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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:9

Which two are effects of connecting a network segment that is running 802.1D to a network

segment that is running 802.1w? (Choose two.)

A. The entire network switches to 802.1D and generates BPDUs to determine root bridge status. B.

A migration delay of three seconds occurs when the port that is connected to the 802.1D bridge

comes up.

C. The entire network reconverges and a unique root bridge for the 802.1D segment, and a root

bridge for the 802.1w segment, is chosen.

D. The first hop 802.1w switch that is connected to the 802.1D runs entirely in 802.1D compatibility

mode and converts the BPDUs to either 802.1D or 802.1w BPDUs to the 802.1D or 802.1w

segments of the network.

E. Classic 802.1D timers, such as forward delay and max-age, will only be used as a backup, and

will not be necessary if point-to-point links and edge ports are properly identified and set by the

administrator.

Answer: B,E

Explanation:

Each port maintains a variable that defines the protocol to run on the corresponding segment. A

migration delay timer of three seconds also starts when the port comes up. When this timer runs,

the current STP or RSTP mode associated to the port is locked. As soon as the migration delay

expires, the port adapts to the mode that corresponds to the next BPDU it receives. If the port

changes its mode of operation as a result of a BPDU received, the migration delay restarts.

802.1D works by the concept that the protocol had to wait for the network to converge before it

transitioned a port into the forwarding state. With Rapid Spanning Tree it does not have to rely on

any timers, the only variables that that it relies on is edge ports and link types.

Any uplink port that has an alternate port to the root can be directly placed into the forwarding

state (This is the Rapid convergence that you speak of “restored quickly when RSTP is already in

use?”). This is what happened when you disconnected the primary look; the port that was ALT,

moved to FWD immediately, but the switch also still needs to create a BDU with the TC bit set to

notify the rest of the network that a topology has occurred and all non-edge designated ports will

transition to BLK, LRN, and then FWD to ensure there are no loops in the rest of the network. This

is why if you have a host on a switchport, and you know for a fact that it is only one host, enable

portfast to configure the port as an edgeport so that it does not have to transition to all the STP

states.

Reference

http://www.cisco.com/en/US/tech/tk389/tk621/technologies_white_paper09186a0080094cfa.shtml


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:13

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

Explanation:


400-101 Study Guide400-101 Exam Questions400-101 Braindumps

QUESTION NO:17

Which three combinations are valid LACP configurations that will set up a channel? (Choose

three.)

A. On/On

B. On/Auto

C. Passive/Active

D. Desirable/Auto

E. Active/Active

F. Desirable/Desirable

Answer: A,C,E

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:22

Refer to the exhibit.

Which path is selected as best path?

A. path 1, because it is learned from IGP B.

path 1, because the metric is the lowest C.

path 2, because it is external

D. path 2, because it has the higher router ID

Answer: B

Explanation:


QUESTION NO:26

Refer to the exhibit.

Why is AS 65333 in parentheses?

A. It is an external AS.

B. It is a confederation AS.

C. It is the AS of a route reflector.

D. It is our own AS.

E. A route map has been applied to this route.

F. The BGP next hop is unreachable.

Answer: B

Explanation:


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