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Q21. Which statement about the OSPF Loop-Free Alternate feature is true?
A. It is supported on routers that are configured with virtual links.
B. It is supported in VRF OSPF instances.
C. It is supported when a traffic engineering tunnel interface is protected.
D. It is supported when traffic can be redirected to a primary neighbor.
Answer: B
Explanation:
Restrictions for OSPF IPv4 Remote Loop-Free Alternate IP Fast Reroute
. The OSPF IPv4 Remote Loop-Free Alternate IP Fast Reroute feature is not supported on devices that are virtual links headends.
. The feature is supported only in global VPN routing and forwarding (VRF) OSPF
Instances.
. The only supported tunneling method is MPLS.
. You cannot configure a traffic engineering (TE) tunnel interface as a protected interface. Use the MPLS Traffic Engineering—Fast Reroute Link and Node Protection feature to protect these tunnels. For more information, see the “MPLS Traffic Engineering—Fast Reroute Link and Node Protection” section in the Multiprotocol Label Switching Configuration Guide.
. You can configure a TE tunnel interface in a repair path, but OSPF will not verify the tunnel’s placement; you must ensure that it is not crossing the physical interface that it is intended to protect.
. Not all routes can have repair paths. Multipath primary routes might have repair paths for all, some, or no primary paths, depending on the network topology, the connectivity of the computing router, and the attributes required of repair paths.
. Devices that can be selected as tunnel termination points must have a /32 address advertised in the area in which remote LFA is enabled. This address will be used as a tunnel termination IP. If the device does not advertise a /32 address, it may not be used for remote LFA tunnel termination.
. All devices in the network that can be selected as tunnel termination points must be configured to accept targeted LDP sessions using the mpls ldp discovery targeted-hello accept command.
Reference: http://www.cisco.com/c/en/us/td/docs/ios-xml/ios/iproute_ospf/configuration/xe-3s/iro-xe-3s-book/iro-ipfrr-lfa.html
Q22. 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
Q23. Refer to the exhibit.
R1, R2, and R3 have full network connectivity to each other, but R2 prefers the path through R3 to reach network 172.17.1.0/24. Which two actions can you take so that R2 prefers the path through R1 to reach 172.17.1.0/24? (Choose two.)
A. Set the reference bandwidth to 10000 on R1, R2, and R3.
B. Configure the cost on the link between R1 and R3 to be greater than 100 Mbps.
C. Set the reference bandwidth on R2 only.
D. Configure a manual bandwidth statement with a value of 1 Gbps on the link between R1 and R3.
E. Modify the cost on the link between R1 and R2 to be greater than 10 Gbps.
F. Configure a manual bandwidth statement with a value of 100 Mbps on the link between R1 and R2.
Answer: A,B
Explanation:
By default, the reference bandwidth used in Cisco routers is 100Mbps, so FastEthernet and above will have a cost of 1, so a gigabit interface and 10GE interface will be equal with a fastethernet. This is not ideal. If we change the reference bandwidth to 100000 then the faster links will be used. Changing the reference bandwidth needs to be done on all routers in the OSPF network. Increasing the cost on the R1-R3 link will also cause the traffic to take the more direct route.
Q24. Which value is the maximum segment size if you start with an MTU of 1500 bytes and then remove the overhead of the Ethernet header, IP header, TCP header, and the MAC frame check sequence?
A. 1434 bytes
B. 1460 bytes
C. 1458 bytes
D. 1464 bytes
Answer: B
Q25. Which bit should be set in the link-state PDU of an IS-IS L1/L2 router to indicate that it is a potential exit point of the area?
A. the ABR bit
B. the ATT bit
C. the down bit
D. the P bit
Answer: B
Explanation:
Default routing is achieved in two distinct ways with Integrated IS-IS:
. Attached bit—Set by a Level 1/Level 2 router in its own Level 1 LSP and used to indicate to all Level 1 routers (within the area) that this router is a potential exit point of the area. Level 1-only routers will default to the nearest attached Level 2 router.
. Default information originate—Can be configured in Level 1 as well as Level 2. The default route (0.0.0.0/0) is inserted in the router LSP (Level 1 or Level 2, according to the configuration command) and the LSP is flooded according to the router type (Level 1 or Level 2). A Level 2 router doesn't need to have a default route to originate a default route.
Reference: http://www.cisco.com/en/US/products/ps6599/products_white_paper09186a00800a3e6f.sh tml
Renovate 400-101 practice test:
Q26. Which packet does a router receive if it receives an OSPF type 4 packet?
A. hello packet
B. database descriptor packet
C. link state update packet
D. link state request packet
E. link state acknowledge packet
Answer: C
Q27. Which two statements about the passive-interface command are true? (Choose two.)
A. A RIP router listens to multicast updates from its neighbor but stops sending multicast updates on the passive interface.
B. In OSPF, configuring passive-interface at the interface level suppresses hello packets for the interface and all sub interfaces.
C. An EIGRP router can form neighbor relationship on the passive interface, but incoming and outgoing multicast updates are disabled on the interface.
D. A RIP router disables all incoming and outgoing multicast updates in the passive interface.
E. In EIGRP, the passive interface stops sending hello packets.
F. In OSPF, the passive interface can receive incoming routing updates and update the device routing table.
Answer: A,E
Q28. How does EIGRP derive the metric for manual summary routes?
A. It uses the best composite metric of any component route in the topology table.
B. It uses the worst composite metric of any component route in the topology table.
C. It uses the best metric vectors of all component routes in the topology table.
D. It uses the worst metric vectors of all component routes in the topology table.
Answer: A
Explanation:
For example if your router has a routing table like this:
D 192.168.8.0/24 [90/2632528] via 192.168.0.1, 00:00:12, Serial0/0
D 192.168.9.0/24 [90/2323456] via 192.168.0.1, 00:00:12, Serial0/0
D 192.168.10.0/24 [90/2195456] via 192.168.0.1, 00:00:12, Serial0/0
D 192.168.11.0/24 [90/2323456] via 192.168.0.1, 00:00:12, Serial0/0
Now suppose you want to manually summarize all the routes above, you can use this command (on the router that advertised these routes to our router):
Router(config-if)#ip summary-address eigrp 1 192.168.8.0 255.255.248.0
After that the routing table of your router will look like this:
D 192.168.8.0/21 [90/2195456] via 192.168.0.1, 00:01:42, Serial0/0
And we can see the manual summary route takes the smallest metric of the specific routes.
Q29. Which statement is true about trunking?
A. Cisco switches that run PVST+ do not transmit BPDUs on nonnative VLANs when using a dot1q trunk.
B. When removing VLAN 1 from a trunk, management traffic such as CDP is no longer passed in that VLAN.
C. DTP only supports autonegotiation on 802.1q and does not support autonegotiation for ISL.
D. DTP is a point-to-point protocol.
Answer: D
Explanation:
Ethernet trunk interfaces support different trunking modes. You can set an interface as trunking or nontrunking or to negotiate trunking with the neighboring interface. To autonegotiate trunking, the interfaces must be in the same VTP domain. Trunk negotiation is managed by the Dynamic Trunking Protocol (DTP), which is a Point-to-Point Protocol. However, some internetworking devices might forward DTP frames improperly, which could cause misconfigurations.
Reference: http://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst3750/software/release/12-2_55_se/configuration/guide/scg3750/swvlan.html
Q30. Which three conditions can cause excessive unicast flooding? (Choose three.)
A. Asymmetric routing
B. Repeated TCNs
C. The use of HSRP
D. Frames sent to FFFF.FFFF.FFFF
E. MAC forwarding table overflow
F. The use of Unicast Reverse Path Forwarding
Answer: A,B,E
Explanation:
Causes of Flooding
The very cause of flooding is that destination MAC address of the packet is not in the L2 forwarding table of the switch. In this case the packet will be flooded out of all forwarding ports in its VLAN (except the port it was received on). Below case studies display most
common reasons for destination MAC address not being known to the switch.
Cause 1: Asymmetric Routing
Large amounts of flooded traffic might saturate low-bandwidth links causing network performance issues or complete connectivity outage to devices connected across such low-bandwidth links.
Cause 2: Spanning-Tree Protocol Topology Changes
Another common issue caused by flooding is Spanning-Tree Protocol (STP) Topology Change Notification (TCN). TCN is designed to correct forwarding tables after the forwarding topology has changed. This is necessary to avoid a connectivity outage, as after a topology change some destinations previously accessible via particular ports might become accessible via different ports. TCN operates by shortening the forwarding table aging time, such that if the address is not relearned, it will age out and flooding will occur. TCNs are triggered by a port that is transitioning to or from the forwarding state. After the TCN, even if the particular destination MAC address has aged out, flooding should not happen for long in most cases since the address will be relearned. The issue might arise when TCNs are occurring repeatedly with short intervals. The switches will constantly be fast-aging their forwarding tables so flooding will be nearly constant. Normally, a TCN is rare in a well-configured network. When the port on a switch goes up or down, there is eventually a TCN once the STP state of the port is changing to or from forwarding. When the port is flapping, repetitive TCNs and flooding occurs.
Cause 3: Forwarding Table Overflow
Another possible cause of flooding can be overflow of the switch forwarding table. In this case, new addresses cannot be learned and packets destined to such addresses are flooded until some space becomes available in the forwarding table. New addresses will then be learned. This is possible but rare, since most modern switches have large enough forwarding tables to accommodate MAC addresses for most designs. Forwarding table exhaustion can also be caused by an attack on the network where one host starts generating frames each sourced with different MAC address. This will tie up all the forwarding table resources. Once the forwarding tables become saturated, other traffic will be flooded because new learning cannot occur. This kind of attack can be detected by examining the switch forwarding table. Most of the MAC addresses will point to the same port or group of ports. Such attacks can be prevented by limiting the number of MAC addresses learned on untrusted ports by using the port security feature.
Reference: http://www.cisco.com/c/en/us/support/docs/switches/catalyst-6000-series-switches/23563-143.html#causes