Section 2 Flashcards
Building Redundant Switched Topologies
1
Q
What does STP stand for?
A
Spanning Tree Protocol
2
Q
What algorithm does STP use?
A
The Spanning Tree Algorithm
3
Q
What is the purpose of STP in a network.
A
To prevent Layer 2 loops
4
Q
What messages does STP use to pass information between bridges?
A
BPDUs (Bridge Protocol Data Units)
5
Q
What is the most common STP standard used by network devices?
A
IEEE 802.1D
6
Q
What is the IEEE 802.1 standard based on?
A
The Digital Equipment Corporation STP
7
Q
What does LAN stand for?
A
Local-Area Network
8
Q
What is a LAN?
A
A high-speed, low error data network covering a relatively small geographic area (office or campus).
9
Q
What do LAN standards specify?
A
Cabling and signaling at the Physical and Data Link layers of the OSI model.
10
Q
What are three examples of LAN technologies?
A
Ethernet, FDDI, and Token Ring.
11
Q
What does MAC address stand for?
A
Media Access Control address
12
Q
What is a MAC address?
A
A standardized data link layer address that is required for every port or device that connects to a LAN.
13
Q
What do other devices use the MAC address for?
A
To locate specific ports, create, and update routing tables and data structures.
14
Q
How many bits/bytes is a MAC address?
A
48 bits (6 bytes)
15
Q
What other terms are commonly used to refer to a MAC address?
A
Hardware address, MAC layer address, and physical address.
16
Q
Why does STP place ports into a Standby state?
A
To ensure they do not listen to, forward, or flood data frames.
17
Q
What is the effect of STP placing ports into a standby state?
A
It ensures there is only one path that is active to each network segment.
18
Q
What does STP do if there is a problem with connectivity to a segment?
A
Re-establish connectivity by automatically activating a previously inactive path, if one exist.
19
Q
What is Ethernet?
A
A baseband LAN specification invented by Xerox Corporation and developed jointly by Xerox, Intel, and Digital Equipment Corporation.
20
Q
What collision avoidance mechanism does Ethernet use?
A
CSMA/CD
21
Q
What speed do Ethernet networks run at?
A
10 Mbps
22
Q
What is the most commonly used LAN technology?
A
Ethernet
23
Q
Ethernet is similar to which IEEE standard(s)?
A
802.3
24
Q
Why is Ethernet one the most commonly used LAN technology?
A
It is easy to implement, manage, and maintain. It allows low cost network implementations, provides topological flexibility, and is standards compliant (manufacturer independent).
25
Why are redundant topologies important?
They eliminate a single point of failure in the network.
26
What issues can occur in a redundant topology without a loop prevention mechanism?
Broadcast storms, multiple frame copies, MAC address table instability
27
What occurs during a broadcast storm?
The switch floods broadcast endlessly due to a layer 2 loop.
28
Why are multiple frame copies arriving at a destination an issue?
Many protocols expect to only receive one copy. Multiple copies of the same frame can cause unrecoverable errors.
29
What causes MAC database (address table) instability?
Different interfaces on the switch receive copies of the same frame.
30
What issue is the result of MAC database instability?
Data forwarding can be impaired due to too much resource usage coping with the constant changes in the MAC address table.
31
What does TTL stand for?
Time to Live
32
What is TTL?
A mechanism that limits the lifespan or lifetime of data in a computer network.
33
At what OSI layer do protocols commonly make use of TTL?
Layer 3 (Network Layer)
34
Do OSI Layer 2 protocols make use of TTL?
No
35
What does IEEE (I triple E) stand for?
Institute of Electrical and Electronics Engineers
36
Why is the IEEE important in regards to networking?
It is a professional organization whose activities include the development of communications and network standards.
37
What is the MAC Bridges standard?
IEEE 802.1D
38
What does the IEEE 802.1D standard specify?
The MAC bridges standard, which includes bridging and spanning tree, among others.
39
Who standardizes IEEE 802.1D?
The IEEE 802.1 working group
40
How does STP provide a loop free topology?
By placing certain ports into the blocking state
41
Where are STP standards published?
In the 802.1D specification.
42
What does STP use for communication between switches?
BPDUs
43
Why does STP place certain ports into a blocked state?
So they do not listen to, forward, or flood data frames.
44
How many paths should exist to each segment when STP is active?
One
45
What does BPDU stand for?
Bridge Protocol Data Unit
46
What is a BPDU?
A Spanning Tree Protocol hello packet that is sent out at configurable intervals to exchange information among bridges in the network.
47
How many fields compose a BPDU bridge ID? What are they?
Two. The Bridge Priority and MAC address.
48
How many bytes are the Bridge Priority and MAC address fields in a BPDU Bridge ID?
Bridge Priority (2 bytes) MAC address field (6 bytes)
49
What four steps does the STP alogrithm follow?
1. Elects a root bridge
2. Elects a root port for each non-root switch
3. Elects a designated port for each segment
4. Ports transition to a forwarding or blocking state
50
What is the value range for the Bridge Priority?
0 - 65,535
51
What is the default value for the Bridge Priority field?
32,768
52
What increment can the Bridge Priority value be changed by?
4096
53
Which bridge is elected the root bridge?
The one with the lowest Bridge ID
54
How many root bridges can there be per network?
One
55
What occurs if two bridges have the same lowest bridge priority?
The one with lowest MAC address will become the root bridge.
56
What is a root port?
The switch port with the best path to the root bridge.
57
How many root ports can exist on a switch?
One
58
Does the root bridge contain any root ports?
No
59
Designated ports are chosen based on what?
The lowest root path cost.
60
Each STP link must contain one port of which type.
Designated
61
If a port does not become a root or designated port, what state will it be placed into?
Blocking
62
What does BID stand for in relation to STP?
Bridge ID
63
What are the four STP port roles?
1. Root
2. Designated
3. Non-Designated (blocking in STP, discarding in RSTP)
4. Disabled
64
What is the STP path cost for a 4 Mbps link?
250 (old), 5,000,000 (new)
65
What is the STP path cost for a 10 Mbps link?
100 (old), 2,000,000 (new)
66
What is the STP path cost for a 16 Mbps link?
62 (old), 1,250,000 (new)
67
What is the STP path cost for a 100 Mbps link?
19 (old), 200,000 (new)
68
What is the STP path cost for a 1 Gbps link?
4 (old), 20,000 (new)
69
What is the STP path cost for a 2 Gbps link?
3 (old), 10,000 (new)
70
What is the STP path cost for a 10 Gbps link?
2 (old), 2,000 (new)
71
Which switch is elected root bridge?
The switch with the lowest bridge ID.
72
What root ID does a switch start with?
Its own root ID.
73
When does a switch replace its root ID with the ID from another switch?
When it receives a BPDU indicating that another switch has a lower root ID.
74
What is the difference between the Root ID field and the Bridge ID field?
The Root ID field contains the Bridge ID value of the current root bridge. The Bridge ID field will contain the Bridge ID of the switch sending the BPDU.
75
What field in a BPDU packet indicated the root bridge?
The Root ID field.
76
How is root path cost determined?
Based on link speed. The higher the bandwidth, the lower the cost.
77
How are ties broken for Designated ports?
Ties are broken based off upstream bridge ID and then lowest port ID if necessary.
78
What state do ports start in at the beginning of the STP algorithm.
Blocking
79
What does CST stand for?
Common Spanning Tree
80
What is CST?
A single spanning-tree instance regardless of the number of VLANs.
81
What does PVST+ stand for?
Per VLAN Spanning Tree Plus
82
What is PVST+
A Cisco proprietary enhancement of STP that provides a separate 802.1D spanning-tree instance for each VLAN that is configured on the network.
83
What does MSTP stand for?
Multiple Spanning Tree Protocol
84
What is MSTP.
An IEEE standard that is inspired by the earlier Cisco proprietary MISTP implmentation. MSTP maps multiple VLANs into the same spanning-tree instance.
85
What does MISTP ("myst-p") stand for?
Multi-Instance Spanning Tree Protocol
86
What does RSTP stand for?
Rapid Spanning Tree Protocol
87
What is RSTP?
An evolution of STP that provides faster convergence of STP. It redefines port roles and link cost.
88
What doest Rapid PVST+ stand for?
Rapid Per-VLAN Spanning Tree Plus
89
What is Rapid PVST+?
A Cisco enhancement of RSTP that uses PVST+. Rapid PVST+ provides a separate instance of 802.1w per VLAN.
90
List the following for STP: standard, resources needed, convergence time speed (slow or fast), number of spanning trees.
802.1d, low, slow (30 seconds), one
91
List the following for PVST+: standard, resources needed (low, medium, or high), convergence time speed (slow or fast), number of spanning trees.
Cisco proprietary, high, slow, one per VLAN
92
List the following for RSTP: standard, resources needed, convergence time speed (slow or fast), number of spanning trees.
802.1w, medium, fast, one
93
List the following for Rapid PVS+: standard, resources needed, convergence time speed (slow or fast), number of spanning trees.
Cisco proprietary, high, fast, one per VLAN
94
List the following for MSTP: standard, resources needed, convergence time speed (slow or fast), number of spanning trees.
802.1s, medium OR high, fast, one for multiple (groups of) VLANs
95
What is the default Spanning Tree configuration for Cisco switches?
PVST+
96
What is the main advantage of PVST+
Load balancing can occur since STP is determined per VLAN.
97
What is the main disadvantage of PVST+
It is resource intensive due to having to calculate the spanning-tree for each VLAN.
98
What does PVST+ modify to keep STP data separate for each VLAN.
The Bridge ID is modified to the Extended Bridge ID in order to contain the VLAN ID.
99
What fields does the Extended Bridge ID contain?
Bridge Priority (4 bits), Extended System ID (12 bits), and the MAC address (48 bits). Extended Bridge ID is still 64 bits (8 bytes) total.
100
What command is used to force a switch to be the root bridge for a specified VLAN?
spanning-tree vlan root primary
101
What does the following command accomplish?
| spanning-tree vlan root primary
It forces the switch to be the root bridge for a specified VLAN.
102
What command configures a switch to be the backup root bridge for a specified VLAN?
spanning-tree vlan root secondary
103
What command is used to view spanning-tree configuration for manually set root bridges?
show running-config | include root
104
What command displays an overview of STP status and topology?
show spanning-tree
105
What does the following command accomplish?
| show spanning-tree
It displays an overview of STP status and topology
106
Which command is used to obtain STP information for a particular VLAN?
show spanning-tree vlan
107
What are the STP port stages?
1. Blocking
2. Listening
3. Learning
4. Forwarding
108
What is the maximum time a port spends in the Blocking state?
20 seconds
109
What is the maximum time a port spends in the Listening state?
15 seconds
110
What is the maximum time a port spends in the Learning state?
15 seconds
111
Does a port in the Listening state populate the MAC address table? Does it forward frames?
No. No.
112
Does a port in the Learning state populate the MAC address table? Does it forward frames?
Yes. No.
113
Does a port in the Forwarding state populate the MAC address table? Does it forward frames?
Yes. Yes.
114
What does DHCP stand for?
Dynamic Host Configuration Protocol
115
What is DHCP used for?
It provides a mechanism for allocating IP address dynamically so that addresses can be reused when the host no longer needs them.
116
What is PortFast?
An STP feature that can be enabled on access ports so that they instantly enter the forwarding state.
117
Where should PortFast be configured?
On access ports only.
118
What is the purpose of BPDU Guard?
BPDU Guard will shut down a port if it receives a BPDU.
119
What is BPDU Guard typically used in conjunction with?
PortFast
120
Where should BPDU guard be configured?
On access ports only
121
What command activates PortFast on an interface?
spanning-tree portfast
122
What command activates PortFast globally on a switch?
spanning-tree portfast default
123
What command activates BPDU Guard on an interface?
spanning-tree bpduguard enable
124
What command activates BPDU Guard globally?
spanning-tree portfast bpdguard default
125
What command indicates whether PortFast and BPDU Guard have been configured on an interface?
show running-config interface [interface]
126
What command indicates whether PortFast has been configured on a port or globally?
show spanning-tree interface [interface] portfast
127
What command indicates whether PortFast and BPDU Guard have been enabled globally?
show spanning-tree summary
