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2021 Apr 400-101 test

Q171. Refer to the exhibit. 

If IS-IS is configured utilizing default metrics, what is the cost for Router 4 to reach the 10.2.2.0/24 network? 

A. 1 

B. 20 

C. 30 

D. 63 

Answer: C 

Explanation: 

By default, every link is an IS-IS network has a metric of 10. 


Q172. Which two statements about the function of a PIM designated router are true? (Choose two.) 

A. It forwards multicast traffic from the source into the PIM network. 

B. It registers directly connected sources to the PIM rendezvous point. 

C. It sends PIM Join/Prune messages for directly connected receivers. 

D. It sends IGMP queries. 

E. It sends PIM asserts on the interfaces of the outgoing interface list. 

Answer: B,C 

Explanation: 

In PIM ASM and SSM modes, the software chooses a designated router (DR) from the routers on each network segment. The DR is responsible for forwarding multicast data for specified groups and sources on that segment. In ASM mode, the DR is responsible for unicasting PIM register packets to the RP. When a DR receives an IGMP membership report from a directly connected receiver, the shortest path is formed to the RP, which may or may not go through the DR. The result is a shared tree that connects all sources transmitting on the same multicast group to all receivers of that group. In SSM mode, the DR triggers (*, G) or (S, G) PIM join messages toward the RP or the source. The path from the receiver to the source is determined hop by hop. The source must be known to the receiver or the DR. 

Reference: http://www.cisco.com/c/en/us/td/docs/switches/datacenter/sw/5_x/nx-os/multicast/configuration/guide/n7k_multic_cli_5x/pim.html#wp1054047 


Q173. Refer to the exhibit. 

Which statement is true? 

A. 2001:DB8::1/128 is a local host route, and it can be redistributed into a dynamic routing protocol. 

B. 2001:DB8::1/128 is a local host route, and it cannot be redistributed into a dynamic routing protocol. 

C. 2001:DB8::1/128 is a local host route that was created because ipv6 unicast-routing is not enabled on this router. 

D. 2001:DB8::1/128 is a route that was put in the IPv6 routing table because one of this router's loopback interfaces has the IPv6 address 2001:DB8::1/128. 

Answer: B 

Explanation: 

The local routes have the administrative distance of 0. This is the same adminstrative distance as connected routes. However, when you configure redistributed connected under any routing process, the connected routes are redistributed, but the local routes are not. This behavior allows the networks to not require a large number of host routes, because the networks of the interfaces are advertised with their proper masks. These host routes are only needed on the router that owns the IP address in order to process packets destined to that IP address. 

It is normal for local host routes to be listed in the IPv4 and IPv6 routing table for IP addresses of the router's interfaces. Their purpose is to create a corresponding CEF entry as a receive entry so that the packets destined to this IP address can be processed by the router itself. These routes cannot be redistributed into any routing protocol. 

Reference: http://www.cisco.com/c/en/us/support/docs/ip/ip-routing/116264-technote-ios-00.html 


Q174. Which command sets the maximum segment size for a TCP packet initiated from a router? 

A. ip mtu 

B. ip tcp adjust-mss 

C. ip tcp mss 

D. ip tcp window-size 

Answer: C 


Q175. Which two mechanisms can be used to eliminate Cisco Express Forwarding polarization? (Choose two.) 

A. alternating cost links 

B. the unique-ID/universal-ID algorithm 

C. Cisco Express Forwarding antipolarization 

D. different hashing inputs at each layer of the network 

Answer: B,D 

Explanation: 

This document describes how Cisco Express Forwarding (CEF) polarization can cause suboptimal use of redundant paths to a destination network. CEF polarization is the effect when a hash algorithm chooses a particular path and the redundant paths remain completely unused. 

How to Avoid CEF Polarization 

. Alternate between default (SIP and DIP) and full (SIP + DIP + Layer4 ports) hashing inputs configuration at each layer of the network. 

. Alternate between an even and odd number of ECMP links at each layer of the network.The CEF load-balancing does not depend on how the protocol routes are inserted in the routing table. Therefore, the OSPF routes exhibit the same behavior as EIGRP. In a hierarchical network where there are several routers that perform load-sharing in a row, they all use same algorithm to load-share. 

The hash algorithm load-balances this way by default: 

1: 1 

2: 7-8 

3: 1-1-1 

4: 1-1-1-2 

5: 1-1-1-1-1 

6: 1-2-2-2-2-2 

7: 1-1-1-1-1-1-1 

8: 1-1-1-2-2-2-2-2 

The number before the colon represents the number of equal-cost paths. The number after the colon represents the proportion of traffic which is forwarded per path. 

This means that: 

For two equal cost paths, load-sharing is 46.666%-53.333%, not 50%-50%. 

For three equal cost paths, load-sharing is 33.33%-33.33%-33.33% (as expected). 

For four equal cost paths, load-sharing is 20%-20%-20%-40% and not 25%-25%-25%-25%. 

This illustrates that, when there is even number of ECMP links, the traffic is not load-balanced. 

.Cisco IOS introduced a concept called unique-ID/universal-ID which helps avoid CEF polarization. This algorithm, called the universal algorithm (the default in current Cisco IOS versions), adds a 32-bit router-specific value to the hash function (called the universal ID - this is a randomly generated value at the time of the switch boot up that can can be manually controlled). This seeds the hash function on each router with a unique ID, which ensures that the same source/destination pair hash into a different value on different routers along the path. This process provides a better network-wide load-sharing and circumvents the polarization issue. This unique -ID concept does not work for an even number of equal-cost paths due to a hardware limitation, but it works perfectly for an odd number of equal-cost paths. In order to overcome this problem, Cisco IOS adds one link to the hardware adjacency table when there is an even number of equal-cost paths in order to make the system believe that there is an odd number of equal-cost links. 

Reference: http://www.cisco.com/c/en/us/support/docs/ip/express-forwarding-cef/116376-technote-cef-00.html 


Up to the minute 400-101 exam:

Q176. While troubleshooting an issue for a remote user, you must capture the communication between the user's computer and a server at your location. The traffic passes through a Cisco IOS-XE capable switch. Which statement about obtaining the capture is true? 

A. The Embedded Packet Capture application in the IOS-XE Software can capture the packets, but there is a performance impact. 

B. The Embedded Packet Capture application in the IOS-XE Software can capture the packets without impacting performance. 

C. The Mini Protocol Analyzer embedded in the IOS-XE Software can capture the packets without impacting performance. 

D. The Mini Protocol Analyzer embedded in the IOS-XE Software can be used to capture the packets, but there is a performance impact. 

E. Wireshark can capture packets through a SPAN port, but there is a performance impact. 

Answer: A 


Q177. 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. 


Q178. How are the Cisco Express Forwarding table and the FIB related to each other? 

A. The FIB is used to populate the Cisco Express Forwarding table. 

B. The Cisco Express Forwarding table allows route lookups to be forwarded to the route processor for processing before they are 

C. There can be only one FIB but multiple Cisco Express Forwarding tables on IOS devices. 

D. Cisco Express Forwarding uses a FIB to make IP destination prefix-based switching decisions. 

Answer: D 


Q179. DRAG DROP 

Drag and drop the NHRP flag on the left to the corresponding meaning on the right. 

Answer: 


Q180. When you implement PfR, which IP SLA probe is used to determine the MOS? 

A. jitter 

B. latency 

C. packet loss 

D. throughput 

Answer: A