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### 2021 Jun 400-101 Study Guide Questions:

Q161. Which statement is true regarding the UDP checksum?

A. It is used for congestion control.

B. It cannot be all zeros.

C. It is used by some Internet worms to hide their propagation.

D. It is computed based on the IP pseudo-header.

Explanation:

The method used to compute the checksum is defined in RFC 768: “Checksum is the 16-bit one's complement of the one's complement sum of a pseudo header of information from the IP header, the UDP header, and the data, padded with zero octets at the end (if necessary) to make a multiple of two octets.” In other words, all 16-bit words are summed using one's complement arithmetic. Add the 16-bit values up. Each time a carry-out (17th bit) is produced, swing that bit around and add it back into the least significant bit. The sum is then one's complemented to yield the value of the UDP checksum field. If the checksum calculation results in the value zero (all 16 bits 0) it should be sent as the one's complement (all 1s).

Reference: http://en.wikipedia.org/wiki/User_Datagram_Protocol

Q162. Which two options are the two underlying protocols on which a DMVPN relies? (Choose two.)

A. IPsec

B. NHRP

C. GDOI

D. ISAKMP

E. SSL

F. NLRI

Q163. Refer to the exhibit.

What is the PHB class on this flow?

A. EF

B. none

C. AF21

D. CS4

Explanation:

This command shows the TOS value in hex, which is 80 in this case. The following chart shows some common DSCP/PHB Class values:

Service

DSCP value

TOS value

Juniper Alias

DSCP - TOS Binary

46

184

ef

B8

101110 - 101110xx

LBE

32

cs1

20

001000 - 001000xx

DWS

32

128

cs4

80

100000 - 100000xx

Network control

48

192

cs6

c0

110000 - 110000xx

Network control 2

56

224

cs7

e0

111000 - 111000xx

Reference: http://www.tucny.com/Home/dscp-tos

### Update 400-101 vce:

Q164. Which two BGP path attributes are visible in Wireshark? (Choose two.)

A. weight

B. AS path

C. local preference

D. route maps

Q165. Which two options are signaling protocols that are used in MPLS? (Choose two.)

A. LDP

B. RSVP

C. BFD

D. LISP

E. CLNS

F. CDP

Explanation:

* Signaling is the means by which LSRs all along the path know that they are a part of a given LSP. It is a signaling function by which the LSR knows that the internal transit path for the LSP depicted goes from Interface 2 to Interface 4.

* Label distribution is the means by which an LSR tells an upstream LSR what label value to use for a particular LSP.

There are four protocols that can perform the label distribution function:

* Label Distribution Protocol (LDP)

* Resource Reservation Protocol with Traffic Engineering Extensions (RSVP-TE)

* Constraint-Based Routed LDP (CR-LDP)

* Multiprotocol BGP

LDP and RSVP-TE are the two most commonly used label distribution protocols

Reference: http://www.networkworld.com/article/2237487/cisco-subnet/understanding-mpls-label-distribution.html

Q166. Which three statements about RIP timers are true? (Choose three.)

A. The default update timer is 30 seconds.

B. The default invalid timer is 180 seconds.

C. The default holddown timer is 180 seconds.

D. The default flush timer is 60 seconds.

E. The default scan timer is 60 seconds.

F. The default hello timer is 5 seconds.

Explanation:

The routing information protocol uses the following timers as part of its operation:

Update Timer

Invalid Timer

Flush Timer

Holddown Timer

Update Timer

The update timer controls the interval between two gratuitous Response Message. By default the value is 30 seconds. The response message is broadcast to all its RIP enabled interface.

Invalid Timer

The invalid timer specifies how long a routing entry can be in the routing table without being updated. This is also called as expiration Timer. By default, the value is 180 seconds. After the timer expires the hop count of the routing entry will be set to 16, marking the destination as unreachable.

Flush Timer

The flush timer controls the time between the route is invalidated or marked as unreachable and removal of entry from the routing table. By default the value is 240 seconds. This is 60 seconds longer than Invalid timer. So for 60 seconds the router will be advertising about this unreachable route to all its neighbors. This timer must be set to a higher value than the invalid timer.

Hold-down Timer

The hold-down timer is started per route entry, when the hop count is changing from lower value to higher value. This allows the route to get stabilized. During this time no update can be done to that routing entry. This is not part of the RFC 1058. This is Cisco's implementation. The default value of this timer is 180 seconds.

Reference: http://en.wikipedia.org/wiki/Routing_Information_Protocol#Timers

### Breathing vce 400-101:

Q167. DRAG DROP

Drag and drop each IPv6 neighbor discovery message type on the left to the corresponding description on the right.

Q168. Which packet does a router receive if it receives an OSPF type 4 packet?

A. hello packet

B. database descriptor packet

Q169. Refer to the exhibit.

Which two issues can cause the interface VLAN10 to be down/down? (Choose two.)

A. The VLAN is inactive or has been removed from the VLAN database.

B. STP is in a forwarding state on the port.

C. A Layer 2 access port is configured with VLAN10, but is in a down/down state.

D. The autostate exclude feature was used on interface VLAN10.

Q170. Refer to the exhibit.

Switch DSW1 should share the same MST region with switch DSW2. Which statement is true?

A. Configure DSW1 with the same version number, and VLAN-to-instance mapping as shown on DSW2.

B. Configure DSW1 with the same region name, number, and VLAN-to-instance mapping as shown on DSW2.

C. DSW2 uses the VTP server mode to automatically propagate the MST configuration to DSW1.

D. DSW1 is in VTP client mode with a lower configuration revision number, therefore, it automatically inherits MST configuration from DSW2.

E. DSW1 automatically inherits MST configuration from DSW2 because they have the same domain name.