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Question 2: 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.
Correct Answer: A
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
Correct Answer: D
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
Correct Answer: C,D
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.
Correct Answer: A,C
Reference:
https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&ved=0CCwQFjAA&url=htt
p%3A%2F%2Fstaffweb.itsligo.ie%2Fstaff%2Fpflynn%2FTelecoms%25203%2FSlides%2FONT%2
520Mod%25204%2520Lesson%25207.ppt&ei=LoDIUfTTGtO3hAeQz4HQCA&usg=AFQjCNGY24
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.
Correct Answer: B,D,F
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
Correct Answer: A,B,C
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.
Correct Answer: C
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)
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