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NO.1 Which statement is true about loop guard?
A. Loop guard only operates
on interfaces that are considered point-to-point by the spanning tree.
B.
Loop guard only operates on root ports.
C. Loop guard only operates on
designated ports.
D. Loop guard only operates on edge ports.
Answer:
A
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Explanation:
Understanding How Loop Guard
Works
Unidirectional link failures may cause a root port or alternate port to
become designated as root if
BPDUs are absent. Some software failures may
introduce temporary loops in the network. Loop
guard checks if a root port or
an alternate root port receives BPDUs. If the port is receiving
BPDUs, loop
guard puts the port into an inconsistent state until it starts receiving BPDUs
again.
Loop guard isolates the failure and lets spanning tree converge to a
stable topology without the
failed link or bridge.
You can enable loop
guard per port with the set spantree guard loop command.
Note When you are in
MST mode, you can set all the ports on a switch with the set
spantree
global-defaults loop-guard command.
When you enable loop guard,
it is automatically applied to all of the active instances or VLANs to
which
that port belongs. When you disable loop guard, it is disabled for the specified
ports.
Disabling loop guard moves all loop-inconsistent ports to the
listening state.
If you enable loop guard on a channel and the first link
becomes unidirectional, loop guard blocks
the entire channel until the
affected port is removed from the channel. Figure 8-6 shows loop
guard in a
triangle switch configuration.
Figure 8-6 Triangle Switch Configuration with
Loop Guard
Figure 8-6 illustrates the following configuration:
Switches A
and B are distribution switches.
Switch C is an access switch.
Loop guard
is enabled on ports 3 /1 and 3 / 2 on Switches A, B, and C.
Use loop guard
only in topologies where there are blocked ports. Topologies that have no
blocked
ports, which are loop free, do not need to enable this feature.
Enabling loop guard on a root switch
has no effect but provides protection
when a root switch becomes a nonroot switch.
Follow these guidelines when
using loop guard:
Do not enable loop guard on PortFast-enabled or dynamic
VLAN ports.
Do not enable PortFast on loop guard-enabled ports.
Do not
enable loop guard if root guard is enabled.
Do not enable loop guard on ports
that are connected to a shared link.
Note: We recommend that you enable loop
guard on root ports and alternate root ports on access
switches.
Loop
guard interacts with other features as follows:
Loop guard does not affect
the functionality of UplinkFast or BackboneFast.
Root guard forces a port to
always be designated as the root port. Loop guard is effective only if
the
port is a root port or an alternate port. Do not enable loop guard and root
guard on a port at
the
same time.
PortFast transitions a port into a
forwarding state immediately when a link is established. Because
a
PortFast-enabled port will not be a root port or alternate port, loop guard and
PortFast cannot be
configured on the same port. Assigning dynamic VLAN
membership for the port requires that the
port is PortFast enabled. Do not
configure a loop guard-enabled port with dynamic VLAN
membership. If your
network has a type-inconsistent port or a PVID-inconsistent port, all BPDUs
are
dropped until the misconfiguration is corrected. The port transitions out
of the inconsistent state
after the message age expires. Loop guard ignores
the message age expiration on type-inconsistent
ports and PVID-inconsistent
ports. If the port is already blocked by loop guard, misconfigured BPDUs
that
are received on the port make loop guard recover, but the port is moved into
the
type-inconsistent state or PVID-inconsistent state.
In
high-availability switch configurations, if a port is put into the blocked state
by loop guard, it
remains blocked even after a switchover to the redundant
supervisor engine. The newly activated
supervisor engine recovers the port
only after receiving a BPDU on that port. Loop guard uses the
ports known to
spanning tree. Loop guard can take advantage of logical ports provided by the
Port
Aggregation Protocol (PAgP). However, to form a channel, all the
physical ports grouped in the
channel must have compatible configurations.
PAgP enforces uniform configurations of root guard
or loop guard on all the
physical ports to form a channel. These caveats apply to loop
guard:
-Spanning tree always chooses the first operational port in the
channel to send the BPDUs. If that
link becomes unidirectional, loop guard
blocks the channel, even if other links in the channel are
functioning
properly. -If a set of ports that are already blocked by loop guard are grouped
together
to form a channel, spanning tree loses all the state information for
those ports and the new channel
port may obtain the forwarding state with a
designated role. -If a channel is blocked by loop guard
and the channel
breaks, spanning tree loses all the state information. The individual physical
ports
may obtain the forwarding state with the designated role, even if one
or more of the links that
formed the channel are unidirectional. You can
enable UniDirectional Link Detection (UDLD) to help
isolate the link failure.
A loop may occur until UDLD detects the failure, but loop guard will not
be
able to detect it. Loop guard has no effect on a disabled spanning tree
instance or a
VLAN.
Reference:
http://www.cisco.com/en/US/docs/switches/lan/catalyst4000/
8.2glx/configuration/guide/stp_enha.
html#wp1048163
2. Which command is
used to enable EtherChannel hashing for Layer 3 IP and Layer 4
port-based
CEF?
A. mpls ip cef
B. port-channel ip cef
C. mpls ip
port-channel cef
D. port-channel load balance
E. mpls ip
load-balance
F. ip cef EtherChannel channel-id XOR L4
G. ip cef connection
exchange
Answer: D
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Explanation:
Port-channel load
balance is normally used for enable etherchannel hashing for Layer 3 IP and
Layer
4 port based CEF.
3. Which two options are contained in a VTP subset
advertisement? (Choose two.)
A. followers field
B. MD5 digest
C. VLAN
information
D. sequence number
Answer: C,D
Cisco Actual
Test 350-001 study guide 350-001 Exam
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Explanation:
Subset Advertisements When you add, delete, or
change a VLAN in a Catalyst, the server Catalyst
where the changes are made
increments the configuration revision and issues a summary
advertisement. One
or several subset advertisements follow the summary advertisement. A
subset
advertisement contains a list of VLAN information.
If there are
several VLANs, more than one subset advertisement can be required in order
to
advertise all the VLANs.
Subset Advertisement Packet Format This
formatted example shows that each VLAN information
field contains information
for a different VLAN. It is ordered so that lowered-valued ISL VLAN IDs
occur
first:
Most of the fields in this packet are easy to understand. These are
two clarifications:
Code - The format for this is 0x02 for subset
advertisement.
Sequence number - This is the sequence of the packet in the
stream of packets that follow a
summary advertisement. The sequence starts
with 1.
Advertisement Requests
A switch needs a VTP advertisement request
in these situations:
The switch has been reset.
The VTP domain name has
been changed.
The switch has received a VTP summary advertisement with a
higher configuration revision than
its own.
Upon receipt of an
advertisement request, a VTP device sends a summary advertisement. One
or
more subset advertisements follow the summary advertisement. This is an
example:
Code-The format for this is 0x03 for an advertisement
request.
Start-Value-This is used in cases in which there are several subset
advertisements. If the first (n)
subset advertisement has been received and
the subsequent one (n+1) has not been received,
the Catalyst only requests
advertisements from the (n+1)th
one.
Reference
http://www.cisco.com/en/US/tech/tk389/tk689/technologies_tech_note09186a0080094c52.shtml
4.
Which two statements are true about traffic shaping? (Choose two.)
A.
Out-of-profile packets are queued.
B. It causes TCP retransmits.
C.
Marking/remarking is not supported.
D. It does not respond to BECN and
ForeSight Messages.
E. It uses a single/two-bucket mechanism for
metering.
Answer:
A,C
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https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&ved=0CCwQFjAA&url=htt
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UkAfy8tKIHlzEm9gfoIjv6fg&sig2=t4UIzkZ12wnO2988dEDyug&bvm=bv.48293060,d.ZG4
(slide 6)
5. Which three options are features of VTP version 3? (Choose
three.)
A. VTPv3 supports 8K VLANs.
B. VTPv3 supports private VLAN
mapping.
C. VTPv3 allows for domain discovery.
D. VTPv3 uses a primary
server concept to avoid configuration revision issues.
E. VTPv3 is not
compatible with VTPv1 or VTPv2.
F. VTPv3 has a hidden password
option.
Answer: B,D,F
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Explanation:
Key
Benefits of VTP Version 3 Much work has gone into improving the usability of VTP
version 3 in
three major areas: The new version of VTP offers better
administrative control over which device is
allowed to update other devices'
view of the VLAN topology. The chance of unintended and
disruptive changes is
significantly reduced, and availability is increased. The reduced risk
of
unintended changes will ease the change process and help speed deployment.
Functionality for the
VLAN environment has been significantly expanded. Two
enhancements are most beneficial for
today's networks:
-
In addition to
supporting the earlier ISL VLAN range from 1 to 1001, the new version
supports
the whole IEEE 802.1Q VLAN range up to 4095.
-
In addition to
supporting the concept of normal VLANs, VTP version 3 can transfer
information
regarding Private VLAN (PVLAN) structures.
The third area of
major improvement is support for databases other than VLAN (for
example,
MST).
Brief Background on VTP Version 1 and VTP Version 2 VTP
version 1 was developed when only 1k
VLANs where available for configuration.
A tight internal coupling of the VLAN implementation, the
VLAN pruning
feature, and the VTP function itself offered an efficient means of
implementation. It
has proved in the field to reliably support Ethernet,
Token Ring, and FDDI networks via VTP . The use
of consistent VLAN naming was
a requirement for successful use of VMPS (Vlan Membership Policy
Server). VTP
ensures the consistency of VLAN names across the VTP domain. Most
VMPS
implementations are likely to be migrated to a newer, more flexible and
feature-rich method. To add
support for Token Ring, VTP version 1 was
enhanced and called VTP version 2. Certain other minor
changes and
enhancements were also added at this time. The functional base in VTP version 3
is left
unchanged from VTP version 2, so backward compatibility is built in.
It is possible, on a per link basis,
to automatically discover and support
VTP version 2 devices.
VTP version 3 adds a number of enhancements to VTP
version 1 and VTP version 2: Support for a
structured and secure VLAN
environment (Private VLAN, or PVLAN) Support for up to 4k VLANs
Feature
enhancement beyond support for a single database or VTP instance Protection
from
unintended database overrides during insertion of new switches Option of
clear text or hidden
password protection Configuration option on a per port
base instead of only a global scheme
Optimized resource handling and more
efficient transfer of information These new requirements
made a new code
foundation necessary. The design goal was to make VTP version 3 a
versatile
vehicle. This was not only for the task of transferring a VLAN DB
but also for transferring other
databases-for example, the MST
database.
Reference
http://www.cisco.com/en/US/prod/collateral/switches/ps5718
/ps708 /solution_guide_c78_508010.
ht ml
6. Which three options are
considered in the spanning-tree decision process? (Choose three.)
A. lowest
root bridge ID
B. lowest path cost to root bridge
C. lowest sender bridge
ID
D. highest port ID
E. highest root bridge ID
F. highest path cost to
root bridge
Answer: A,B,C
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Explanation:
Configuration
bridge protocol data units (BPDUs) are sent between switches for each port.
Switches
use s four step process to save a copy of the best BPDU seen on
every port. When a port receives a
better BPDU, it stops sending them. If the
BPDUs stop arriving for 20 seconds (default), it begins
sending them
again.
Step 1 Lowest Root Bridge ID (BID) Step 2 Lowest Path Cost to Root
Bridge Step 3 Lowest Sender BID
Step 4 Lowest Port ID
Reference Cisco
General Networking Theory Quick Reference Sheets
7. In 802.1s, how is the
VLAN to instance mapping represented in the BPDU?
A. The VLAN to instance
mapping is a normal 16-byte field in the MST BPDU.
B. The VLAN to instance
mapping is a normal 12-byte field in the MST BPDU.
C. The VLAN to instance
mapping is a 16-byte MD5 signature field in the MST BPDU.
D. The VLAN to
instance mapping is a 12-byte MD5 signature field in the MST BPDU.
Answer:
C
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Answers
Explanation:
MST Configuration and MST Region Each switch
running MST in the network has a single MST
configuration that consists of
these three attributes:
1.An alphanumeric configuration name (32
bytes)
NO.2 A 4096-element table that associates each of the potential
4096 VLANs supported on the chassis
to a given instance.
In order to be
part of a common MST region, a group of switches must share the same
configuration
attributes. It is up to the network administrator to properly
propagate the configuration throughout
the region. Currently, this step is
only possible by the means of the command line interface (CLI) or
through
Simple Network Management Protocol (SNMP). Other methods can be envisioned, as
the
IEEE specification does not explicitly mention how to accomplish that
step. Note: If for any reason
two switches differ on one or more
configuration attribute, the switches are part of different
regions. For more
information refer to the Region Boundary section of this document.
Region
Boundary In order to ensure consistent VLAN-to-instance mapping, it is necessary
for the
protocol to be able to exactly identify the boundaries of the
regions. For that purpose, the
characteristics of the region are included in
the BPDUs. The exact VLANs-to-instance mapping is not
propagated in the BPDU,
because the switches only need to know whether they are in the same
region as
a neighbor. Therefore, only a digest of the VLANs-toinstance mapping table is
sent, along
with the revision number and the name. Once a switch receives a
BPDU, the switch extracts the
digest (a numerical value derived from the
VLAN-to-instance mapping table through a mathematical
function) and compares
this digest with its own computed digest. If the digests differ, the port
on
which the BPDU was received is at the boundary of a region. In generic
terms, a port is at the
boundary of a region if the designated bridge on its
segment is in a different region or if it receives
legacy 802.1d BPDUs. In
this diagram, the port on B1 is at the boundary of region A, whereas
the
ports on B2 and B3 are internal to region B:
MST
Instances
According to the IEEE 802.1s specification, an MST bridge must be
able to handle at least these
two instances:
One Internal Spanning Tree
(IST)
One or more Multiple Spanning Tree Instance(s) (MSTIs)
The
terminology continues to evolve, as 802.1s is actually in a pre-standard phase.
It is likely
these names will change in the final release of 802.1s. The
Cisco implementation supports 16
instances: one IST (instance 0) and 15
MSTIs.
show vtp status
Cisco switches "show vtp status" Field Descriptions
has a MD5 digest field that is a 16-byte
checksum of the
VTP configuration
as shown below
Router# show vtp status
VTP Version: 3
(capable)
Configuration Revision: 1
Maximum VLANs supported locally:
1005
Number of existing VLANs: 37
VTP Operating Mode: Server
VTP Domain
Name: [smartports]
VTP Pruning Mode: Disabled
VTP V2 Mode: Enabled VTP
Traps Generation: Disabled MD5 digest : 0x26 0xEE 0x0D 0x84 0x73
0x0E 0x1B
0x69 Configuration last modified by 172.20.52.19 at 7-25-08 14:33:43 Local
updater ID is
172.20.52.19 on interface Gi5 / 2 (first layer3 interface fou)
VTP version running:
2
Reference
http://www.cisco.com/en/US/tech/tk389/tk621
/technologies_white_paper09186a0080094cfc.shtm
l
http://www.cisco.com/en/US/docs/ios-xml/ios/lanswitch/command/lsw-cr-book.pdf
8.
Which statement is true about TCN propagation?
A. The originator of the TCN
immediately floods this information through the network.
B. The TCN
propagation is a two step process.
C. A TCN is generated and sent to the root
bridge.
D. The root bridge must flood this information throughout the
network.
Answer: C
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Explanation:
Explanation New Topology Change Mechanisms
When an 802.1D bridge detects a topology change,
it uses a reliable mechanism
to first notify the root bridge. This is shown in this diagram:
Once the root
bridge is aware of a change in the topology of the network, it sets the TC flag
on the
BPDUs it sends out, which are then relayed to all the bridges in the
network. When a bridge
receives a BPDU with the TC flag bit set, it reduces
its bridging-table aging time to forward delay
seconds. This ensures a
relatively quick flush of stale information. Refer to
Understanding
Spanning-Tree Protocol Topology Changes for more information on
this process. This topology
change mechanism is deeply remodeled in RSTP .
Both the detection of a topology change and its
propagation through the
network evolve.
Topology Change Detection
In RSTP , only non-edge ports
that move to the forwarding state cause a topology change. This
means that a
loss of connectivity is not considered as a topology change any more, contrary
to
802.1D (that is, a port that moves to blocking no longer generates a TC).
When a RSTP bridge
detects a topology change, these occur:
It starts the
TC While timer with a value equal to twice the hello-time for all its
non-edge
designated ports and its root port, if necessary.
It flushes the
MAC addresses associated with all these ports.
Note: As long as the TC While
timer runs on a port, the BPDUs sent out of that port have the TC
bit
set.
BPDUs are also sent on the root port while the timer is
active.
Topology Change Propagation
When a bridge receives a BPDU with the
TC bit set from a neighbor, these occur:
It clears the MAC addresses learned
on all its ports, except the one that receives the topology
change.
It
starts the TC While timer and sends BPDUs with TC set on all its designated
ports and root port
(RSTP no longer uses the specific TCN BPDU, unless a
legacy bridge needs to be notified).
This way, the TCN floods very quickly
across the whole network. The TC propagation is now a one
step process. In
fact, the initiator of the topology change floods this information throughout
the
network, as opposed to 802.1D where only the root did. This mechanism is
much faster than the
802.1D equivalent. There is no need to wait for the root
bridge to be notified and then maintain the
topology change state for the
whole network for <max age plus forward delay> seconds.
In just a few
seconds, or a small multiple of hello-times, most of the entries in the CAM
tables of the
entire network (VLAN) flush. This approach results in
potentially more temporary flooding, but on
the other hand it clears
potential stale information that prevents rapid connectivity
restitution.
Reference
http://www.cisco.com/en/US/tech/tk389/tk621
/technologies_white_paper09186a0080094cfa.shtm
l
9. While you are
troubleshooting network performance issues, you notice that a switch
is
periodically flooding all unicast traffic. Further investigation reveals
that periodically the switch is
also having spikes in CPU utilization,
causing the MAC address table to be flushed and relearned.
What is the most
likely cause of this issue?
A. a routing protocol that is flooding
updates
B. a flapping port that is generating BPDUs with the TCN bit
set
C. STP is not running on the switch
D. a user that is downloading the
output of the show-tech command
E. a corrupted switch CAM table
Answer:
B
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Explanation:
Spanning-Tree Protocol Topology Changes
Another common issue caused by flooding is
Spanning-Tree Protocol (STP)
Topology Change Notification (TCN). TCN is designed to correct
forwarding
tables after the forwarding topology has changed. This is necessary to avoid
a
connectivity outage, as after a topology change some destinations
previously accessible via
particular ports might become accessible via
different ports. TCN operates by shortening the
forwarding table aging time,
such that if the address is not relearned, it will age out and flooding
will
occur. TCNs are triggered by a port that is transitioning to or from the
forwarding state. After
the TCN, even if the particular destination MAC
address has aged out, flooding should not happen
for long in most cases since
the address will be relearned. The issue might arise when TCNs are
occurring
repeatedly with short intervals. The switches will constantly be fast-aging
their forwarding
tables so flooding will be nearly constant. Normally, a TCN
is rare in a well-configured network.
When the port on a switch goes up or
down, there is eventually a TCN once the STP state of the port
is changing to
or from forwarding. When the port is flapping, repetitive TCNs and flooding
occurs.
Ports with the STP portfast feature enabled will not cause TCNs when
going to or from the
forwarding state. Configuration of portfast on all
end-device ports (such as printers, PCs, servers,
and so on) should limit
TCNs to a low amount. Refer to this document for more information on
TCNs:
Understanding Spanning-Tree Protocol Topology Changes Note: In MSFC
IOS, there is an
optimization that will trigger VLAN interfaces to repopulate
their ARP tables when there is a TCN in
the respective VLAN. This limits
flooding in case of TCNs, as there will be an ARP broadcast and the
host MAC
address will be relearned as the hosts reply to ARP
.
Reference
http://www.cisco.com/en/US/products/hw/switches/ps700/products_tech_note09186a00801d080
8.
shtml
10. Your network is suffering from regular outages. After
troubleshooting, you learn that the
transmit lead of a fiber uplink was
damaged. Which two features can prevent the same issues in the
future?
(Choose two.)
A. root guard
B. loop guard
C. BPDU guard
D.
UDLD
E. BPDU skew detection
Answer: B,D
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Explanation:
STP Loop Guard The STP loop
guard feature provides additional protection against Layer 2
forwarding loops
(STP loops). An STP loop is created when an STP blocking port in a
redundant
topology erroneously transitions to the forwarding state. This
usually happens because one of the
ports of a physically redundant topology
(not necessarily the STP blocking port) no longer receives
STP BPDUs. In its
operation, STP relies on continuous reception or transmission of BPDUs based
on
the port role. The designated port transmits BPDUs, and the non-designated
port receives BPDUs.
When one of the ports in a physically redundant topology
no longer receives BPDUs, the STP
conceives that the topology is loop free.
Eventually, the blocking port from the alternate or backup
port becomes
designated and moves to a forwarding state. This situation creates a loop. The
loop
guard feature makes additional checks. If BPDUs are not received on a
non-designated port, and
loop guard is enabled, that port is moved into the
STP loop-inconsistent blocking state, instead of
the listening / learning /
forwarding state. Without the loop guard feature, the port assumes
the
designated port role. The port moves to the STP forwarding state and
creates a loop.
Loop Guard versus UDLD Loop guard and Unidirectional Link
Detection (UDLD) functionality overlap,
partly in the sense that both protect
against STP failures caused by unidirectional links. However,
these two
features differ in functionality and how they approach the problem. This table
describes
loop guard and UDLD functionality:
Based on the various design
considerations, you can choose either UDLD or the loop guard feature.
In
regards to STP , the most noticeable difference between the two features is the
absence of
protection in UDLD against STP failures caused by problems in
software. As a result, the designated
switch does not send BPDUs. However,
this type of failure is (by an order of magnitude) more rare
than failures
caused by unidirectional links. In return, UDLD might be more flexible in the
case of
unidirectional links on EtherChannel. In this case, UDLD disables
only failed links, and the channel
should remain functional with the links
that remain. In such a failure, the loop guard puts it into
loop-inconsistent
state in order to block the whole channel.
Additionally, loop guard does not
work on shared links or in situations where the link has been
unidirectional
since the link-up. In the last case, the port never receives BPDU and
becomes
designated. Because this behavior could be normal, this particular
case is not covered by loop guard.
UDLD provides protection against such a
scenario. As described, the highest level of protection is
provided when you
enable UDLD and loop
guard.
Reference
http://www.cisco.com/en/US/tech/tk389/tk621
/technologies_tech_note09186a0080094640.
shtml#loop_guard_vs_uld
NO.3
A configuration revision number (two bytes)
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