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Exam Code: 400-101
Exam Name: CCIE Routing and Switching (v5.0)
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Q391. Which two OSPF network types require the use of a DR and BDR? (Choose two.)
A. non-broadcast networks
B. point-to-point networks
C. point-to-multipoint networks
D. broadcast networks
E. point-to-multipoint non-broadcast networks
Q392. Which switching technology can be used to solve reliability problems in a switched network?
A. fragment-free mode
B. cut-through mode
C. check mode
D. store-and-forward mode
Characteristics of Store-and-Forward Ethernet Switching
This section provides an overview of the functions and features of store-and-forward Ethernet switches.
Figure 1 shows a store-and-forward switch receiving an Ethernet frame in its entirety. At the end of that frame, the switch will compare the last field of the datagram against its own frame-check-sequence (FCS) calculations, to help ensure that the packet is free of physical and data-link errors. The switch then performs the forwarding process. Whereas a store-and-forward switch solves reliability issues by dropping invalid packets, cut-through devices forward them because they do not get a chance to evaluate the FCS before transmitting the packet.
Figure 1. Ethernet Frame Entering a Store-and-Forward Bridge or Switch (from Left to Right)
Q393. Which term describes an EIGRP route that has feasible successors?
A topology table entry for a destination can have one of two states. A route is considered in the Passive state when a router is not performing a route recomputation. The route is in Active state when a router is undergoing a route recomputation. If there are always feasible successors, a route never has to go into Active state and avoids a route recomputation.
When there are no feasible successors, a route goes into Active state and a route recomputation occurs. A route recomputation commences with a router sending a query packet to all neighbors. Neighboring routers can either reply if they have feasible successors for the destination or optionally return a query indicating that they are performing a route recomputation. While in Active state, a router cannot change the next-hop neighbor it is using to forward packets. Once all replies are received for a given query, the destination can transition to Passive state and a new successor can be selected.
Q394. 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
Q395. Which object tracking function tracks the combined states of multiple objects?
Q396. What is the goal of Unicast Reverse Path Forwarding?
A. to verify the reachability of the destination address in forwarded packets
B. to help control network congestion
C. to verify the reachability of the destination address in multicast packets
D. to verify the reachability of the source address in forwarded packets
Network administrators can use Unicast Reverse Path Forwarding (Unicast RPF) to help limit the malicious traffic on an enterprise network. This security feature works by enabling a router to verify the reachability of the source address in packets being forwarded. This capability can limit the appearance of spoofed addresses on a network. If the source IP address is not valid, the packet is discarded.
Q397. Which statement describes Cisco PfR link groups?
A. Link groups enable Cisco PfR Fast Reroute when NetFlow is enabled on the external interfaces of the border routers.
B. Link groups define a strict or loose hop-by-hop path pReference:
C. Link groups are required only when Cisco PfR is configured to load-balance all traffic.
D. Link groups are enabled automatically when Cisco PfR is in Fast Reroute mode.
E. Link groups set a preference for primary and fallback (backup) external exit interfaces.
The Performance Routing - Link Groups feature introduced the ability to define a group of exit links as a preferred set of links, or a fallback set of links for PfR to use when optimizing traffic classes specified in an PfR policy. PfR currently selects the best link for a traffic class based on the preferences specified in a policy and the traffic class performance—using parameters such as reachability, delay, loss, jitter or MOS—on a path out of the specified link.
Q398. Which statement describes the effect of the configuration line redistribute maximum-prefix 1500 90 withdraw?
A. After the 1500th route is redistributed, a warning is posted in the log file and 90 more routes are redistributed before further routes are discarded.
B. After the 1350th route is redistributed, a warning is posted in the log file until the 1500th route is redistributed, and then further routes are discarded.
C. After the 1500th route is redistributed, further routes are discarded only if the CPU is above 90%.
D. The routing protocol receives 1500 routes. After the routing process has redistributed 90% of the routes, the process supernets routes and injects a NULL route to prevent black-hole routing.
Q399. Which statement is true when using a VLAN ID from the extended VLAN range (1006–4094)?
A. VLANs in the extended VLAN range can be used with VTPv2 in either client or server mode.
B. VLANs in the extended VLAN range can only be used as private VLANs.
C. STP is disabled by default on extended-range VLANs.
D. VLANs in the extended VLAN range cannot be pruned.
Enabling VTP pruning on a VTP server enables pruning for the entire management domain. Making VLANs pruning-eligible or pruning-ineligible affects pruning eligibility for those VLANs on that device only (not on all switches in the VTP domain). VTP pruning takes effect several seconds after you enable it. VTP pruning does not prune traffic from VLANs that are pruning-ineligible. VLAN 1 and VLANs 1002 to 1005 are always pruning-ineligible; traffic from these VLANs cannot be pruned. Extended-range VLANs (VLAN IDs higher than 1005) are also pruning-ineligible.
Q400. What is a cause for unicast flooding?
A. Unicast flooding occurs when multicast traffic arrives on a Layer 2 switch that has directly connected multicast receivers.
B. When PIM snooping is not enabled, unicast flooding occurs on the switch that interconnects the PIM-enabled routers.
C. A man-in-the-middle attack can cause the ARP cache of an end host to have the wrong MAC address. Instead of having the MAC address of the default gateway, it has a MAC address of the man-in-the-middle. This causes all traffic to be unicast flooded through the man-in-the-middle, which can then sniff all packets.
D. Forwarding table overflow prevents new MAC addresses from being learned, and packets destined to those MAC addresses are flooded until space becomes available in the forwarding table.
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
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.
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