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QUESTION NO:2

A branch router is configured with an egress QoS policy that was designed for a total number of

10 concurrent VOIP calls.

Due to expansion, 15 VOIP calls are now running over the link, but after the 14th call was

established, all calls were affected and the voice quality was dramatically degraded.

Assuming that there is enough bandwidth on the link for all of this traffic, which part of the QoS

configuration should be updated due to the new traffic profile?

A. Increase the shaping rate for the priority queue. B.

Remove the policer applied on the priority queue. C.

Remove the shaper applied on the priority queue. D.

Increase the policing rate for the priority queue.

Answer: D

Explanation:


QUESTION NO:6

Why would a rogue host that is running a DHCP server on a campus LAN network present a

security risk?

A. It may allocate IP addresses from an unknown subnet to the users.

B. All multicast traffic can be sniffed by using the DHCP multicast capabilities.

C. The CPU utilization of the first hop router can be overloaded by exploiting DHCP relay open

ports.

D. A potential man-in-the-middle attack can be used against the clients.

Answer: D

Explanation:


QUESTION NO:8

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

Explanation:

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:


QUESTION NO:11

When you are troubleshooting duplex mismatches, which two errors are typically seen on the full-

duplex end? (Choose two.)

A. runts

B. FCS errors

C. interface resets

D. late collisions

Answer: A,B

Explanation:


QUESTION NO:16

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

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)

2. A configuration revision number (two bytes)

3. 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.shtml

http://www.cisco.com/en/US/docs/ios-xml/ios/lanswitch/command/lsw-cr-book.pdf


400-101 Practice Test400-101 Study Guide400-101 Braindumps

QUESTION NO:18

Refer to the exhibit.

Which statement is correct about the prefix 160.0.0.0/8?

A. The prefix has encountered a routing loop.

B. The prefix is an aggregate with an as-set.

C. The prefix has been aggregated twice, once in AS 100 and once in AS 200.

D. None of these statements is true.

Answer: B

Explanation:


QUESTION NO:19

Which two options does Cisco PfR use to control the entrance link selection with inbound

optimization? (Choose two.)

A. Prepend extra AS hops to the BGP prefix.

B. Advertise more specific BGP prefixes (longer mask).

C. Add (prepend) one or more communities to the prefix that is advertised by BGP.

D. Have BGP dampen the prefix.

Answer: A,C

Explanation: PfR Entrance Link Selection Control Techniques

The PfR BGP inbound optimization feature introduced the ability to influence inbound traffic. A

network advertises reachability of its inside prefixes to the Internet using eBGP advertisements to

its ISPs. If the same prefix is advertised to more than one ISP, then the network is multihoming.

PfR BGP inbound optimization works best with multihomed networks, but it can also be used with

a network that has multiple connections to the same ISP. To implement BGP inbound

optimization, PfR manipulates eBGP advertisements to influence the best entrance selection for

traffic bound for inside prefixes. The benefit of implementing the best entrance selection is limited

to a network that has more than one ISP connection.

To enforce an entrance link selection, PfR offers the following methods:

BGP Autonomous System Number Prepend When an entrance link goes out-of-policy (OOP) due

to delay, or in images prior to Cisco IOS Releases 15.2(1) T1 and 15.1(2)S, and PfR selects a

best entrance for an inside prefix, extra autonomous system hops are prepended one at a time (up

to a maximum of six) to the inside prefix BGP advertisement over the other entrances. In Cisco

IOS Releases 15.2(1)T1, 15.1(2)S, and later releases, when an entrance link goes out-of policy

(OOP) due to unreachable or loss reasons, and PfR selects a best entrance for an inside prefix,

six extra autonomous system hops are prepended immediately to the inside prefix BGP

advertisement over the other entrances. The extra autonomous system hops on the other

entrances increase the probability that the best entrance will be used for the inside prefix. When

the entrance link is OOP due to unreachable or loss reasons, six extra autonomous system hops

are added immediately to allow the software to quickly move the traffic away from the old entrance

link. This is the default method PfR uses to control an inside prefix, and no user configuration is

required.

BGP Autonomous System Number Community Prepend

When an entrance link goes out-of-policy (OOP) due to delay, or in images prior to Cisco IOS

Releases 15.2

(1)T1 and 15.1(2)S, and PfR selects a best entrance for an inside prefix, a BGP prepend

community is attached one at a time (up to a maximum of six) to the inside prefix BGP

advertisement from the network to another autonomous system such as an ISP. In Cisco IOS

Releases 15.2(1)T1, 15.1(2)S, and later releases, when an entrance link goes out-of-policy (OOP)

due to unreachable or loss reasons, and PfR selects a best entrance for an inside prefix, six BGP

prepend communities are attached to the inside prefix BGP advertisement. The BGP prepend

community will increase the number of autonomous system hops in the advertisement of the

inside prefix from the ISP to its peers. Autonomous system prepend BGP community is the

preferred method to be used for PfR BGP inbound optimization because there is no risk of the

local ISP filtering the extra autonomous system hops. There are some issues, for example, not all

ISPs support the BGP prepend community, ISP policies may ignore or modify the autonomous

system hops, and a transit ISP may filter the autonomous system path. If you use this method of

inbound optimization and a change is made to an autonomous system, you must issue an

outbound reconfiguration using the “clear ip bgp” command.

Reference

http://www.cisco.com/en/US/docs/ios-xml/ios/pfr/configuration/15-2s/pfr-bgp-inbound.html#GUID-

F8A59E241D59-

4924-827D-B23B43D9A8E0

http://www.cisco.com/en/US/products/ps8787/products_ios_protocol_option_home.html


QUESTION NO:21

Refer to the exhibit.

A packet from RTD with destination RTG, is reaching RTB. What is the path this packet will take

from RTB to reach RTG?

A. RTB – RTA – RTG

B. RTB – RTD – RTC – RTA – RTG

C. RTB – RTF – RTE – RTA – RTG

D. RTB will not be able to reach RTG since the OSPF configuration is wrong.

Answer: C

Explanation:


QUESTION NO:24

Refer to the exhibit.

R1 is not learning about the 172.16.10.0 subnet from the BGP neighbor R2 (209.165.202.130).

What can be done so that R1 will learn about this network?

A. Disable auto-summary on R2.

B. Configure an explicit network command for the 172.16.10.0 subnet on R2.

C. Subnet information cannot be passed between IBGP peers.

D. Disable auto-summary on R1.

Answer: B

Explanation:

By default, BGP does not accept subnets redistributed from IGP. To advertise and carry subnet

routes in BGP, use an explicit network command or the no auto-summary command. If you disable

auto-summarization and have not entered a network command, you will not advertise network

routes for networks with subnet routes unless they contain a summary route.

Reference

http://www.cisco.com/en/US/docs/ios/11_3/np1/command/reference/1rbgp.html


QUESTION NO:27

Refer to the exhibit.

What triggered the first SPF recalculation?

A. changes in a router LSA, subnet LSA, and external LSA

B. changes in a router LSA, summary network LSA, and external LSA

C. changes in a router LSA, summary network LSA, and summary ASBR LSA

D. changes in a router LSA, summary ASBR LSA, and external LSA

Answer: B

Explanation:

OSPFv2

Is built around links, and any IP prefix change in an area will trigger a full SPF. It advertises IP

information in Router and Network LSAs. The routers thus, advertise both the IP prefix information

(or the connected subnet information) and topology information in the same LSAs. This implies

that if an IP address attached to an interface changes, OSPF routers would have to originate a

Router LSA or a Network LSA, which btw also carries the topology information. This would trigger

a full SPF on all routers in that area, since the same LSAs are flooded to convey topological

change information. This can be an issue with an access router or the one sitting at the edge,

since many stub links can change regularly.

Only changes in interarea, external and NSSA routes result in partial SPF calculation (since type

3, 4, 5 and 7 LSAs only advertise IP prefix information) and thus IS-IS


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