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2021 Mar 400-101 exam answers
Q181. Which two features does the show ipv6 snooping features command show information about? (Choose two.)
A. RA guard
B. DHCP guard
C. ND inspection
D. source guard
Answer: A,C
Explanation:
The show ipv6 snooping features command displays the first-hop features that are configured on the router. Examples
The following example shows that both IPv6 NDP inspection and IPv6 RA guard are configured on the router:
Router# show ipv6 snooping features
Feature name priority state
RA guard 100 READY
NDP inspection 20 READY
Reference: http://www.cisco.com/c/en/us/td/docs/ios-xml/ios/ipv6/command/ipv6-cr-book/ipv6-s5.html
Q182. With which ISs will an IS-IS Level 1 IS exchange routing information?
A. Level 1 ISs
B. Level 1 ISs in the same area
C. Level 1 and Level 2 ISs
D. Level 2 ISs
Answer: B
Explanation:
IS-IS differs from OSPF in the way that "areas" are defined and routed between. IS-IS routers are designated as being: Level 1 (intra-area); Level 2 (inter area); or Level 1-2 (both). Level 2 routers are inter area routers that can only form relationships with other Level 2 routers. Routing information is exchanged between Level 1 routers and other Level 1 routers, and Level 2 routers only exchange information with other Level 2 routers. Level 1-2 routers exchange information with both levels and are used to connect the inter area routers with the intra area routers.
Reference: http://en.wikipedia.org/wiki/IS-IS
Q183. Refer to the exhibit.
Which action must you take to enable the WAN link to function properly?
A. Enter a clock rate on the DCE interface.
B. Enter a clock rate on the DTE interface.
C. Enter a compression algorithm on both interfaces.
D. Configure both interfaces for HDLC encapsulation.
Answer: A
Explanation:
When connecting a serial cable to the serial interface of the router, clocking is provided by an external device, such as a CSU/DSU device. A CSU/DSU (Channel Service Unit/Data Service Unit) is a digital-interface device used to connect a router to a digital circuit. The router is the DTE (Data Terminal Equipment) and the external device is the DCE (Data Communications Equipment), where the DCE provides the clocking. However, in some cases we might connect two routers back-to-back using the routers’ serial interfaces (Example. Inside the router labs). Each router is a DTE by default. Since clocking is required to enable the interface, one of the two routers should function as DCE and should provide clocking. This can be done by using the "clock rate" command, from the interface configuration mode.
Reference: http://www.omnisecu.com/cisco-certified-network-associate-ccna/how-to-configure-router-serial-interfaces.php
Q184. Refer to the exhibit.
All switches have default bridge priorities, and originate BPDUs with MAC addresses as indicated. The numbers shown are STP link metrics. Which two ports are in blocking state after STP converges? (Choose two.)
A. the port on switch SWD that connects to switch SWE
B. the port on switch SWF that connects to switch SWG
C. the port on switch SWD that connects to switch SWC
D. the port on switch SWB that connects to switch SWD
Answer: C,D
Explanation:
This is a scenario that wants you to demonstrate understanding of the Root switch and Root port election process. So, it’s best to start with where the root switch will be and work down from there. It’s setup nicely because the lowest MAC address switch starts at the top and then the lower priority/higher mac addresses move down the architecture. SWA wins the root election and of course all ports in SWA are forwarding. SWB introduces the possibility for a switching loop so it’s important to understand which ports will be put into the blocking state. Since SWD is a higher MAC address it will end up with a blocked port connected to SWB to prevent a loop: and this is one of the correct answers. To prevent the possibility of another potential switching loop, SWD again ends up with the higher MAC address so blocking the link between D and C prevents a B/C/D switching loop.
Q185. Which statement is true regarding UDLD and STP timers?
A. The UDLD message timer should be two times the STP forward delay to prevent loops.
B. UDLD and STP are unrelated features, and there is no relation between the timers.
C. The timers need to be synced by using the spanning-tree udld-sync command.
D. The timers should be set in such a way that UDLD is detected before the STP forward delay expires.
Answer: D
Explanation:
UDLD is designed to be a helper for STP. Therefore, UDLD should be able to detect an unidirectional link before STP would unblock the port due to missed BPDUs. Thus, when you configure UDLD timers, make sure your values are set so that unidirectional link is detected before “STP MaxAge + 2xForwardDelay” expires.
Reference: http://blog.ine.com/tag/stp/
Improved 400-101 exam answers:
Q186. Which statement about the feasibility condition in EIGRP is true?
A. The prefix is reachable via an EIGRP peer that is in the routing domain of the router.
B. The EIGRP peer that advertises the prefix to the router has multiple paths to the destination.
C. The EIGRP peer that advertises the prefix to the router is closer to the destination than the router.
D. The EIGRP peer that advertises the prefix cannot be used as a next hop to reach the destination.
Answer: C
Explanation:
The advertised metric from an EIGRP neighbor (peer) to the local router is called Advertised Distance (or reported distance) while the metric from the local router to that network is called Feasible Distance. For example, R1 advertises network 10.10.10.0/24 with a metric of 20 to R2. For R2, this is the advertised distance. R2 calculates the feasible distance by adding the metric from the advertised router (R1) to itself. So in this case the feasible distance to network 10.10.10.0/24 is 20 + 50 = 70.
Before a router can be considered a feasible successor, it must pass the feasibility condition rule. In short, the feasibility condition says that if we learn about a prefix from a neighbor, the advertised distance from that neighbor to the destination must be lower than our feasible distance to that same destination. Therefore we see the Advertised Distance always smaller than the Feasible Distance to satisfy the feasibility condition.
Q187. Which protocol uses a proprietary 2-byte Type field for multiple protocol support?
A. HDLC
B. PPP
C. CHAP
D. PAP
Answer: A
Q188. What is the destination multicast MAC address for BPDUs on the native VLAN, for a switch that is running 802.1D?
A. 0185. C400. 0000
B. 0100.0CCC.CCCC
C. 0100.0CCC.CCCD
D. 0180.C200.0000
Answer: D
Explanation:
If the native vlan is 1:
A STP BPDU for VLAN 1 will be sent untagged to MAC 0180.c200.0000 (this is the
common spanning tree)
A PVST+ BPDU for VLAN 1 will be sent untagged to MAC 0100.0ccc.cccd
A PVST+ BPDU for all other vlans will be sent with a 802.1Q tag to MAC 0100.0ccc.cccd
(with a PVID = to the VLAN)
If the native vlan is not 1:
A STP BPDU for VLAN 1 will be sent untagged (on the native vlan) to MAC
0180.c200.0000 (this is the common spanning tree)
A PVST+ BPDU for VLAN1 will be sent with a 802.1Q tag to MAC 0100.0ccc.cccd (with a PVID=1)
A PVST+ BPDU for the native vlan will be sent untagged to MAC 0100.0ccc.cccd (with a PVID=native vlan)
A PVST+ BPDU for all other vlans will be sent with a 802.1Q tag to MAC 0100.0ccc.cccd (with a PVID = to the VLAN)
Q189. Refer to the exhibit.
What does the return code 3 represent in this output?
A. The mapping of the replying router for the FEC is different.
B. The packet is label-switched at stack depth.
C. The return code is reserved.
D. The upstream index is unknown.
E. The replying router was the proper egress for the FEC.
Answer: E
Explanation:
Return Codes The Return Code is set to zero by the sender. The receiver can set it to one of the values listed below. The notation <RSC> refers to the Return Subcode. This field is filled in with the stack-depth for those codes that specify that. For all other codes, the Return Subcode MUST be set to zero.
Value Meaning
0 No return code
1 Malformed echo request received
2 One or more of the TLVs was not understood
3 Replying router is an egress for the FEC at stack-depth <RSC>
4 Replying router has no mapping for the FEC at stack-depth <RSC>
Reference: https://www.ietf.org/rfc/rfc4379.txt
Q190. The no ip unreachables command is configured on interfaces to protect the control plane of a router.
Which mechanism is impacted by using this command?
A. ICMP redirects
B. path MTU discovery
C. source routing
D. ICMP router discovery protocol
Answer: B