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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: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: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


QUESTION NO:14

Which three options are features of VTP version 3? (Choose three.)

A. VTPv3 supports 8K VLANs.

B. VTPv3 supports private VLAN mapping.

C. VTPv3 allows for domain discovery.

D. VTPv3 uses a primary server concept to avoid configuration revision issues.

E. VTPv3 is not compatible with VTPv1 or VTPv2.

F. VTPv3 has a hidden password option.

Answer: B,D,F

Explanation:

Key Benefits of VTP Version 3

Much work has gone into improving the usability of VTP version 3 in three major areas:

The new version of VTP offers better administrative control over which device is allowed to update

other devices\’ view of the VLAN topology. The chance of unintended and disruptive changes is

significantly reduced, and availability is increased. The reduced risk of unintended changes will

ease the change process and help speed deployment.

Functionality for the VLAN environment has been significantly expanded. Two enhancements are

most beneficial for today\’s networks:


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:


Latest 400-101 Dumps400-101 Study Guide400-101 Exam Questions

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


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: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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