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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


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


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



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


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.



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


Answer: B,E


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




Which command is used to enable EtherChannel hashing for Layer 3 IP and Layer 4 port-based


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



When you are troubleshooting duplex mismatches, which two errors are typically seen on the full-

duplex end? (Choose two.)

A. runts

B. FCS errors

C. interface resets

D. late collisions

Answer: A,B


Latest 400-101 Dumps400-101 VCE Dumps400-101 Study Guide


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



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


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


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 at 7-25-08 14:33:43

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

VTP version running: 2



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?




D. RTB will not be able to reach RTG since the OSPF configuration is wrong.

Answer: C



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



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


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