how many questions of 400-101 examcollection?

Answer: C,D,E 

Explanation: 

Key Advantages of using PfR for Load balancing: 

. Utilization based load-balancing: PfR takes real-time link utilization into account when load balancing the links. This will ensure that a link will not go beyond a certain percentage of its maximum capacity (75% by default). 

. Application Performance based Load Balancing: PfR does not randomly forward traffic through one link or another. It takes application performance requirements into consideration and then forwards the traffic through a link which meets the performance policy requirements. PfR also load balances the link at the same time. 

. Bi-directional Solution: PfR is a bi-directional load balancing solution which influences outbound as well as in-bound traffic. 

. Consolidated Centralized View: PfR offers consolidated and centralized view of the state of all external links in the network. At any given time, the network administrator can see the current link utilization (in kbps and percentage of its capacity), maximum link threshold, and the policies applied to the links in the network. 

Reference: http://docwiki.cisco.com/wiki/PfR:Solutions:InternetOutboundLoadBalancing 

Answer: A,B 

Explanation: 

Neighbor solicitation messages are sent on the local link when a node wants to determine the link-layer address of another node on the same local link (see the figure below). When a node wants to determine the link-layer address of another node, the source address in a neighbor solicitation message is the IPv6 address of the node sending the neighbor solicitation message. The destination address in the neighbor solicitation message is the solicited-node multicast address that corresponds to the IPv6 address of the destination node. The neighbor solicitation message also includes the link-layer address of the source node. 

Figure 1. IPv6 Neighbor Discovery: Neighbor Solicitation Message 

After receiving the neighbor solicitation message, the destination node replies by sending a neighbor advertisement message, which has a value of 136 in the Type field of the ICMP packet header, on the local link. The source address in the neighbor advertisement message is the IPv6 address of the node (more specifically, the IPv6 address of the node interface) sending the neighbor advertisement message. The destination address in the neighbor advertisement message is the IPv6 address of the node that sent the neighbor solicitation message. The data portion of the neighbor advertisement message includes the link-layer address of the node sending the neighbor advertisement message. After the source node receives the neighbor advertisement, the source node and destination node can communicate. 

Reference: http://www.cisco.com/c/en/us/td/docs/ios-xml/ios/ipv6_basic/configuration/xe-3s/ip6b-xe-3s-book/ip6-neighb-disc-xe.html 

Answer: D 

Explanation: 

For IP PIM multicast, Cisco recommends Sparse-Mode over Dense-Mode. In the midst of our network migration, we have a new network operating in Sparse-Mode with Anycast rendezvous point (RP) but our existing network is still operating in Dense-Mode. To bridge two different modes across both PIM domains, we should use the ip pim dense-mode proxy-register command on the interface leading toward the bordering dense mode region. This configuration will enable the border router to register traffic from the dense mode region (which has no concept of registration) with the RP in the sparse mode domain. 

Reference: http://networkerslog.blogspot.com/2010/12/bridging-dense-mode-pim-to-sparse-mode.html 

Answer:  

Answer: B 

Explanation: 

The following example shows how to configure the router (and downstream routers) to drop all options packets that enter the network: Router(config)# ip options drop 

Reference: http://www.cisco.com/c/en/us/td/docs/ios/12_0s/feature/guide/sel_drop.html 

Answer: A,B,C 

Answer: A,D 

Explanation: 

There are two ways we can create EIGRP neighbor relationship: 

+ Use “network ” command. This is the more popular way to create EIGRP neighbor relationship. That router will check which interfaces whose IP addresses belong to the and turn EIGRP on that interface. EIGRP messages are sent via multicast packets. 

+ Use “neighbor” commanD. The interface(s) that have this command applied no longer send or receive EIGRP multicast packets. EIGRP messages are sent via unicast. The router only accepts EIGRP packets from peers that are explicitly configured with a neighbor statement. Consequently, any messages coming from routers without a corresponding neighbor statement are discarded. This helps prevent the insertion of unauthorized routing peers -> A and D are correct. 

Answer:  

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 

Answer: A,B