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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: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

Latest 400-101 Dumps 400-101 VCE Dumps 400-101 Exam Questions

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

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

QUESTION NO:32

Which two tunneling techniques support IPv6 multicasting? (Choose two.)

A. 6to4

B. 6over4

C. ISATAP

D. 6PE

E. GRE

Answer: B,E

Explanation:

When IPv6 multicast is supported (over a 6to4 tunnel), an IPv6 multicast routing protocol must be

used

Restrictions for Implementing IPv6 Multicast

IPv6 multicast for Cisco IOS software uses MLD version 2. This version of MLD is fully backward-

compatible with MLD version 1 (described in RFC 2710). Hosts that support only MLD version 1

will interoperate with a router running MLD version 2. Mixed LANs with both MLD version 1 and

MLD version 2 hosts are likewise supported.

IPv6 multicast is supported only over IPv4 tunnels in Cisco IOS Release 12.3(2)T, Cisco IOS

Release 12.2

(18)S, and Cisco IOS Release 12.0(26)S.

When the bidirectional (bidir) range is used in a network, all routers in that network must be able to

understand the bidirectional range in the bootstrap message (BSM).

IPv6 multicast routing is disabled by default when the ipv6 unicast-routing command is configured.

On Cisco Catalyst 6500 and Cisco 7600 series routers, the ipv6 multicast-routing also must be

enabled in order to use IPv6 unicast routing

Reference http://www.cisco.com/web/about/ac123/ac147/ac174/ac197/

about_cisco_ipj_archive_article09186a00800c830a.html

http://www.cisco.com/en/US/docs/ios/ipv6/configuration/guide/ip6-multicast.html

https://supportforums.cisco.com/thread/183386

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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