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2021 Apr 400-101 answers
Q201. What is Nagle's algorithm used for?
A. To increase the latency
B. To calculate the best path in distance vector routing protocols
C. To calculate the best path in link state routing protocols
D. To resolve issues caused by poorly implemented TCP flow control.
Answer: D
Explanation:
Silly window syndrome is a problem in computer networking caused by poorly implemented TCP flow control. A serious problem can arise in the sliding window operation when the sending application program creates data slowly, the receiving application program consumes data slowly, or both. If a server with this problem is unable to process all incoming data, it requests that its clients reduce the amount of data they send at a time (the window setting on a TCP packet). If the server continues to be unable to process all incoming data, the window becomes smaller and smaller, sometimes to the point that the data transmitted is smaller than the packet header, making data transmission extremely inefficient. The name of this problem is due to the window size shrinking to a "silly" value. When there is no synchronization between the sender and receiver regarding capacity of the flow of data or the size of the packet, the window syndrome problem is created. When the silly window syndrome is created by the sender, Nagle's algorithm is used. Nagle's solution requires that the sender sends the first segment even if it is a small one, then that it waits until an ACK is received or a maximum sized segment (MSS) is accumulated.
Reference: http://en.wikipedia.org/wiki/Silly_window_syndrome
Q202. Which two statements about logging are true? (Choose two.)
A. Log messages are sent to the console port by default.
B. Log messages are displayed in a Telnet session by default.
C. Interface status changes are logged at the Notification level.
D. Interface status changes are logged at the Informational level.
E. System restart messages are logged at the Critical level.
F. Reload requests are logged at the Notification level.
Answer: A,C
Explanation:
By default, switches send the output from system messages and debug privileged EXEC commands to a logging process. The logging process controls the distribution of logging messages to various destinations, such as the logging buffer, terminal lines, or a UNIX syslog server, depending on your configuration. The process also sends messages to the console.
Table 29-3 Message Logging Level Keywords
Level Keyword
Level
Description
Syslog Definition
emergencies
0
System unstable
LOG_EMERG
alerts
1
Immediate action needed
LOG_ALERT
critical
2
Critical conditions
LOG_CRIT
errors
3
Error conditions
LOG_ERR
warnings
4
Warning conditions
LOG_WARNING
notifications
5
Normal but significant condition
LOG_NOTICE
informational
6
Informational messages only
LOG_INFO
debugging
7
Debugging messages
LOG_DEBUG
The software generates four other categories of messages:
. Error messages about software or hardware malfunctions, displayed at levels warnings through emergencies. These types of messages mean that the functionality of the switch is affected. For information on how to recover from these malfunctions, see the system message guide for this release.
. Output from the debug commands, displayed at the debugging level. Debug commands are typically used only by the Technical Assistance Center.
Interface up or down transitions and system restart messages, displayed at the notifications level. This message is only for information; switch functionality is not affected.
. Reload requests and low-process stack messages, displayed at the informational level. This message is only for information; switch functionality is not affected.
References: http://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst2950/software/release/12-
1_9_ea1/configuration/guide/scg/swlog.html
http://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst2960/software/release/12-2_55_se/configuration/guide/scg_2960/swlog.html
Q203. Which option is the Cisco recommended method to secure access to the console port?
A. Configure the activation-character command.
B. Configure a very short timeout (less than 100 milliseconds) for the port.
C. Set the privilege level to a value less than 15.
D. Configure an ACL.
Answer: A
Explanation:
The activation-character command defines a session activation character. Entering this character at a vacant terminal begins a terminal session. The default activation character is the Return key
To secure the console port, you should change this character to a different one as most people simply hit the enter key when trying to access the console.
Q204. Which statement about traffic management when PIM snooping is enabled is true?
A. Traffic is restricted to host ports.
B. All multicast traffic is flooded to the designated router.
C. Join message are flooded to all routers.
D. Designated routers receive traffic only from groups through which a join message is received.
Answer: D
Q205. Which three options are sources from which a SPAN session can copy traffic? (Choose three.)
A. ports
B. EtherChannels
C. VLANs
D. subnets
E. primary IP addresses
F. secondary IP addresses
Answer: A,B,C
Explanation:
. SPAN Sources
The interfaces from which traffic can be monitored are called SPAN sources. Sources designate the traffic to monitor and whether to copy ingress, egress, or both directions of traffic. SPAN sources include the following:
. Ethernet ports
. Port channels
. The inband interface to the control plane CPU — You can monitor the inband interface only from the default VDC. Inband traffic from all VDCs is monitored.
. VLANs — When a VLAN is specified as a SPAN source, all supported interfaces in the VLAN are SPAN sources.
. Remote SPAN (RSPAN) VLANs
. Fabric port channels connected to the Cisco Nexus 2000 Series Fabric Extender .
Satellite ports and host interface port channels on the Cisco Nexus 2000 Series Fabric Extender
— These interfaces are supported in Layer 2 access mode, Layer 2 trunk mode, and Layer 3 mode.
Reference: http://www.cisco.com/c/en/us/td/docs/switches/datacenter/sw/5_x/nx-os/system_management/configuration/guide/sm_nx_os_cg/sm_14span.html#wp1239492
Replace 400-101 study guide:
Q206. Refer to the exhibit.
The device with this configuration is unable to reach network 172.31.31.0/24. The next hop router has been verified to have full connectivity to the network. Which two actions can you take to establish connectivity to the network? (Choose two.)
A. Create a static route to 172.16.199.0 using the address of the next hop router.
B. Create a default route to the link address of the next hop router.
C. Create a static route to the loopback address of the next hop router.
D. Create a default route to 172.16.199.9.
E. Modify the existing static route so that the next hop is 0.0.0.0.
F. Replace the ip default-network command with the ip default-gateway command.
Answer: A,B
Explanation:
Unlike the ip default-gateway command, you can use ip default-network when ip routing is enabled on the Cisco router. When you configure ip default-network the router considers routes to that network for installation as the gateway of last resort on the router.
For every network configured with ip default-network, if a router has a route to that network, that route is flagged as a candidate default route. However, in this case if the router does not a route to the drfault network of 172.16.199.9, then you would need to ensure that this route exisits by creating a static route to 172.16.199.0 using the address of the next hop router, or simply create a default route using the address of the next hop router.
Q207. Which technology can be used to prevent flooding of IPv6 multicast traffic on a switch?
A. IGMP snooping
B. IGMP filtering
C. MLD snooping
D. MLD filtering
Answer: C
Explanation:
MLD snooping allows the switch to examine MLD packets and make forwarding decisions based on their content. You can configure the switch to use MLD snooping in subnets that receive MLD queries from either MLD or the MLD snooping querier. MLD snooping constrains IPv6 multicast traffic at Layer 2 by configuring Layer 2 LAN ports dynamically to forward IPv6 multicast traffic only to those ports that want to receive it.
Reference: http://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst6500/ios/12-2SX/configuration/guide/book/snoopmld.html
Q208. Which two statements about the OSPF two-way neighbor state are true? (Choose two.)
A. Each neighbor receives its own router ID in a hello packet from the other neighbor.
B. Each neighbor receives a hello message from the other neighbor.
C. It is valid only on NBMA networks.
D. It is valid only on point-to-point networks.
E. Each neighbor receives an acknowledgement of its own hello packet from the other neighbor.
F. Each neighbor receives the router ID of the other neighbor in a hello packet from the other neighbor.
Answer: A,E
Q209. Which two options are mandatory components of a multiprotocol BGP VPN-IPv4 address? (Choose two.)
A. a route distinguisher
B. an IPv4 address
C. a route target
D. an MPLS label
E. a system ID
F. an area ID
Answer: A,B
Explanation:
The IP prefix is a member of the IPv4 address family. After the PE device learns the IP prefix, the PE converts it into a VPN-IPv4 prefix by combining it with an 8-byte route distinguisher (RD). The generated prefix is a member of the VPN-IPv4 address family. It uniquely identifies the customer address, even if the customer site is using globally nonunique (unregistered private) IP addresses. The route distinguisher used to generate the VPN-IPv4 prefix is specified by a configuration command associated with the virtual routing and forwarding (VRF) instance on the PE device.
Reference: http://www.cisco.com/c/en/us/td/docs/ios-xml/ios/mp_l3_vpns/configuration/15-mt/mp-l3-vpns-15-mt-book/mp-bgp-mpls-vpn.html
Q210. Which statement about OSPF loop prevention is true?
A. The discard route is generated automatically on the ABR to prevent routing loops.
B. The ASBR uses type 3 LSAs from non-backbone areas to prevent control-plane routing loops.
C. The ABR can filter type 3 LSPs to prevent routing loops.
D. The DN bit ignores LSA types 2, 3, and 5 to prevent routing loops.
Answer: A