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QUESTION NO:4

Refer to the exhibit.

R1 has an EBGP session to ISP 1 and an EBGP session to ISP 2. R1 receives the same prefixes

through both links.

Which configuration should be applied so that the link between R1 and ISP 2 will be preferred for

outgoing traffic (R1 to ISP 2)?

A. Increase local preference on R1 for received routes

B. Decrease local preference on R1 for received routes

C. Increase MED on ISP 2 for received routes

D. Decrease MED on ISP 2 for received routes

Answer: A

Explanation: Explanation

Local preference is an indication to the AS about which path has preference to exit the AS in order

to reach a certain network. A path with higher local preference is preferred more. The default value

of preference is 100.

Reference

http://www.cisco.com/en/US/tech/tk872/technologies_configuration_example09186a0080b82d1f.s

html?

referring_site=smartnavRD


QUESTION NO:5

Refer to the exhibit.

A small enterprise connects its office to two ISPs, using separate T1 links. A static route is used

for the default route, pointing to both interfaces with a different administrative distance, so that one

of the default routes is preferred.

Recently the primary link has been upgraded to a new 10 Mb/s Ethernet link.

After a few weeks, they experienced a failure. The link did not pass traffic, but the primary static

route remained active. They lost their Internet connectivity, even though the backup link was

operating.

Which two possible solutions can be implemented to avoid this situation in the future? (Choose

two.)

A. Implement HSRP link tracking on the branch router R1.

B. Use a track object with an IP SLA probe for the static route on R1.

C. Track the link state of the Ethernet link using a track object on R1.

D. Use a routing protocol between R1 and the upstream ISP.

Answer: B,D

Explanation:

Interface Tracking

Interface tracking allows you to specify another interface on the router for the HSRP process to

monitor in order to alter the HSRP priority for a given group.

If the specified interface\’s line protocol goes down, the HSRP priority of this router is reduced,

allowing another HSRP router with higher priority can become active (if it has preemption

enabled).

To configure HSRP interface tracking, use the standby [group] track interface [priority] command.

When multiple tracked interfaces are down, the priority is reduced by a cumulative amount. If you

explicitly set the decrement value, then the value is decreased by that amount if that interface is

down, and decrements are cumulative. If you do not set an explicit decrement value, then the

value is decreased by 10 for each interface that goes down, and decrements are cumulative.

The following example uses the following configuration, with the default decrement value of 10.

Note: When an HSRP group number is not specified, the default group number is group 0.

interface ethernet0

ip address 10.1.1.1 255.255.255.0

standby ip 10.1.1.3

standby priority 110

standby track serial0

standby track serial1

The HSRP behavior with this configuration is:

0 interfaces down = no decrease (priority is 110)

1 interface down = decrease by 10 (priority becomes100)

2 interfaces down = decrease by 10 (priority becomes 90)

Reference

http://www.cisco.com/en/US/tech/tk648/tk362/technologies_tech_note09186a0080094a91.shtml#i

ntracking


QUESTION NO:7

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

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 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.shtml


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:12

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

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


Latest 400-101 Dumps400-101 PDF Dumps400-101 Exam Questions

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: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:25

Refer to the exhibit.

After a link flap in the network, which two EIGRP neighbors will not be queried for alternative

paths? (Choose two.)

A. 192.168.1.1

B. 192.168.3.7

C. 192.168.3.8

D. 192.168.3.6

E. 192.168.2.1

F. 192.168.3.9

Answer: B,C

Explanation:

Explanation

Both 192.168.3.7 and 192.168.3.8 are in an EIGRP Stub area

The Enhanced Interior Gateway Routing Protocol (EIGRP) Stub Routing feature improves network

stability, reduces resource utilization, and simplifies stub router configuration.

Stub routing is commonly used in a hub and spoke network topology. In a hub and spoke network,

one or more end (stub) networks are connected to a remote router (the spoke) that is connected to

one or more distribution routers (the hub). The remote router is adjacent only to one or more

distribution routers. The only route for IP traffic to follow into the remote router is through a

distribution router. This type of configuration is commonly used in WAN topologies where the

distribution router is directly connected to a WAN. The distribution router can be connected to

many more remote routers. Often, the distribution router will be connected to 100 or more remote

routers. In a hub and spoke topology, the remote router must forward all nonlocal traffic to a

distribution router, so it becomes unnecessary for the remote router to hold a complete routing

table. Generally, the distribution router need not send anything more than a default route to the

remote router.

When using the EIGRP Stub Routing feature, you need to configure the distribution and remote

routers to use EIGRP, and to configure only the remote router as a stub. Only specified routes are

propagated from the remote (stub) router. The router responds to queries for summaries,

connected routes, redistributed static routes, external routes, and internal routes with the message

“inaccessible.” A router that is configured as a stub will send a special peer information packet to

all neighboring routers to report its status as a stub router. Any neighbor that receives a packet

informing it of the stub status will not query the stub router for any routes, and a router that has a

stub peer will not query that peer. The stub router will depend on the distribution router to send the

proper updates to all peers.

Reference

http://www.cisco.com/en/US/docs/ios/12_0s/feature/guide/eigrpstb.html#wp1021949


QUESTION NO:30

What is the flooding scope of an OSPFv3 LSA, if the value of the S2 bit is set to 1 and the S1 bit is

set to 0?

A. link local

B. area wide

C. AS wide

D. reserved

Answer: C

Explanation:

The Type 1 router LSA is now link local and the Type 2 Network LSA is AS Wide

S2 and S1 indicate the LSA\’s flooding scope. Table 9-1 shows the possible values of these two

bits and the associated flooding scopes.

Table 9-1 S bits in the OSPFv3 LSA Link State Type field and their associated flooding scopes

LSA Function Code, the last 13 bits of the LS Type field, corresponds to the OSPFv2 Type field.

Table 9-2 shows the common LSA types used by OSPFv3 and the values of their corresponding

LS Types. If you decode the hex values, you will see that the default U bit of all of them is 0. The S

bits of all LSAs except two indicate area scope. Of the remaining two, AS External LSAs have an

AS flooding scope and Link LSAs have a linklocal flooding scope. Most of the OSPFv3 LSAs have

functional counterparts in OSPFv2; these OSPFv2 LSAs and their types are also shown in Table

9-2.

Table 9-2 OSPFv3 LSA types and their OSPFv2 counterparts

Reference

http://www.networkworld.com/subnets/cisco/050107-ch9-ospfv3.html?page=1


QUESTION NO:33

Which two OSPF LSA types are new in OSPF version 3? (Choose two.)

A. Link

B. NSSA external

C. Network link

D. Intra-area prefix

E. AS domain

Answer: A,D

Explanation:

New LSA Types

OSPFv3 carries over the seven basic LSA types we\’re familiar with from OSPFv2. However, the

type 1 and 2 LSAs have been re-purposed, as will be discussed in a bit. OSPFv3 also introduces

two new LSA types: Link and Intra-area Prefix.

Reference

http://packetlife.net/blog/2010/mar/2/ospfv2-versus-ospfv3/


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