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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: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: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: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:16

In 802.1s, how is the VLAN to instance mapping represented in the BPDU?

A. The VLAN to instance mapping is a normal 16-byte field in the MST BPDU.

B. The VLAN to instance mapping is a normal 12-byte field in the MST BPDU.

C. The VLAN to instance mapping is a 16-byte MD5 signature field in the MST BPDU.

D. The VLAN to instance mapping is a 12-byte MD5 signature field in the MST BPDU.

Answer: C

Explanation:

MST Configuration and MST Region

Each switch running MST in the network has a single MST configuration that consists of these

three attributes:

1. An alphanumeric configuration name (32 bytes)

2. A configuration revision number (two bytes)

3. A 4096-element table that associates each of the potential 4096 VLANs supported on the

chassis to a given instance.

In order to be part of a common MST region, a group of switches must share the same

configuration attributes.

It is up to the network administrator to properly propagate the configuration throughout the region.

Currently, this step is only possible by the means of the command line interface (CLI) or through

Simple Network

Management Protocol (SNMP). Other methods can be envisioned, as the IEEE specification does

not explicitly mention how to accomplish that step.

Note: If for any reason two switches differ on one or more configuration attribute, the switches are

part of different regions. For more information refer to the Region Boundary section of this

document.

Region Boundary

In order to ensure consistent VLAN-to-instance mapping, it is necessary for the protocol to be able

to exactly identify the boundaries of the regions. For that purpose, the characteristics of the region

are included in the BPDUs. The exact VLANs-to-instance mapping is not propagated in the BPDU,

because the switches only need to know whether they are in the same region as a neighbor.

Therefore, only a digest of the VLANs-toinstance mapping table is sent, along with the revision

number and the name. Once a switch receives a BPDU, the switch extracts the digest (a

numerical value derived from the VLAN-to-instance mapping table through a mathematical

function) and compares this digest with its own computed digest. If the digests differ, the port on

which the BPDU was received is at the boundary of a region.

In generic terms, a port is at the boundary of a region if the designated bridge on its segment is in

a different region or if it receives legacy 802.1d BPDUs. In this diagram, the port on B1 is at the

boundary of region A, whereas the ports on B2 and B3 are internal to region B:

MST Instances

According to the IEEE 802.1s specification, an MST bridge must be able to handle at least these

two instances:

One Internal Spanning Tree (IST)

One or more Multiple Spanning Tree Instance(s) (MSTIs)

The terminology continues to evolve, as 802.1s is actually in a pre-standard phase. It is likely

these names will change in the final release of 802.1s. The Cisco implementation supports 16

instances: one IST (instance 0) and 15 MSTIs.

show vtp status

Cisco switches “show vtp status” Field Descriptions has a MD5 digest field that is a 16-byte

checksum of the

VTP configuration as shown below

Router# show vtp status

VTP Version: 3 (capable)

Configuration Revision: 1

Maximum VLANs supported locally: 1005

Number of existing VLANs: 37

VTP Operating Mode: Server

VTP Domain Name: [smartports]

VTP Pruning Mode: Disabled

VTP V2 Mode: Enabled

VTP Traps Generation: Disabled

MD5 digest : 0x26 0xEE 0x0D 0x84 0x73 0x0E 0x1B 0x69

Configuration last modified by 172.20.52.19 at 7-25-08 14:33:43

Local updater ID is 172.20.52.19 on interface Gi5/2 (first layer3 interface fou)

VTP version running: 2

Reference

http://www.cisco.com/en/US/tech/tk389/tk621/technologies_white_paper09186a0080094cfc.shtml

http://www.cisco.com/en/US/docs/ios-xml/ios/lanswitch/command/lsw-cr-book.pdf


Latest 400-101 Dumps400-101 PDF Dumps400-101 Study Guide

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:18

Refer to the exhibit.

Which statement is correct about the prefix 160.0.0.0/8?

A. The prefix has encountered a routing loop.

B. The prefix is an aggregate with an as-set.

C. The prefix has been aggregated twice, once in AS 100 and once in AS 200.

D. None of these statements is true.

Answer: B

Explanation:


QUESTION NO:20

Refer to the exhibit.

What is the potential issue with this configuration?

A. There is no potential issue; OSPF will work fine in any condition.

B. Sub-optimal routing may occur since there is no area 1 adjacency between the ABRs.

C. This is a wrong OSPF configuration because all routers must be in area 0 only.

D. This is a wrong OSPF configuration because /30 requires 0.0.0.3 wild card.

Answer: B

Explanation:


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:


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