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2021 Mar 400-101 practice
Q71. Which three statements about bridge assurance are true? (Choose three.)
A. Bridge assurance must be enabled on both ends of a link.
B. Bridge assurance can be enabled on one end of a link or on both ends.
C. Bridge assurance is enabled on STP point-to-point links only.
D. Bridge assurance is enabled on STP multipoint links only.
E. If a bridge assurance port fails to receive a BPDU after a timeout, the port is put into a blocking state.
F. If a bridge assurance port fails to receive a BPDU after a timeout, the port is put into an error disabled state.
Answer: A,C,E
Explanation:
Bridge Assurance is enabled by default and can only be disabled globally. Also, Bridge Assurance can be enabled only on spanning tree network ports that are point-to-point links.
Finally, both ends of the link must have Bridge Assurance enabled.
With Bridge Assurance enabled, BPDUs are sent out on all operational network ports, including alternate and backup ports, for each hello time period. If the port does not receive a BPDU for a specified period, the port moves into the blocking state and is not used in the root port calculation. Once that port receives a BPDU, it resumes the normal spanning tree transitions.
Reference:
http://www.cisco.com/c/en/us/td/docs/switches/datacenter/nexus5000/sw/configuration/guid e/cli/CLIConfigurationGuide/SpanningEnhanced.html
Q72. A company is multihomed to several Internet providers using EBGP. Which two measures guarantee that the network of the company does not become a transit AS for Internet traffic? (Choose two.)
A. Prepend three times the AS number of the company to the AS path list.
B. Add the community NO_EXPORT when sending updates to EBGP neighbors.
C. Write AS-path access-list which permits one AS long paths only and use it to filter updates sent to EBGP neighbors.
D. Add the community NO_EXPORT when receiving updates from EBGP neighbors.
Answer: C,D
Explanation:
By default BGP will advertise all prefixes to EBGP (External BGP) neighbors. This means that if you are multi-homed (connected to two or more ISPs) that you might become a transit AS. Let me show you an example:
R1 is connected to ISP1 and ISP2 and each router is in a different AS (Autonomous System). Since R1 is multi-homed it’s possible that the ISPs will use R1 to reach each other. In order to prevent this we’ll have to ensure that R1 only advertises prefixes from its own autonomous system. As far as I know there are 4 methods how you can prevent becoming a transit AS:
Filter-list with AS PATH access-list.
No-Export Community.
Prefix-list Filtering
Distribute-list Filtering
Reference: http://networklessons.com/bgp/bgp-prevent-transit-as/
Q73. Which two statements about redistribution are true? (Choose two.)
A. EIGRP requires the route to have a default metric defined.
B. EIGRP and OSPF use their router IDs to prevent loops.
C. When OSPF is redistributed into IS-IS, the default metric must be configured under the IS-IS process.
D. When traffic is redistributed into OSPF, the subnets command is needed to redistribute classful subnets.
E. The default seed metric for OSPF redistributed routes is 30.
Answer: A,B
Q74. Refer to the exhibit.
Which statement is true?
A. This is an MPLS TE point-to-multipoint LSP in an MPLS network.
B. This is an MPLS TE multipoint-to-point LSP in an MPLS network.
C. This is a point-to-multipoint LSP in an MPLS network.
D. This is a multipoint-to-multipoint LSP in an MPLS network.
Answer: D
Explanation:
Same example of this provided on slide 24 at the reference link below:
Reference: “mVPN Deployment Models” Cisco Live Presentation
http://d2zmdbbm9feqrf.cloudfront.net/2014/eur/pdf/BRKIPM-2011.pdf, slide 24
Q75. 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
Far out 400-101 exam guide:
Q76. 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
Q77. What does a nonzero forwarding address indicate in a type-5 LSA?
A. It indicates that this link-state ID is eligible for ECMP.
B. It indicates that this router should have an OSPF neighbor relationship with the forwarding address before using this link-state ID.
C. It indicates that the receiving router must check that the next hop is reachable in its routing table before using this link-state ID.
D. It indicates that traffic can be directly routed to this next hop in shared segment scenarios where the external route source is directly connected.
Answer: D
Explanation:
The value of the forwarding address specified by the autonomous system boundary router (ASBR) can be either 0.0.0.0 or non-zero. The 0.0.0.0 address indicates that the originating router (the ASBR) is the next hop. The forwarding address is determined by these conditions:
. The forwarding address is set to 0.0.0.0 if the ASBR redistributes routes and OSPF is not enabled on the next hop interface for those routes.
. These conditions set the forwarding address field to a non-zero address: ASBR's next hop interface address falls under the network range specified in the router ospf command.
Reference: http://www.cisco.com/c/en/us/support/docs/ip/open-shortest-path-first-ospf/13682-10.html
Q78. Which two statements about BGP best-path selection are true? (Choose two.)
A. The route with the highest local preference is preferred.
B. The weight attribute is advertised to peers.
C. The route with the lowest MED is preferred.
D. A route that originates from iBGP peers is preferred.
E. A route that originates from a router with a higher BGP router ID is preferred.
F. The lowest weight advertised is preferred.
Answer: A,C
Q79. Which two options are advantages of NetFlow version 9 over NetFlow version 5? (Choose two.)
A. NetFlow version 9 adds support for IPv6 headers.
B. NetFlow version 9 adds support for MPLS labels.
C. NetFlow version 9 adds support for the Type of Service field.
D. NetFlow version 9 adds support for ICMP types and codes.
Answer: A,B
Explanation:
NetFlow version 9 includes support for all of these fields that version 5 supports and can optionally include additional information such as Multiprotocol Label Switching (MPLS) labels and IPv6 addresses and ports.
Q80. Which two statements about OSPFv3 are true? (Choose two.)
A. It supports unicast address families for IPv4 and IPv6.
B. It supports unicast address families for IPv6 only.
C. It supports only one address family per instance.
D. It supports the use of a cluster ID for loop prevention.
E. It supports multicast address families for IPv4 and IPv6.
F. It supports multicast address families for IPv6 only.
Answer: A,C