Mod 5.1-5.2: Purpose and Operations of STP Flashcards
____ is a loop-prevention network protocol that allows for redundancy while creating a loop-free Layer 2 topology.
Spanning Tree Protocol (STP)
____is the original IEEE MAC Bridging standard for STP.
IEEE 802.1D
Issues with Redundant Switch Links:
____ provides multiple network services by eliminating the possibility of a single point of failure.
Path redundancy
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 Layer 2 loop can result in ___instability, link saturation, and ___ on switches and end-devices, resulting in the network becoming unusable.
MAC address table
high CPU utilization
True or false
Unlike the Layer 3 protocols, IPv4 and IPv6, Layer 2 Ethernet does not include a mechanism to recognize and eliminate endlessly looping frames.
True
Both IPv4 and IPv6 include a mechanism that limits the number of times a Layer 3 networking device can retransmit a packet via TTL (time to live) which is decremented by routers or hop limit in ipv6
____ was developed specifically as a loop prevention mechanism for Layer 2 Ethernet.
STP
Without STP enabled, Layer 2 loops can form, causing broadcast, multicast and unknown unicast frames to ___.
loop endlessly
An____ frame 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.
unknown unicast
A ____is an abnormally high number of broadcasts overwhelming the network during a specific amount of time.
broadcast storm
Broadcast storms can be caused by a hardware problem such as a___ or from a___ in the network.
faulty NIC
Layer 2 loop
SPANNING TREE ALGORITHM:
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.”
Radia Perlman
Radia Perlman’s ____ 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)
STA PROCESS:
Step 1:STP prevents loops from occurring by configuring a loop-free path through the network using strategically placed ____” ports
“blocking-state
STA PROCESS:
Step 1: The switches running STP are able to compensate for failures by ___ the previously blocked ports and permitting traffic to traverse the alternate paths.
dynamically unblocking
STA
Step 2: The spanning tree algorithm begins by selecting a ___
single root bridge
Note: The STA and STP refers to ___ as bridges
switches
STA
Step 3: STP ensures that there is only one logical path between all destinations on the network by intentionally ____ that could cause a loop
blocking redundant paths
STA
Step 4: A blocked port has the effect of making that link a non-forwarding link between the two switches. Thus, this creates a ___ topology.
Loop-Free
STA
Step 5: 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.
recalculates
True or false
STP recalculation happens only during link failure.
False
STP recalculations can also occur any time a new switch or new inter-switch link is added to the network.
Reorder the following:
Steps to a Loop-Free Topology
Elect Alternate (blocked) ports
Elect root ports
Elect root bridge
Elect designated ports
Elect the root bridge.
Elect the root ports.
Elect designated ports.
Elect alternate (blocked) ports.
During STA and STP functions, switches use ___ to share information about themselves and their connections.
Bridge Protocol Data Units (BPDUs)
BPDUs are used to elect the___.
root bridge, root ports, designated ports, and alternate ports
Each BPDU contains a ___ that identifies which switch sent the BPDU.
bridge ID (BID)
The ___is involved in making many of the STA decisions including root bridge and port roles.
BID
The lowest BID value is determined by the combination of these three fields:
bridge priority value, an extended system ID, and the MAC address of the switch
Bridge Priority has ___ bits
4
Extended System Id has __ bits
12
MAC address has __ bits
48
Bridge priority:
The default priority value for all Cisco switches is the decimal value ___.
32768
Bridge Priority:
The range is 0 to 61440 in increments of ___
4096
True or false.
higher bridge priority is preferable because the highest value takes precedence.
False
A lower bridge priority is preferable. A bridge priority of 0 takes precedence over all other bridge priorities
The ___value is a decimal value added to the bridge priority value in the BID to identify the VLAN for this BPDU.
extended system ID
The extended system ID allows later implementations of STP to have different ___ for different sets of VLANs
root bridges
The extended system ID allows later implementations of STP to have different ___ for different sets of VLANs
root bridges
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.
MAC address
- Elect the Root bridge
True or false
The STA designates multiple switch as the root bridge and uses it as the reference point for all path calculations.
False
Should be single
- Elect the Root bridge
True or false
All switches in the broadcast domain participate in the election process.
True
After a switch boots, it begins to send out BPDU frames every two seconds.
The BPDU frames sent by switches contain the BID of the sending switch and the BID of the root bridge, known as the ___.
Root ID
The switch with the ___ BID will become the root bridge.
lowest
Is it possible for two or more switches to have the same priority?
yes, because the default BID is 32768
In this scenario, where the priorities are the same, the switch with the ___ will become the root bridge.
lowest MAC address
When the root bridge has been elected for a given spanning tree instance, the STA starts the process of determining the___ to the root bridge from all destinations in the broadcast domain.
best paths
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.
internal root path cost
The default port costs are defined by the___.
speed at which the port operates
Link Speed STP Cost RSTP Cost
10 Gbps———2 ____________-
_________________4 ———–20,000
100 Mpbs ______________ __________
_______________ 1000__________________
10 Gbps 2 2,000
1 Gbps 4 20,000
100 Mbps 19 200,000
10 Mbps 100 2,000,000
Are port cost configurable?
Yes. The ability to configure individual port costs gives the administrator the flexibility to manually control the spanning tree paths to the root bridge.
- Elect the Root Ports
Every non-root switch will select how many root ports?
Only one
The___ is the port closest to the root bridge in terms of overall cost (best path) to the root bridge
root port
Paths with the ___ cost become preferred, and all other redundant paths are blocked.
lowest
- Elect Designated Ports
Every segment between two switches will have how many designated ports?
one
The ____ is a port on the segment (with two switches) that has the internal root path cost to the root bridge
designated port
True or false
The root port has the best path to receive traffic leading to the root bridge.
False. It should be designated port
What happens to ports that are not a designated port nor root port?
It becomes an alternate or blocked port.
- Elect Designated Ports
All ports on the ___ are designated port
root bridge
- Elect Designated Ports
If one end of a segment is a root port, then the other end is a ___
designated port.
- Elect Designated Ports
In this case, the port on the switch with the____path to the root bridge is the designated port for the segment
least-cost
- Elect Alternate (Blocked) Ports
If a port is not a root port or a designated port, then it becomes an alternate (or ____) port.
backup
- Elect Alternate (Blocked) Ports
Alternate ports and backup ports are in __ or ___ state to prevent loops.
discarding
blocking
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
Lowest sender port priority
Lowest sender port ID
STP TIMERS:
The ** is the interval between BPDUs. The default is 2 seconds but can be modified to between 1 and 10 seconds.
Hello timer
Hello time
STP TIMERS
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.
Forward Delay Timer
forward delay
STP TIMERS
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.
Forward Delay Timer
forward delay
STP 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 be modified to between 6 and 40 seconds.
Max Age Timer
Max age
PORT STATE
The port is an alternate port and does not participate in frame forwarding.
Blocking
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.
PORT STATE
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.
PORT STATE
A switch port transitions to the ____ after the listening state.
Learning
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.
PORT STATE
In the ___ state, a switch port is considered part of the active topology.
forwarding
PORT STATE
A switch port in the ___ does not participate in spanning tree and does not forward frames.
disabled state
Operational details of each port state
Port State |BPDU | MAC Table |Forwarding Data Frames?
Blocking |Receive _____ No
No update
Operational details of each port state
Port State | BPDU |MAC Table| Forwarding Data Frames?
Listening _____ _____ No
Receive and send
No update
Operational details of each port state
Port State |BPDU | MAC Table |Forwarding Data Frames?
Listening _____ _____ No
Receive and send
No update
Operational details of each port state
Port State | BPDU | MAC Table |Forwarding Data Frames?
Learning ______ _____ No
Receive and Send
Updating Table
Operational details of each port state
Port State | BPDU | MAC Table | Forwarding Data Frames?
Forwarding Receive and send _____ ___
Updating table
Yes
Operational details of each port state
Port State | BPDU | MAC Table | Forwarding Data Frames?
______ none sent or received | No Update _____
Disabled
No
True or false
STP can only be configured to operate in an environment with only one VLANs
False
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.
