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CCIE-ROUTING AND SWITCHING QUALIFICATION (Written exam) : 350-001 Exam

Exam Number/Code: 350-001
Exam Name: CCIE-ROUTING AND SWITCHING QUALIFICATION (Written exam)
VUE Code: 350-001
Questions Type: Single choice,
Real Exam Question Numbers: 100 questions
Exam Language(s): English

“CCIE-ROUTING AND SWITCHING QUALIFICATION (Written exam)”, also known as 350-001 exam, is a Cisco certification.
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With the complete collection of questions and answers Q&as with Expert Explanations, Pass4sure has assembled to take you through 408 Q&As to your 350-001 Exam preparation. In the 350-001 exam resources, you will cover every field and category in CCIE helping to ready you for your successful Cisco Certification.

QUESTION 46
The following was executed on switch PassGuide C:
If all switches in the PassGuide network run the same type of spanning tree, what is
the total number of spanning tree topology changes that occurred in this network?
A. 7
B. 35
C. Can not be determined. Only the root bridge tracks the complete amount of topology
changes
D. 0
E. 2
Answer: A
Explanation:
The role of the TC mechanism is to correct L2 forwarding tables after the forwarding
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topology has changed. This is necessary to avoid a connectivity outage because, after a
TC, some MAC addresses previously accessible through particular ports might become
accessible through different ports. TC shortens the forwarding table aging time on all
switches in the VLAN where the TC occurs; thus, if the address is not relearned, it will
age-out and flooding will occur to ensure packets reach the destination MAC address.
TC is triggered by the change of a port’s STP state to or from the STP forwarding state.
Note:With Rapid STP or Multiple STP (IEEE 802.1w and IEEE 802.1s), TC is triggered
by a change of the port’s state to forwarding, as well as the role change from designated
to root. With Rapid STP, the L2 forwarding table is immediately flushed, as opposed to
802.1d, which shortens the aging time. The immediate flushing of the forwarding table
restores connectivity faster, but will cause more flooding.
In the output shown above, the total number of topology changes for VLAN 2 is shown
to be 7, with the last change occurring 3 weeks and 5 days ago.
QUESTION 47
The PassGuide switched LAN is shown below:
Due to hardware failure on the link between switches PassGuide A and PassGuide B,
Spanning Tree BPDUs from switch PassGuide A are no longer received by switch
PassGuide B, but the link remains up (see the drawing) Provided LoopGuard feature
is configured on all ports, which port will be put into ‘Loop-inconsistent’ state?
A. Port on switch PassGuide C connecting to switch PassGuide B
B. Port on switch PassGuide B connecting to switch PassGuide C
C. LoopGuard would not detect any issue in this scenario
D. Port on switch PassGuide A connecting to switch PassGuide B and port on switch
PassGuide B connecting to switch PassGuide A
E. Port on switch PassGuide B connecting to switch PassGuide C
Answer: C
Understanding How Loop Guard Works:
Loop guard helps prevent bridging loops that could occur because of a uni-directional
link failure on a point-to-point link. When enabled globally, the loop guard applies to all
point-to-point ports on the system. Loop guard detects root ports and blocked ports and
ensures that they keep receiving BPDUs from their designated port on the segment. If a
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loop guard enabled root or blocked port stop a receiving BPDUs from its designated port,
it transitions to the loop-inconsistent blocking state, assuming there is a physical link
error on this port. The port recovers from this loop-inconsistent state as soon as it
receives a BPDU.
You can enable loop guard on a per-port basis. 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.
Example:
The figure above 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.
Enabling loop guard on a root switch has no effect but provides protection when a root
switch becomes a nonroot switch.
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.
Note
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/products/hw/switches/ps708/products_configuration_guide_chapter09186a008016

0
QUESTION 48
On a PassGuide bridge running the rapid spanning tree protocol, which port will
send a BPDU with the proposal flag?
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A. Designated port in forwarding state
B. Designated port in non-forwarding state or the Root port in forwarding state
C. Root port in blocking state
D. Alternate port
E. None of the above
Answer: B
Explanation:
Proposal/Agreement Sequence:
When a port has been selected by the STA to become a designated port, 802.1d still waits
twice seconds (2×15 by default) before transitioning it to the forwarding
state. In RSTP, this condition corresponds to a port with a designated role but a blocking
state. The diagrams below illustrate how fast transition is achieved step-by-step. Suppose
a new link is created between the root and Switch
A. Both ports on this link are put in a
designated blocking state until they receive a BPDU from their counterpart.
When a designated port is in a discarding or learning state (and only in this case), it sets
the proposal bit on the BPDUs it sends out. This is what happens for port p0 of the root
bridge, as shown in Step 1 of the diagram above. Because Switch A receives superior
information, it immediately knows that p1 is going to be its new root port. Switch A then
starts a sync to ensure that all of its ports are in-sync with this new information. A port is
in-sync if it meets either of the following criteria:
1. The port is in blocking state (which means discarding, in a stable topology).
2. The port is an edge port.
In order to illustrate the effect of the sync mechanism on different kind of ports, suppose
there exists an alternate port p2, a designated forwarding port p3, and an edge port p4 on
Switch
A. Notice that p2 and p4 already meet one of the criteria listed above. In order to
be in sync (Step 2 of the diagram above), Switch A just needs to block port p3, assigning
it the discarding state. Now that all of its ports are in sync, Switch A can now unblock its
newly selected root port p1 and reply to the root by sending an agreement message (Step
3). This message is a copy of the proposal BPDU, with the agreement bit set instead of
the proposal bit. This ensures that port p0 knows exactly to which proposal the agreement
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it receives corresponds to.
Once p0 receives that agreement, it can immediately transition to forwarding. This is
Step 4 of the figure above. Notice that port p3 was left in a designated discarding state
after the sync. In Step 4, that port is in the exact same situation as was port p0 during
Step 1. It then starts proposing to its neighbor, attempting to quickly transition to
forwarding.
1. The proposal agreement mechanism is very fast, as it does not rely on any timers. This
wave of handshakes propagates quickly towards the edge of the network, and quickly
restores connectivity after a change in the topology.
2. If a designated discarding port does not receive an agreement after having sent a
proposal, it slowly transitions to the forwarding state, falling back to the traditional
802.1d listening-learning sequence. This could happen for instance if the remote bridge
doesn’t understand RSTP BPDUs, or if the remote bridge’s port is blocking.
3. Cisco introduced an enhancement to the sync mechanism that allows a bridge to put
only its former root port in the discarding state when syncing. Detailing the way this
mechanism works is beyond the scope of this document. However, one can safely assume
that it will be invoked in most common reconvergence cases. The scenario described in
the Convergence with 802.1w section of this document now becomes extremely efficient,
as only the ports on the path to the final blocked port are temporarily confused.
Reference: http://www.cisco.com/warp/public/473/146.html#agree
QUESTION 49
In RSTP (Rapid Spanning Tree Protocol) what is a port that provides an alternate
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path to the leaves of the Spanning Tree and what state is this port in when it is not
in the active topology?
A. Root port and listening
B. Alternate port and forwarding
C. Alternate port and learning
D. Designated port and learning
E. Backup port and discarding
F. None of the above
Answer: E
Explanation:
The RSTP provides rapid convergence of the spanning tree by assigning port roles and
by determining the active topology. The RSTP builds upon the IEEE 802.1D STP to
select the switch with the highest switch priority (lowest numerical priority value) as the
root switch. Then the RSTP assigns one of these port roles to individual ports:
1. Root port-provides the best path (lowest cost) when the switch forwards packets to the
root switch.
2.
Designated port-connects to the designated switch, which incurs the lowest path cost
when forwarding packets from that LAN to the root switch. The port through which the
designated switch is attached to the LAN is called the designated port.
3. Alternate port-offers an alternate path toward the root switch to that provided by the
current root port.
4. Backup port-acts as a backup for the path provided by a designated port toward the
leaves of the spanning tree. A backup port can exist only when two ports are connected
together in a loopback by a point-to-point link or when a switch has two or more
connections to a shared LAN segment.
5. Disabled port-has no role within the operation of the spanning tree.
A port with the root or a designated port role is included in the active topology. A port
with the alternate or backup port role is excluded from the active topology.
In a stable topology with consistent port roles throughout the network, the RSTP ensures
that every root port and designated port immediately transition to the forwarding state
while all alternate and backup ports are always in the discarding state (equivalent to
blocking in 802.1D).
Reference:

http://www.cisco.com/en/US/partner/products/hw/switches/ps628/products_configuration_guide_chapter09186

a
QUESTION 50
Exhibit:
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In the diagram shown above, Switch PassGuide 1 is the Root of Spanning Tree. If
there is a Unidirectional link failure between switches PassGuide 1 and PassGuide 3,
and Switch PassGuide 3 stops receiving BPDUs from Switch PassGuide 1, it will
transition its blocked port to the forwarding state and we can have a Spanning Tree
loop.
What feature can we use to prevent this from happening? (Select all that apply)
A. Portfast
B. Portfast BPDU filter
C. Portfast BPDU guard
D. UDLD
E. Loopguard
F. None of the above
Answer: D, E