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2021 Apr 300-101 exam prep
Q51. Which three TCP enhancements can be used with TCP selective acknowledgments? (Choose three.)
A. header compression
B. explicit congestion notification
C. keepalive
D. time stamps
E. TCP path discovery
F. MTU window
Answer: B,C,D
Explanation:
TCP Selective Acknowledgment
The TCP Selective Acknowledgment feature improves performance if multiple packets are lost from one
TCP window of data.
Prior to this feature, because of limited information available from cumulative acknowledgments, a TCP
sender could learn about only one lost packet per-round-trip
time. An aggressive sender could choose to resend packets early, but such re-sent segments might have
already been successfully received.
The TCP selective acknowledgment mechanism helps improve performance. The receiving TCP host
returns selective acknowledgment packets to the sender,
informing the sender of data that has been received. In other words, the receiver can acknowledge packets
received out of order. The sender can then resend only
missing data segments (instead of everything since the first missing packet).
Prior to selective acknowledgment, if TCP lost packets 4 and 7 out of an 8-packet window, TCP would
receive acknowledgment of only packets 1, 2, and 3. Packets
4 through 8 would need to be re-sent. With selective acknowledgment, TCP receives acknowledgment of
packets 1, 2, 3, 5, 6, and 8. Only packets 4 and 7 must be
re-sent.
TCP selective acknowledgment is used only when multiple packets are dropped within one TCP window.
There is no performance impact when the feature is
enabled but not used. Use the ip tcp selective-ack command in global configuration mode to enable TCP
selective acknowledgment.
Refer to RFC 2021 for more details about TCP selective acknowledgment.
TCP Time Stamp
The TCP time-stamp option provides improved TCP round-trip time measurements. Because the time
stamps are always sent and echoed in both directions and the time-stamp value in the header is always
changing, TCP header compression will not compress the outgoing packet. To allow TCP header
compression over a serial link, the TCP time-stamp option is disabled. Use the ip tcp timestamp command
to enable the TCP time-stamp option.
TCP Explicit Congestion Notification
The TCP Explicit Congestion Notification (ECN) feature allows an intermediate router to notify end hosts of
impending network congestion. It also provides enhanced support for TCP sessions associated with
applications, such as Telnet, web browsing, and transfer of audio and video data that are sensitive to delay
or packet loss. The benefit of this feature is the reduction of delay and packet loss in data transmissions.
Use the ip tcp ecn command in global configuration mode to enable TCP ECN.
TCP Keepalive Timer
The TCP Keepalive Timer feature provides a mechanism to identify dead connections. When a TCP
connection on a routing device is idle for too long, the device sends a TCP keepalive packet to the peer
with only the Acknowledgment (ACK) flag turned on. If a response packet (a TCP ACK packet) is not
received after the device sends a specific number of probes, the connection is considered dead and the
device initiating the probes frees resources used by the TCP connection. Reference: http://www.cisco.com/
c/en/us/td/docs/ios-xml/ios/ipapp/configuration/xe-3s/asr1000/iap-xe-3s-asr1000-book/iap-tcp.html#GUID-22A82C5F-631F-4390-9838-F2E48FFEEA01
Q52. CORRECT TEXT
ROUTE.com is a small IT corporation that has an existing enterprise network that is running IPv6 0SPFv3. Currently OSPF is configured on all routers. However, R4's loopback address (FEC0:4:4) cannot be seen in R1's IPv6 routing table. You are tasked with identifying the cause of this fault and implementing the needed corrective actions that uses OPSF features and does not change the current area assignments. You will know that you have corrected the fault when R4's loopback address (FEC0:4:4) can be seen in RTs IPv6 routing table.
Special Note: To gain the maximum number of points you must remove all incorrect or unneeded configuration statements related to this issue.
Answer: Here is the solution below:
Explanation:
To troubleshoot the problem, first issue the show running-config on all of 4 routers. Pay more attention to the outputs of routers R2 and R3 The output of the "show running-config" command of R2:
The output of the "show running-config" command of R3:
We knew that all areas in an Open Shortest Path First (OSPF) autonomous system must be physically connected to the backbone area (Area 0). In some cases, where this is not possible, we can use a virtual link to connect to the backbone through a non-backbone area. The area through which you configure the virtual link is known as a transit area. In this case, the area 11 will become the transit area. Therefore, routers R2 and R3 must be configured with the area <area id> virtual-link <neighbor router-id>command. + Configure virtual link on R2 (from the first output above, we learned that the OSPF process ID of R2 is 1):
R2>enable
R2#configure terminal
R2(config)#ipv6 router ospf 1
R2(config-rtr)#area 11 virtual-link 3.3.3.3
Save the configuration:
R2(config-rtr)#end
R2#copy running-config startup-config
(Notice that we have to use neighbor router-id 3.3.3.3, not R2's router-id 2.2.2.2) + Configure virtual link on R3 (from the second output above, we learned that the OSPF process ID of R3 is 1 and we have to disable the wrong configuration of "area 54 virtual-link 4.4.4.4"):
R3>enable
R3#configure terminal
R3(config)#ipv6 router ospf 1
R3(config-rtr)#no area 54 virtual-link 4.4.4.4
R3(config-rtr)#area 11 virtual-link 2.2.2.2
Save the configuration:
R3(config-rtr)#end
R3#copy running-config startup-config
You should check the configuration of R4, too. Make sure to remove the incorrect configuration statements to get the full points.
R4(config)#ipv6 router ospf 1
R4(config-router)#no area 54 virtual-link 3.3.3.3
R4(config-router)#end
After finishing the configuration doesn’t forget to ping between R1 and R4 to make sure they work.
Note. If you want to check the routing information, use the show ipv6 route command, not "show ip route".
Q53. What does the following access list, which is applied on the external interface FastEthernet 1/0 of the perimeter router, accomplish?
router(config)#access-list 101 deny ip 10.0.0.0 0.255.255.255 any log
router (config)#access-list 101 deny ip 192.168.0.0 0.0.255.255 any log
router (config)#access-list 101 deny ip 172.16.0.0 0.15.255.255 any log
router (config)#access-list 101 permit ip any any
router (config)#interface fastEthernet 1/0
router (config-if)#ip access-group 101 in
A. It prevents incoming traffic from IP address ranges 10.0.0.0-10.0.0.255, 172.16.0.0-172.31.255.255, 192.168.0.0-192.168.255.255 and logs any intrusion attempts.
B. It prevents the internal network from being used in spoofed denial of service attacks and logs any exit to the Internet.
C. It filters incoming traffic from private addresses in order to prevent spoofing and logs any intrusion attempts.
D. It prevents private internal addresses to be accessed directly from outside.
Answer: C
Explanation:
The private IP address ranges defined in RFC 1918 are as follows:
10.0.0.0 - 10.255.255.255
172.16.0.0 - 172.31.255.255
192.168.0.0 - 192.168.255.255
These IP addresses should never be allowed from external networks into a
corporate network as they would only be able to reach the network from the outside via routing problems or
if the IP addresses were spoofed. This ACL is used to prevent all packets with a spoofed reserved private
source IP address to enter the network. The log keyword also enables logging of this intrusion attempt.
Q54. When using SNMPv3 with NoAuthNoPriv, which string is matched for authentication?
A. username
B. password
C. community-string
D. encryption-key
Answer: A
Explanation:
The following security models exist: SNMPv1, SNMPv2, SNMPv3. The following security
levels exits: "noAuthNoPriv" (no authentiation and no encryption noauth keyword in CLI),
"AuthNoPriv" (messages are authenticated but not encrypted auth keyword in CLI), "AuthPriv" (messages
are authenticated and encrypted priv keyword in CLI). SNMPv1 and SNMPv2 models only support the
"noAuthNoPriv" model since they use plain community string to match the incoming packets. The SNMPv3
implementations could be configured to use either of the models on per-group basis (in case if
"noAuthNoPriv" is configured, username serves as a replacement for community string). Reference: http://
blog.ine.com/2008/07/19/snmpv3-tutorial/
Q55. You have been asked to evaluate how EIGRP is functioning in a customer network.
Which key chain is being used for authentication of EIGRP adjacency between R4 and R2?
A. CISCO
B. EIGRP
C. key
D. MD5
Answer: A
Explanation: R4 and R2 configs are as shown below:
Clearly we see the actual key chain is named CISCO.
Up to the immediate present 300-101 exam topics:
Q56. Refer to the exhibit.
Based on this FIB table, which statement is correct?
A. There is no default gateway.
B. The IP address of the router on FastEthernet is 209.168.201.1.
C. The gateway of last resort is 192.168.201.1.
D. The router will listen for all multicast traffic.
Answer: C
Explanation:
The 0.0.0.0/0 route is the default route and is listed as the first CEF entry. Here we see the next hop for this default route lists 192.168.201.1 as the default router (gateway of last resort).
Q57. A network engineer is configuring a solution to allow failover of HSRP nodes during maintenance windows, as an alternative to powering down the active router and letting the network respond accordingly. Which action will allow for manual switching of HSRP nodes?
A. Track the up/down state of a loopback interface and shut down this interface during maintenance.
B. Adjust the HSRP priority without the use of preemption.
C. Disable and enable all active interfaces on the active HSRP node.
D. Enable HSRPv2 under global configuration, which allows for maintenance mode.
Answer: A
Explanation:
The standby track command allows you to specify another interface on the router for the
HSRP process to monitor in order to alter the HSRP priority for a given group. If the line protocol of the
specified interface goes down, the HSRP priority is reduced. This means that another HSRP router with
higher priority can become the active router if that router has standby preempt enabled. Loopback
interfaces can be tracked, so when this interface is shut down the HSRP priority for that router will be
lowered and the other HSRP router will then become the active one. Reference: http://www.cisco.com/c/
en/us/support/docs/ip/hot-standby-router-protocol- hsrp/13780-6.html
Q58. A network engineer is configuring a routed interface to forward broadcasts of UDP 69, 53, and 49 to 172.20.14.225. Which command should be applied to the configuration to allow this?
A. router(config-if)#ip helper-address 172.20.14.225
B. router(config-if)#udp helper-address 172.20.14.225
C. router(config-if)#ip udp helper-address 172.20.14.225
D. router(config-if)#ip helper-address 172.20.14.225 69 53 49
Answer: A
Explanation:
To let a router forward broadcast packet the command ip helper-address can be used. The broadcasts will
be forwarded to the unicast address which is specified with the ip helper command.
ip helper-address {ip address}
When configuring the ip helper-address command, the following broadcast packets will be forwarded by
the router by default:
TFTP - UDP port 69
Domain Name System (DNS) UDP port 53
Time service - port 37
NetBIOS Name Server - port 137
NetBIOS Datagram Server - port 138
Bootstrap Protocol (BOOTP) - port 67
TACACS UDP port 49 Reference: http://www.cisco-faq.com/163/forward_udp_broadcas.html
Topic 6, Infrastructure Services
61. A network engineer is configuring SNMP on network devices to utilize one-way SNMP notifications. However, the engineer is not concerned with authentication or encryption. Which command satisfies the requirements of this scenario?
A. router(config)#snmp-server host 172.16.201.28 traps version 2c CISCORO
B. router(config)#snmp-server host 172.16.201.28 informs version 2c CISCORO
C. router(config)#snmp-server host 172.16.201.28 traps version 3 auth CISCORO
D. router(config)#snmp-server host 172.16.201.28 informs version 3 auth CISCORO
Answer: A
Explanation:
Most network admins and engineers are familiar with SNMPv2c which has become the
dominant SNMP version of the past decade. It's simple to configure on both the router/switch-side and just
as easy on the network monitoring server. The problem of course is that the SNMP statistical payload is
not encrypted and authentication is passed in cleartext. Most companies have decided that the information
being transmitted isn't valuable enough to be worth the extra effort in upgrading to SNMPv3, but I would
suggest otherwise. Like IPv4 to IPv6, there are some major changes under the hood. SNMP version 2
uses community strings (think cleartext passwords, no encryption) to authenticate polling and trap delivery.
SNMP version 3 moves away from the community string approach in favor of user- based authentication
and view-based access control. The users are not actual local user accounts, rather they are simply a
means to determine who can authenticate to the device. The view is used to define what the user account
may access on the IOS device. Finally, each user is added to a group, which determines the access policy
for its users. Users, groups, views. Reference: http://www.ccnpguide.com/snmp-version-3/
Q59. The following configuration is applied to a router at a branch site:
ipv6 dhcp pool dhcp-pool
dns-server 2001:DB8:1:B::1
dns-server 2001:DB8:3:307C::42
domain-name example.com
!
If IPv6 is configured with default settings on all interfaces on the router, which two dynamic IPv6 addressing mechanisms could you use on end hosts to provide end-to-end connectivity? (Choose two.)
A. EUI-64
B. SLAAC
C. DHCPv6
D. BOOTP
Answer: A,B
Explanation:
Q60. Refer to the following configuration command.
router (config-line)# ntp master 10
Which statement about this command is true?
A. The router acts as an authoritative NTP clock and allows only 10 NTP client connections.
B. The router acts as an authoritative NTP clock at stratum 10.
C. The router acts as an authoritative NTP clock with a priority number of 10.
D. The router acts as an authoritative NTP clock for 10 minutes only.
Answer: B
Explanation: