Week 6: STP Flashcards
What are the two purposes for using Switching?
Breaks up Collision Domains (each switch port is its own separate Collision Domain)
Cost-effective way to connect workstations (switches have replaced hubs)
What are the advantages of using Layer 2 Switching?
Hardware-Based Bridging (ASICs)
Low Latency
Low Cost
What are the three Switch functions at Layer 2
Address Learning
Forward/Filter Decisions
Loop Avoidance
What is Switch Address Learning?
Layer 2 switches remember the source hardware address of each frame received on an interface and entered into the Forward/Filter MAC Database Table
Describe Forward/Filter Decisions for a Switch
Switch looks at the destination hardware address, then chooses the appropriate exit interface for it in the MAC Database
What is Switch Loop Avoidance?
If multiple connections between switches are created for redundancy purposes, network loops can occur
What protocol is used to prevent network loops while maintaining redundancy?
Spanning Tree Protocol
What are the three Forward/Filter Decisions for a Switch when a Frame arrives?
- If a source hardware address is not in the forwarding table -> add it
- if a destination hardware address is known -> Frame Filtering
- If the destination hardware address is not known -> frame is flooded to ALL active interfaces EXCEPT source
What types of links are desired between switches?
Redundant Links
What are the benefits of Redundant Links?
Help prevent irrecoverable network failures in case one link stops working
Helps improve a network’s fault tolerance
What can loops in switching lead to?
Broadcast Storm
What is a Broadcast Storm?
Flood of endless broadcast packets
What MUST switches avoid?
Logical Loops
What are the three laters in the Cisco Hierarchical Network Model?
Core Layer
Distribution Layer
Access Layer
What is the function of the Core Layer in the Cisco Hierarchical Network Model?
Backbone of the model
Transport data reliably & quickly
What is the function of the Distribution Layer in the Cisco Hierarchical Network Model?
Routing
filtering
WAN Access
What is the function of the Access Layer in the Cisco Hierarchical Network Model?
Switching
Separating Collision Domains
In which network design is Redundancy introduced?
Hierarchical Network Design
What type of loop will disable a network?
A Logical Loop (due to redundancy)
What does Spanning Tree Protocol (STP) prevent?
Stops loops in Layer 2 Switch Networks
What are the properties of the Spanning Tree Protocol?
Layer 2 Protocol
Uses the Spanning Tree Algorithm (STA)
What are the main standards of Spanning Tree Protocol (STP)?
STP: IEEE802.1D Rapid STP (RSTP): IEEE802.1W
What is the purpose of the Spanning Tree Algorithm?
To interrupt the logical loop created by physical loops in a bridged/switched environment
How does STP prevent Logical Loops?
By ensuring that certain ports on some of the bridges/switches do not forward frames
What is a Bridge Protocol Data Unit (BPDU)?
Used to exchange selection information of the root switch as well as subsequent network configuration
How does a BPDU function?
Each switch compares the parameters in the BPDU that it sends to one neighbour with the ones that it receives from other neighbours
What type of network hardware sends BPDU’s and how frequently?
Sent by Bridges periodically (usually every 2 seconds)
what is Port Cost?
The cumulative value of link costs towards the root
What is Port Cost used for?
Determining the best path when multiple links are used between two switches
How is Port Cost calculated?
It is calculated via the bandwidth of the links along the path
What is a Bridge ID?
The combination of the Bridge Priority and the base MAC Address
What is the default Bridge Priority for all Cisco Switches?
32,768
What are Bridge ID’s used for?
For STP to keep track of all network-related switches
What is a Root Bridge?
The Bridge with the lowest Bridge ID
What is a Non-root Bridge?
All bridges that are not the Root Bridge
What is the main function of Non-Root Bridges?
To exchange BPDU’s with all bridges & update the STP Topology database on all switches
What is a Root Port?
The port that provides the best path to the root bridge
What are two properties of the Root Port?
Root Port is always the link directly connected to the root bridge
OR
The Lowest Path Cost to the root bridge
What is a Designated Port?
A port that has been determined as having the best (lowest) cost to the Root Bridge via the Root Port
What can a Designated Port be marked as?
A Forwarding Port
What is a Non-Designated Port?
A port with a higher cost than the Designated Port
What is a Non-Designated Port marked as?
They are marked with Blocking Mode - they are not forwarding ports
What is a Forwarding Port?
A port that forwards frames; can be a Root Port or a Designated Port
What is a Blocked Port?
A port that, in order to prevent loops, will NOT forward Frames
What is an exception to a Blocked Port?
A Blocked Port will always listen to BPDU Frames but drop any & all other Frames
How do Switches & Bridges configure a single logical path?
via election
What is the election process that Switches & Bridges undergo to configure a single logical path?
- A Root Bridge (root device) is selected; all other Bridges/Switches configure their ports using the root Bridge as a point of reference
- Bridges use STP (BPDUs) to transfer the information about each Bridge’s MAC Address & Priority Number
- Each Bridge or Switch determines which of its own ports offers the best path to the Root Bridge
What process is triggered when there is a Root Failure?
The device in an STP-enabled network (that has stopped receiving BPDUs) will assume:
i. The Root Bridge has failed
ii. Claim to be the Root Bridge
iii. Will begin sending BPDUs describing itself as such
iv. Begin Root Bridge re-election
What is a Stable State?
The normal operating state of all ports when the Root Bridge is available and all paths are functioning as expected
Normal Data Transfer occurs
What is a Transitory State?
Prevents logical loops during a period of transition from one root bridge to another
NO DATA PACKET TRANSFER OCCURS
What are the three Stable States?
Blocking
Forwarding
Disabled
What are the two Transitory States?
Listening
Learning
When are Transitory States used?
When an STP devices are waiting on a new bridge to be elected
What is a Disabled State?
Virtually non-operational port
What is a Listening State?
Switch processe BPDUs & awaits possible new information that would cause it to return to the blocking state
What are two things are ignored by an STP-device in Listening State?
Does not learn MAC Address
Does not forward Frames
What are the properties of an STP-device in Learning State?
Processes BPDUs
Learns MAC Address
Does not forward packets
What is a Forwarding State?
Normal port operation; sending and receiving data
What is a Blocking State?
A port that would cause a Switching Loop; no user data is sent or received
What is one property of a Blocking State?
BPDU data is still received
Under what conditions will a Blocking State go into Forwarding Mode?
- Other links in use were to fail
2. STA determines the port may transition to the Forwarding State
How many bytes long is the Bridge ID?
8 Bytes (2 Byte Priority, 6 Byte MAC Address)
What is used in addition to the Bridge ID in the case of multiple VLANs in a Switch?
VLAN Number is used in the Extended System ID field for distinction
System ID Extension is what?
The VLAN Number
What are the three STP Timers?
Hello Time
Forward Delay
Maximum Age
What is the Hello Time STP timer?
The time between each BPDU Frame that is sent on a port
2 seconds by default, can be 1 - 10 seconds
What is the Forward Delay STP timer?
The time spent in the listening & learning state
15 seconds for each state by default, can be 4 - 30 seconds
What is the Maximum Age STP Timer?
The maximum amount of time a switch saves the configuration BPDU information
20 seconds by default, can be 6 - 40 seconds
