethernet LAN can cause continued propagation of Ethernet frames until a link is disrupted and breaks the loop

Stp

is a loop-prevention network protocol that allows for redundancy while creating a loop-free Layer 2 topology. It logically blocks physical loops in a Layer 2 network, preventing frames from circling the network forever

recalculating

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

Does not

Layer 2 Ethernet [do, does not] include a mechanism to recognize and eliminate endlessly looping frames.

retransmit

Both IPv4 and IPv6 include a mechanism that limits the number of times a Layer 3 networking device can __________ a packet.

Time to live, hop limit field

A router will decrement the TTL (____ __ ____) in every IPv4 packet, and the ___ _____ _____ in every IPv6 packet. When these fields are decremented to 0, a router will drop the packet.

Loop prevention

STP was developed specifically as a ____ __________ mechanism for Layer 2 Ethernet

Loop

Without STP enabled, Layer 2 loops can form, causing broadcast, multicast and unknown unicast frames to ____ endlessly. This can bring down a network quickly.

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.

destination

An unknown unicast frame is when the switch does not have the ___________ 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. It can disable a network within seconds by overwhelming switches and end devices. It can be caused by a hardware problem such as a faulty NIC or from a Layer 2 loop in the network.

MAC address table

A host caught in a Layer 2 loop is not accessible to other hosts on the network. Additionally, due to the constant changes in its ___ _______ _____, the switch does not know out of which port to forward unicast frames.

spanning tree

To prevent these issues from occurring in a redundant network, some type of ________ ____ must be enabled on the switches. It is enabled, by default, on Cisco switches to prevent Layer 2 loops from occurring

Radia Perlman

STP is based on an algorithm invented by _____ _______ while working for Digital Equipment Corporation, and published in the 1985 paper "An Algorithm for Distributed Computation of a Spanning Tree in an Extended LAN." Her 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

Spanning tree algorithm

sta

Blocking-state

STP prevents loops from occurring by configuring a loop-free path through the network using strategically placed "________-_____" ports. The switches running STP are able to compensate for failures by dynamically unblocking the previously blocked ports and permitting traffic to traverse the alternate paths

Root Bridge

Selecting a ____ ______: This bridge (switch) is the reference point for the entire network to build a spanning tree around

logical path

Block Redundant Paths: STP ensures that there is only one _______ ____ 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.

Non-forwarding

Create a Loop-Free Topology: A blocked port has the effect of making that link a ___-__________ 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.

Recalculate

___________ in case of Link Failure: The physical paths still exist to provide redundancy, but these paths are disabled to prevent the loops from occurring. If the path is ever needed to compensate for a network cable or switch failure, STP ___________ the paths and unblocks the necessary ports to allow the redundant path to become active. It can also occur any time a new switch or new inter-switch link is added to the network.

Root bridge, root ports, designated ports, alternate ports

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

1. Elect the ____ ______

2. Elect the ____ _____

3. Elect __________ _____.

4. Elect _________ (blocked) _____

Bridge protocol data unit

bpdu

Bpdu

During STA and STP functions, switches use ____s to share information about themselves and their connections. They are used to elect the root bridge, root ports, designated ports, and alternate ports.

Bridge id

bid

Bid

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

Lowest

The BID contains a priority value, the MAC address of the switch, and an extended system ID. The ______ BID value is determined by the combination of these three fields

32768

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

Extended System ID

The ________ ______ __ value is a decimal value added to the bridge priority value in the BID to identify the VLAN for this BPDU.

MAC address

When two switches are configured with the same priority and have the same extended system ID, the switch having the ___ _______ with the lowest value, expressed in hexadecimal, will have the lower BID

root bridge

The STA designates a single switch as the ____ ______ and uses it as the reference point for all path calculations. Switches exchange BPDUs to build the loop-free topology beginning with selecting the root bridge.

2, Root ID

All switches in the broadcast domain participate in the election process. After a switch boots, it begins to send out BPDU frames every _ seconds. These BPDU frames contain the BID of the sending switch and the BID of the root bridge, known as the____ __.

BPDU

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 ____s which switch has the lowest BID and will agree on one root bridge.

MAC address

Because the default BID 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 lowest ___ _______ will become the root bridge.

When the root bridge has been elected for a given spanning tree instance, the STA starts 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.

internal root path cost

When a switch receives the BPDU, it adds the ingress port cost of the segment to determine its ________ ____ ____ ____

short path cost

The default port costs are defined by the speed at which the port operates. Cisco switches by default use the values as defined by the IEEE 802.1D standard, also known as the _____ ____ ____, for both STP and RSTP.

Configurable

Although switch ports have a default port cost associated with them, the port cost is ____________. The ability to do it gives the administrator the flexibility to manually control the spanning tree paths to the root bridge

root port

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

sum of all the port costs

The internal root path cost is equal to the ___ __ ___ ___ ____ _____ along the path to the root bridge, as shown in the figure. Paths with the lowest cost become preferred, and all other redundant paths are blocked.

designated port

Every segment between two switches will have one designated port. The designated port is a port on the segment that has the internal root path cost to the root bridge. In other words, the __________ ____ has the best path to receive traffic leading to the root bridge.

alternate

What is not a root port or a designated port becomes an _________ or blocked port

Designated

All ports on the root bridge are __________ ports.

designated

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

designated

All ports attached to end devices are __________ ports.

east-cost path

On segments between two switches where neither of the switches is the root bridge, the port on the switch with the _____-____ ____ to the root bridge is a designated port.

discarding

Alternate ports are in __________ or blocking state to prevent loops. This is the loop prevention part of STP

BID, port priority

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

• Lowest sender ___

• Lowest sender ____ ________

Rapid spanning tree protocol

rstp

RSTP

increases the speed of the recalculation of the spanning tree when the Layer 2 network topology changes. ____ can achieve much faster convergence in a properly configured network, sometimes in as little as a few hundred milliseconds. If a port is configured to be an alternate port it can immediately change to a forwarding state without waiting for the network to converge

alternate port

has an alternate path to the root bridge.

backup port

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

Listening, learning, 15

When a device is connected to a switch port or when a switch powers up, the switch port goes through both the _________ and ________ states, each time waiting for the Forward Delay timer to expire. This delay is __ seconds for each state for a total of 30 seconds. This can present a problem for DHCP clients trying to discover a DHCP server because the DHCP process may timeout. The result is that an IPv4 client will not receive a valid IPv4 address.

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 it on access ports to allow devices connected to these ports to access the network immediately. It should only be used on access ports. If you enable it on a port connecting to another switch, you risk creating a spanning tree loop.

BPDU

A PortFast-enabled switch port should never receive ____s because that would indicate that switch is connected to the port, potentially causing a spanning tree loop.

BPDU guard

Cisco switches support a feature called ____ _____. When enabled, it immediately puts the switch port in an errdisabled (error-disabled) state upon receipt of any BPDU. This protects against potential loops by effectively shutting down the port. The administrator must manually put the interface back into service.