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2021 Nov ccie written dumps 400-101:
Q261. Which two statements about BPDU guard are true? (Choose two.)
A. The global configuration command spanning-tree portfast bpduguard default shuts down interfaces that are in the PortFast-operational state when a BPDU is received on that port.
B. The interface configuration command spanning-tree portfast bpduguard enable shuts down only interfaces with PortFast enabled when a BPDU is received.
C. BPDU guard can be used to prevent an access port from participating in the spanning tree in the service provider environment.
D. BPDU guard can be used to protect the root port.
E. BPDU guard can be used to prevent an invalid BPDU from propagating throughout the network.
Answer: A,C
Q262. 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
Q263. Refer to the exhibit.
Which statement about configuring the switch to manage traffic is true?
A. The switchport priority extend cos command on interface FastEthernet0/0 prevents traffic to and from the PC from taking advantage of the high-priority data queue that is assigned to the IP phone.
B. The switchport priority extend cos command on interface FastEthernet0/0 enables traffic to and from the PC to use the high priority data queue that is assigned to the IP phone.
C. When the switch is configured to trust the CoS label of incoming traffic, the trusted boundary feature is disabled automatically.
D. The mls qos cos override command on interface FastEthernet0/0 configures the port to trust the CoS label of traffic to and from the PC.
Answer: A
Explanation:
In some situations, you can prevent a PC connected to the Cisco IP Phone from taking advantage of a high-priority data queue. You can use the switchport priority extend cos interface configuration command to configure the telephone through the switch CLI to override the priority of the traffic received from the PC.
Reference: http://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst2950/software/release/12-1_22_ea2/configuration/guide/2950scg/swqos.html
Q264. DRAG DROP
Drag each routing protocol on the left to the matching statement on the right.
Answer:
Q265. DRAG DROP
Drag and drop each SNMP security model and level on the left to the corresponding mode of authentication on the right.
Answer:
Improved ccie dumps 400-101:
Q266. Which multicast protocol uses source trees and RPF?
A. DVMRP
B. PIM sparse mode
C. CBT
D. mOSPF
Answer: A
Explanation:
DVMRP builds a parent-child database using a constrained multicast model to build a forwarding tree rooted at the source of the multicast packets. Multicast packets are initially flooded down this source tree. If redundant paths are on the source tree, packets are not forwarded along those paths. Forwarding occurs until prune messages are received on those parent-child links, which further constrains the broadcast of multicast packets.
Reference: DVMRP and dense-mode PIM use only source trees and use RPF as previously described.
Reference: http://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst3550/software/release/12-1_19_ea1/configuration/guide/3550scg/swmcast.html
Q267. Which two statements about the command distance bgp 90 60 120 are true? (Choose two.)
A. Implementing the command is a Cisco best practice.
B. The external distance it sets is preferred over the internal distance.
C. The internal distance it sets is preferred over the external distance.
D. The local distance it sets may conflict with the EIGRP administrative distance.
E. The internal distance it sets may conflict with the EIGRP administrative distance.
F. The local distance it sets may conflict with the RIP administrative distance.
Answer: C,F
Explanation:
To allow the use of external, internal, and local administrative distances that could be a better route than other external, internal, or local routes to a node, use the distance bgp command in address family or router configuration mode. To return to the default values, use the no form of this command. distance bgp external-distance internal-distance local-distance no distance bgp
. Syntax Description
external-distance
Administrative distance for BGP external routes. External routes are routes for which the best path is learned from a neighbor external to the autonomous system. Accept table values are from 1 to 255. The default is 20. Routes with a distance of 255 are not installed in the routing table.
internal-distance
Administrative distance for BGP internal routes. Internal routes are those routes that are learned from another BGP entity within the same autonomous system. Accept table values are from 1 to 255. The default is 200. Routes with a distance of 255 are not installed in the routing table.
local-distance
Administrative distance for BGP local routes. Local routes are those networks listed with a network router configuration command, often as back doors, for that router or for networks that are being redistributed from another process. Accept table values are from 1 to 255. The default is 200. Routes with a distance of 255 are not installed in the routing table.
Defaults
external-distance: 20
internal-distance: 200
local-distance: 200
In this case, the internal distance is 60 and the external is 90, and the local distance is 120 (same as RIP).
Reference: http://www.cisco.com/c/en/us/td/docs/ios/12_2/iproute/command/reference/fiprrp_r/1rfbgp1. html#wp1113874
Q268. Refer to the exhibit.
ICMP Echo requests from host A are not reaching the intended destination on host B. What is the problem?
A. The ICMP payload is malformed.
B. The ICMP Identifier (BE) is invalid.
C. The negotiation of the connection failed.
D. The packet is dropped at the next hop.
E. The link is congested.
Answer: D
Explanation:
Here we see that the Time to Live (TTL) value of the packet is one, so it will be forwarded to the next hop router, but then dropped because the TTL value will be 0 at the next hop.
Q269. Which three statements about the differences between Cisco IOS and IOS-XE functionality are true? (Choose three.)
A. Only IOS-XE Software can host applications outside of the IOS context.
B. Only the IOS-XE Services Plane has multiple cores.
C. Only the IOS-XE Data Plane has multiple cores.
D. Only the IOS-XE Control Plane has multiple cores.
E. Only IOS-XE module management integrates with packet processing.
F. Only IOS-XE configuration and control is integrated with the kernel.
Answer: A,B,C
Q270. Refer to the exhibit.
Your network is suffering excessive output drops. Which two actions can you take to resolve the problem? (Choose two.)
A. Install a switch with larger buffers.
B. Configure a different queue set.
C. Reconfigure the switch buffers.
D. Configure the server application to use TCP.
E. Update the server operating system.
Answer: A,B
Explanation:
Installing a switch with larger buffers and correctly configuring the buffers can solve output queue problems.
For each queue we need to configure the assigned buffers. The buffer is like the ‘storage’ space for the interface and we have to divide it among the different queues. This is how to do it:
mls qos queue-set output <queue set> buffers Q1 Q2 Q3 Q4
In this example, there is nothing hitting queue 2 or queue 3 so they are not being utilized.