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Why would a rogue host that is running a DHCP server on a campus LAN network present a

security risk?

A. It may allocate IP addresses from an unknown subnet to the users.

B. All multicast traffic can be sniffed by using the DHCP multicast capabilities.

C. The CPU utilization of the first hop router can be overloaded by exploiting DHCP relay open


D. A potential man-in-the-middle attack can be used against the clients.

Answer: D



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 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



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


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


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:


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 PDF Dumps400-101 Practice Test


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


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:



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



Which three combinations are valid LACP configurations that will set up a channel? (Choose


A. On/On

B. On/Auto

C. Passive/Active

D. Desirable/Auto

E. Active/Active

F. Desirable/Desirable

Answer: A,C,E



Which two options does Cisco PfR use to control the entrance link selection with inbound

optimization? (Choose two.)

A. Prepend extra AS hops to the BGP prefix.

B. Advertise more specific BGP prefixes (longer mask).

C. Add (prepend) one or more communities to the prefix that is advertised by BGP.

D. Have BGP dampen the prefix.

Answer: A,C

Explanation: PfR Entrance Link Selection Control Techniques

The PfR BGP inbound optimization feature introduced the ability to influence inbound traffic. A

network advertises reachability of its inside prefixes to the Internet using eBGP advertisements to

its ISPs. If the same prefix is advertised to more than one ISP, then the network is multihoming.

PfR BGP inbound optimization works best with multihomed networks, but it can also be used with

a network that has multiple connections to the same ISP. To implement BGP inbound

optimization, PfR manipulates eBGP advertisements to influence the best entrance selection for

traffic bound for inside prefixes. The benefit of implementing the best entrance selection is limited

to a network that has more than one ISP connection.

To enforce an entrance link selection, PfR offers the following methods:

BGP Autonomous System Number Prepend When an entrance link goes out-of-policy (OOP) due

to delay, or in images prior to Cisco IOS Releases 15.2(1) T1 and 15.1(2)S, and PfR selects a

best entrance for an inside prefix, extra autonomous system hops are prepended one at a time (up

to a maximum of six) to the inside prefix BGP advertisement over the other entrances. In Cisco

IOS Releases 15.2(1)T1, 15.1(2)S, and later releases, when an entrance link goes out-of policy

(OOP) due to unreachable or loss reasons, and PfR selects a best entrance for an inside prefix,

six extra autonomous system hops are prepended immediately to the inside prefix BGP

advertisement over the other entrances. The extra autonomous system hops on the other

entrances increase the probability that the best entrance will be used for the inside prefix. When

the entrance link is OOP due to unreachable or loss reasons, six extra autonomous system hops

are added immediately to allow the software to quickly move the traffic away from the old entrance

link. This is the default method PfR uses to control an inside prefix, and no user configuration is


BGP Autonomous System Number Community Prepend

When an entrance link goes out-of-policy (OOP) due to delay, or in images prior to Cisco IOS

Releases 15.2

(1)T1 and 15.1(2)S, and PfR selects a best entrance for an inside prefix, a BGP prepend

community is attached one at a time (up to a maximum of six) to the inside prefix BGP

advertisement from the network to another autonomous system such as an ISP. In Cisco IOS

Releases 15.2(1)T1, 15.1(2)S, and later releases, when an entrance link goes out-of-policy (OOP)

due to unreachable or loss reasons, and PfR selects a best entrance for an inside prefix, six BGP

prepend communities are attached to the inside prefix BGP advertisement. The BGP prepend

community will increase the number of autonomous system hops in the advertisement of the

inside prefix from the ISP to its peers. Autonomous system prepend BGP community is the

preferred method to be used for PfR BGP inbound optimization because there is no risk of the

local ISP filtering the extra autonomous system hops. There are some issues, for example, not all

ISPs support the BGP prepend community, ISP policies may ignore or modify the autonomous

system hops, and a transit ISP may filter the autonomous system path. If you use this method of

inbound optimization and a change is made to an autonomous system, you must issue an

outbound reconfiguration using the “clear ip bgp” command.





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


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