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QUESTION NO:1

Which two commands are required to enable multicast on a router, knowing that the receivers only

support IGMPv2? (Choose two.)

A. ip pim rp-address

B. ip pim ssm

C. ip pim sparse-mode

D. ip pim passive

Answer: A,C

Explanation:

Sparse mode logic (pull mode) is the opposite of Dense mode logic (push mode), in Dense mode

it is supposed that in every network there is someone who is requesting the multicast traffic so

PIM-DM routers begin by flooding the multicast traffic out of all their interfaces except those from

where a prune message is received to eliminate the

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:10

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

Answer: D

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 PDF Dumps400-101 Practice Test400-101 Study Guide

QUESTION NO:15

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

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

QUESTION NO:23

What action will a BGP route reflector take when it receives a prefix marked with the community

attribute NO ADVERTISE from a client peer?

A. It will advertise the prefix to all other client peers and non-client peers.

B. It will not advertise the prefix to EBGP peers.

C. It will only advertise the prefix to all other IBGP peers.

D. It will not advertise the prefix to any peers.

Answer: D

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:

QUESTION NO:28

Which two orders in the BGP Best Path Selection process are correct? (Choose two.)

A. Higher local preference, then lowest MED, then eBGP over iBGP paths

B. Higher local preference, then highest weight, then lowest router ID

C. Highest weight, then higher local preference, then shortest AS path

D. Lowest origin type, then higher local preference, then lowest router ID

E. Highest weight, then higher local preference, then highest MED

Answer: A,C

Explanation:

QUESTION NO:31

How will EIGRPv6 react if there is an IPv6 subnet mask mismatch between the Global Unicast

addresses on a point-to-point link?

A. EIGRPv6 will form a neighbor relationship.

B. EIGRPv6 will not form a neighbor relationship.

C. EIGRPv6 will form a neighbor relationship, but with the log MSG: “EIGRPv6 neighbor not on a

common subnet.”

D. EIGRPv6 will form a neighbor relationship, but routes learned from that neighbor will not be

installed in the routing table.

Answer: A Explanation:

http://www.ietf.org/rfc/rfc3587.txt

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