CCNA3 Exploration Chapter 5. Study questions. Answers
5.1
Why is it an advantage to have redundancy in a switched Ethernet network?
The additional devices and links provide alternative paths for data if some
paths become unavailable because of equipment failure. This increases the
reliability and availability of the network.
What is the purpose of Spanning Tree Protocol?
To prevent loops in a level 2 switched network where there is redundancy.
What mechanism is available in the packet header to prevent endless routing
loops, but is not available in the frame header to do the same for switching
loops.
Time to live field.
If frames are caught in a loop, how can you break the loop?
You need to break the loop physically by removing a connection or powering
off a switch.
What is a broadcast storm?
A large number of broadcasts looping endlessly in a network, taking all the
bandwidth so that normal traffic cannot be sent.
What are the consequences of having a switching loop?
Broadcast storms, duplicate unicast frames arriving, discrepancies in switch
MAC address tables.
How can a physical loop occur?
It may be created on purpose to give redundancy, or it may be created by
accident if there is a confusion with cables.
How does Spanning Tree Protocol manage physical loops to allow
redundancy without allowing frames to travel in loops?
It shuts down certain ports so that no loops are active. If a link fails and the
blocked port is needed, then Spanning Tree Protocol will recalculate and
unblock ports as required.
SW
3/9/2016
106756911
1
What is the name of the type of frame that STP sends in order to carry out its
function?
Bridge Protocol Data Unit (BPDU)
What is the Root Bridge in a switched network?
The switch that is used as a starting point for the STP calculations. It is
chosen by an election.
What is a BID and what does it consist of?
A bridge identifier. It identifies a switch for STP purposes. It consists of a
priority number, the switch’s base MAC address, and it may also include an
extended system ID.
How is the BID used in the choice of a root bridge?
The switch with the lowest BID becomes the root bridge. The switches
exchange BID information in their BPDUs.
What is a root port?
Every switch except the root bridge has one root port. It is the port closest to
the root bridge – the one with the lowest cost route to the root bridge. Root
ports are allowed to forward frames.
What is a designated port?
A port that is allowed to forward frames, but is not a root port.
What happens to a non-designated port?
It is closed down by STP. It enters the blocking state.
How often, by default, does a switch send out BPDUs?
Every 2 seconds.
When a switch first starts up, which BID does it put in its BPDU as the root
ID?
Its own BID. It is saying that it is the root bridge.
What would make a switch change the root ID that it puts in its BPDUs?
If it receives a BPDU that has a lower value as the root ID then it uses this
lower value. It is saying that it is not the root bridge, another switch is the root
bridge.
SW
3/9/2016
106756911
2
If there are several switches between a certain port and the root bridge, how
is the cost to the root bridge worked out?
It is the sum of the costs of all the links on the route. The cost for a link
depends on the bandwidth of the link.
What is the revised IEEE value for the cost of a Fast Ethernet link?
19
How can you configure a switch so that it wins the election for root bridge?
Change its priority so that it has the lowest priority value of any switch on the
network.
Why was the extended system ID added to the BID?
It allows STP to support VLANs by running a separate process for each
VLAN. The extended system ID contains the VLAN number.
How is the root bridge chosen if all the switches have the default priority?
The switch with the lowest base MAC address is chosen.
If the default priority of a switch is 32768, why may the priority be shown as
32769?
The VLAN number is added on.
What does the command spanning-tree vlan 1 root primary do?
It sets the switch priority to 24576, or to 4096 less than the lowest priority
detected on the network – whichever of these is lower. The effect is to make
this the switch with the lowest priority so that it becomes root bridge.
Why do priority values go up in increments of 4096?
Originally they went up in increments of 1 from 1 to 65536, using 16 bits.
When the extended system ID was introduced, it took 12 of the original 16
bits. Only 4 bits remained. In order to keep the same maximum value, the
priority needs to go up in steps of 4096 (212) You could try it out and see.
Why might you use the command spanning-tree vlan 1 root secondary?
It sets the switch priority to 24576. Assuming that you have used spanningtree vlan 1 root primary on your chosen root bridge, and that all the other
switches are left with the default priority, this switch will win the election if the
root bridge fails. It therefore lets you choose a backup root bridge.
SW
3/9/2016
106756911
3
What is a disabled port?
A port that has been administratively shut down. It does not participate in
STP.
If a switch has two ports with the same cost route to the root bridge, which of
these ports will become the root port?
Each port has a port priority that can be configured, and the one with the
lower priority becomes root port. If both have the same priority then the one
with the lower port ID becomes root port. (E.g. Fa0/1 would win over Fa0/2.)
If port Fa0/6 has the default port priority, how would its priority be written?
128.6
Why do all active ports on the root bridge become designated ports?
Each segment has a designated port, and this is the port closer to the root
bridge. If one of the ports on a segment is on the root bridge then it has to be
the closer port, so it is designated.
What are the five port states in the original STP?
Blocking, listening, learning, forwarding, disabled.
Which of these states are temporary and are not seen once the network has
converged?
Listening and learning.
What is a port doing while it is in the Learning state?
Learning MAC addresses and building up its switching table.
What are the purposes of the Hello, Forward delay and Maximum age timers?
The Hello timer, default 2 seconds, controls the interval at which BPDUs are
sent out. The Forward delay timer, default 15 seconds, controls the time the
switch spends in the listening and learning states. The Maximum age timer,
default 20 seconds, controls the time BPDU information is kept. After 20
seconds with no BPDU received, the switch will assume that the link is down
and STP calculations will have to be run again.
Can the timers be changed?
They can, but it is not recommended. They are optimized for a network
diameter of 7. If it is really necessary then the network diameter can be
changed and the timers will automatically change with it, Even this is not
recommended in normal conditions.
SW
3/9/2016
106756911
4
What is PortFast?
It is a Cisco proprietary method of speeding up STP convergence. Access
ports that are connected to end devices, such as workstations, can never be
involved in loops. It makes sense to bring them to the forwarding state
immediately without spending time in listening and learning. A port configured
with PortFast will do this. It is important not to configure a port as PortFast if it
is connected to another switch,
What are the three steps in STP convergence?
1. Elect a root bridge
2. Elect root ports
3. Elect designated and non-designated ports
Why does STP use timers to keep switches in the blocking, listening and
learning states for given lengths of time?
It allows the network time to converge before user data frames are forwarded.
This is based on a network diameter of 7 switches.
Once a network has converged, BPDUs are normally sent outwards from the
root bridge but not back towards it. How do switches inform the root bridge if a
link goes down?
They send a topology change notification (TCN) frame, which is a special
BPDU.
Why did Cisco introduce PVST and PVST+ as variations of STP?
They provide support for VLANs, so that each VLAN can have its own
instance of STP. Ports may be blocking for some VLANs but not for others.
PVST used only ISL for trunking, but PVST+ supports both ISL and
IEEE802.1Q.
Why was RSTP introduced?
Rapid STP was introduced to avoid having to wait up to 50 seconds for the
network to converge.
What three port types are defined by RSTP?
Discarding, Learning and Forwarding.
What happens if you have some switches running STP and some running
RSTP?
They are compatible and will work correctly together.
SW
3/9/2016
106756911
5
How does the BPDU for RSTP compare with the BPDU for STP?
They have the same format – same fields of the same length. RSTP has the
version field set to 2.
A switch running STP waits for 20 seconds (10 missed BPDUs) before
assuming that the link is down. How long does a switch running RSTP wait?
3 missed BPDUs or 6 seconds by default.
What is an edge port in RSTP?
A port that is not intended to be connected to another switching device. It will
only be connected to an end device such as a workstation.
What is the advantage of having a port configured as an edge port?
It will change to the forwarding state at once when it is enabled, and will not
spend time in intermediate states.
Which Cisco proprietary enhancement to STP is similar to the edge port in
RSTP?
PortFast
What happens if an RSTP edge port receives a BPDU? Is its behaviour the
same as that of a PortFast port used with STP?
If an RSTP edge port receives a BPDU, this means that it has been
connected to a switch. It immediately stops being an edge port. It reverts to
normal and takes part in spanning tree activities. A PortFast port does not do
this by default, though there is a feature that can allow it to do so.
On a Cisco switch, what is the command that configures a port as an RSTP
edge port, and why was this command chosen?
spanning-tree portfast
This command was chosen to make the transition from STP to RSTP easier
because the command is unchanged from the old PortFast configuring
command.
How can non-edge ports be classified?
It depends on the type of link. They can be point-to-point or shared. The type
can be found automatically or it can be configured.
Which type of port makes most use of this classification?
Designated ports.
SW
3/9/2016
106756911
6
RSTP has a discarding port state that is not found in STP. Which STP port
states correspond to the discarding port state?
Blocking, listening and disabled.
In STP, root ports and designated ports are able to forward frames. RSTP has
these port roles, but it introduces roles for ports that are not forwarding. What
are these roles and what is their purpose?
Alternate ports are discarding (closed down) but can take over from
designated ports quickly when necessary. Backup ports are discarding, but
can take over from root ports quickly when necessary.
How can RSTP converge more quickly than STP, and without the use of
timers?
It works on one link at a time, closing it down briefly and determining the port
roles, using a proposal and agreement process. Then it moves on to the next
link.
What is Rapid PVST+ ?
It is Cisco’s implementation of RSTP. It supports the use of VLANs.
Do all the switches in a network have to be running Rapid PVST+ ?
Not necessarily. At least one switch on each loop in a VLAN must be running
it, otherwise there will be a broadcast storm.
If a switch is running PVST+ by default, how can it be configured to run Rapid
PVST+ ?
S1(config)#spanning-tree mode rapid-pvst
What is the best choice of a root bridge?
A powerful switch in the middle of the network with a direct connection to the
servers and routers. This keeps the average distance traveled by frames as
small as possible.
What is the advantage of using some layer 3 switches in a network?
They break up broadcast domains (as routers do), so there can be redundant
links without forming switching loops that cause broadcast storms. They keep
the advantage of forwarding very quickly (unlike routers).
SW
3/9/2016
106756911
7
You have designed a network without switching loops. Should you disable
STP?
No. There is too much risk that a loop could be created by accident and cause
a broadcast storm. It is better to leave STP running as it does not make too
much demand on processing or bandwidth.
Why is it essential for the network administrator to know the topology of the
network, including all redundant links, which switch is the root bridge, and
which ports are closed down by STP?
If there is a problem, this information is required for troubleshooting.
SW
3/9/2016
106756911
8