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Q491. An access switch at a remote location is connected to the spanning-tree root with redundant uplinks. A network engineer notices that there are issues with the physical cabling of the current root port. The engineer decides to force the secondary link to be the desired forwarding root port. Which action accomplishes this task?
A. Adjust the secondary link to have a lower priority than the primary link.
B. Change the link type to point-to-point.
C. Apply a BPDU filter on the primary interface of the remote switches.
D. Enable Rapid Spanning Tree to converge using the secondary link.
Answer: A
Q492. DRAG DROP
Drag and drop the BGP attribute on the left to the correct category on the right.
Answer:
Q493. 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 forwarding traffic after STP converges? (Choose two.)
A. The port connecting switch SWD with switch SWE
B. The port connecting switch SWG with switch SWF
C. The port connecting switch SWC with switch SWE
D. The port connecting switch SWB with switch SWC
Answer: C,D
Explanation:
Here, we know SWB to SWC are forwarding because we already identified the blocking port. So for the last correct answer let’s consider what must be done to prevent a switch loop between SWC/SWD/SWE. SWE to SWD will be blocked because SWC has a lower MAC address so it wins the forwarding port. And to look at it further, you could try to further understand what would happen with ports on SWG. Would the ports on SWG try to go through SWE or SWF? SWE has the lower MAC address so the port from SWG to SWE would win the forwarding election. Therefore, answer B could never be correct.
Q494. Refer to the exhibit.
Why is the router out of memory?
A. The router is experiencing a BGP memory leak software defect.
B. The BGP peers have been up for too long.
C. The amount of BGP update traffic in the network is too high.
D. The router has insufficient memory due to the size of the BGP database.
Answer: D
Explanation:
Here we can see that this router is running out of memory due to the large size of the BGP routing database. In this case, this router is receiving over 200,000 routes from each of the 4 peers.
Q495. Refer to the exhibit.
Which statement is true?
A. R2 is directly connected to the receiver for this group and is the winner of an assert mechanism.
B. R2 is directly connected to the receiver for this group, and it forwards the traffic onto Ethernet3/0, but it is forwarding duplicate traffic onto Ethernet3/0.
C. R2 has the A flag (Accept flag) set on Ethernet 3/0. This is fine, since the group is in BIDIR-PIM mode.
D. R2 is directly connected to the receiver for this group and is the loser of an assert mechanism.
E. The A flag is set until the SPT threshold is reached for this multicast group.
Answer: A
Explanation:
show ip mroute Field Descriptions
Field
Description
RPF neighbor or RPF nbr
IP address of the upstream router to the source. Tunneling indicates that this router is sending data to the RP encapsulated in register packets. The hexadecimal number in parentheses indicates to which RP it is registering. Each bit indicates a different RP if multiple RPs per group are used. If an asterisk (*) appears after the IP address in this field, the RPF neighbor has been learned through an assert.
Reference: http://www.cisco.com/c/en/us/td/docs/ios/12_2/ipmulti/command/reference/fiprmc_r/1rfmult 3.html
Q496. 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
Q497. Refer to the exhibit.
What is a reason for the RIB-failure?
A. CEF is not enabled on this router.
B. The route 10.100.1.1/32 is in the routing table, but not as a BGP route.
C. The routing table has yet to be updated with the BGP route.
D. The BGP route is filtered inbound and hence is not installed in the routing table.
Answer: B
Explanation:
A rib-failure occurs when BGP tries to install the bestpath prefix into the RIB, but the RIB rejects the BGP route because a route with better administrative distance already exists in the routing table. An inactive Border Gateway Protocol (BGP) route is a route that is not installed in the RIB, but is installed in the BGP table as rib-failure. Example Topology Router 1 (R1) and router 2 (R2) have two parallel links; one links runs BGP AS 65535 and the other link runs Enhanced Interior Gateway Routing Protocol (EIGRP) AS 1. Both BGP and EIGRP are advertising the network 10.1.1.1/32 on R1.
R2 learns about the 1.1.1.1/32 route through both EIGRP and BGP, but installs only the EIGRP route in the routing table because of the lower administrative distance. Since the BGP route is not installed in the R2 routing table, the route appears as a rib-failure in the R2 BGP table.
Reference: http://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/116146-config-bgp-next-hop-00.html
Q498. What is the hop limit for an MLD message?
A. 1
B. 2
C. 15
D. 255
Answer: A
Explanation:
MLD uses the Internet Control Message Protocol (ICMP) to carry its messages. All MLD messages are link-local with a hop limit of 1, and they all have the alert option set. The alert option implies an implementation of the hop-by-hop option header.
Reference: http://www.cisco.com/c/en/us/td/docs/ios-xml/ios/ipmulti_lsm/configuration/xe-3s/imc-lsm-xe-3s-book/ipv6-mcast-mld-xe.html
Q499. Which statement about VRRP is true?
A. It supports load balancing.
B. It can be configured with HSRP on a switch or switch stack.
C. It supports IPv4 and IPv6.
D. It supports encrypted authentication.
Answer: B
Explanation:
VRRP Limitations
. You can configure both HSRP and VRRP on a switch or switch stack. However, you cannot add a switch model that supports only one protocol to a stack that is configured for both protocols.
. The VRRP implementation on the switch does not support the MIB specified in RFC 2787.
. The VRRP implementation on the switch supports only text -based authentication.
. The switch supports VRRP only for IPv4.
Reference: http://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst3750x_3560x/software/release/1 2-2_58_se/configuration/guide/3750xscg/swhsrp.html#pgfId-1107127