Cisco 400-101 ExamCCIE Routing and Switching (v5.0)

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2016 Aug 400-101 cisco ccie written:

Q181. 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 in blocking state after STP converges? (Choose two.) 

A. the port on switch SWD that connects to switch SWE 

B. the port on switch SWF that connects to switch SWG 

C. the port on switch SWD that connects to switch SWC 

D. the port on switch SWB that connects to switch SWD 

Answer: C,D 

Explanation: 

This is a scenario that wants you to demonstrate understanding of the Root switch and Root port election process. So, it’s best to start with where the root switch will be and work down from there. It’s setup nicely because the lowest MAC address switch starts at the top and then the lower priority/higher mac addresses move down the architecture. SWA wins the root election and of course all ports in SWA are forwarding. SWB introduces the possibility for a switching loop so it’s important to understand which ports will be put into the blocking state. Since SWD is a higher MAC address it will end up with a blocked port connected to SWB to prevent a loop: and this is one of the correct answers. To prevent the possibility of another potential switching loop, SWD again ends up with the higher MAC address so blocking the link between D and C prevents a B/C/D switching loop. 


Q182. Which two loop-prevention mechanisms are implemented in BGP? (Choose two.) 

A. A route with its own AS in the AS_PATH is dropped automatically if the route reenters its own AS. 

B. A route with its own cluster ID in the CLUSTER_LIST is dropped automatically when the route reenters its own AS. 

C. The command bgp allowas-in enables a route with its own AS_PATH to be dropped when it reenters its own AS. 

D. The command bgp bestpath as-path ignore enables the strict checking of AS_PATH so that they drop routes with their own AS in the AS_PATH. 

E. The command bgp bestpath med missing-as-worst assigns the smallest possible MED, which directly prevents a loop. 

Answer: A,B 

Explanation: 

When dealing with the possibility of routing updates making their way back into an AS, BGP relies on the information in the AS_path for loop detection. An update that tries to make its way back into the AS it was originated from will be dropped by the border router. With the introduction of route reflectors, there is a potential for having routing loops within an AS. A routing update that leaves a cluster might find its way back inside the cluster. Loops inside the AS cannot be detected by the traditional AS_path approach because the routing updates have not left the AS yet. BGP offers two extra measures for loop avoidance inside an AS when route reflectors are configured. 

Using an Originator ID 

The originator ID is a 4-byte, optional, nontransitive BGP attribute (type code 9) that is created by the route reflector. This attribute carries the router ID of the originator of the route in the local AS. If, because of poor configuration, the update comes back to the originator, the originator ignores it. 

Using a Cluster List 

The cluster list is an optional, nontransitive BGP attribute (type code 10). Each cluster is represented with a cluster ID. 

A cluster list is a sequence of cluster IDs that an update has traversed. When a route reflector sends a route from its clients to nonclients outside the cluster, it appends the local cluster ID to the cluster list. If the route reflector receives an update whose cluster list contains the local cluster ID, the update is ignored. This is basically the same concept as the AS_path list applied between the clusters inside the AS. 

Reference: http://borg.uu3.net/cisco/inter_arch/page11.html 


Q183. Refer to the exhibit. 


What is the PHB class on this flow? 

A. EF 

B. none 

C. AF21 

D. CS4 

Answer: D 

Explanation: 

This command shows the TOS value in hex, which is 80 in this case. The following chart shows some common DSCP/PHB Class values: 

Service 

DSCP value 

TOS value 

Juniper Alias 

TOS hexadecimal 

DSCP - TOS Binary 

Premium IP 

46 

184 

ef 

B8 

101110 - 101110xx 

LBE 

32 

cs1 

20 

001000 - 001000xx 

DWS 

32 

128 

cs4 

80 

100000 - 100000xx 

Network control 

48 

192 

cs6 

c0 

110000 - 110000xx 

Network control 2 

56 

224 

cs7 

e0 

111000 - 111000xx 

Reference: http://www.tucny.com/Home/dscp-tos 


Q184. Which EIGRP packet types are sent as unicast packets? 

A. hello, update, query 

B. query, SIA query, reply 

C. SIA query, reply, ACK 

D. query, SIA query, SIA reply 

Answer: C 


Q185. Which two services are used to transport Layer 2 frames across a packet-switched network? (Choose two.) 

A. Frame Relay 

B. ATM 

C. AToM 

D. L2TPv3 

Answer: C,D 

Explanation: 

Both AToM and L2TPv3 have the common objective of transmitting packet switched traffic of L2 frames (Frame Relay, ATM, and Ethernet) across a packet-switched network. 

Reference: Layer 2 VPN Architectures - Google Books Result Wei Luo, Carlos Pignataro, Anthony Chan 

https://books.google.com/books?isbn=0132796864 


400-101 torrent

Update 400-101 cisco ccie written:

Q186. Which two actions can you take to recover an interface in a errdisable state? (Choose two.) 

A. Enable UDLD on the switch. 

B. Enable errdisable recovery on the switch. 

C. Execute the shutdown command on the interface, followed by the no shutdown command. 

D. Remove the related commands from the configuration and reenter them. 

E. Enable loop guard on the switch. 

Answer: B,C 


Q187. Which two statements about NetFlow are true? (Choose two.) 

A. It must be configured on each router in a network. 

B. It supports ATM LAN emulation. 

C. The existing network is unaware that NetFlow is running. 

D. It uses SIP to establish sessions between neighbors. 

E. It provides resource utilization accounting. 

Answer: C,E 

Explanation: 

NetFlow identifies packet flows for both ingress and egress IP packets. It does not involve any connection-setup protocol, either between routers or to any other networking device or end station. NetFlow does not require any change externally--either to the packets themselves or to any networking device. NetFlow is completely transparent to the existing network, including end stations and application software and network devices like LAN switches. Also, NetFlow capture and export are performed independently on each internetworking device; NetFlow need not be operational on each router in the network. NetFlow data provides fine-grained metering for highly flexible and detailed resource utilization accounting. For example, flow data includes details such as IP addresses, packet and byte counts, timestamps, type-of-service, and application ports. Service providers might utilize the information for billing based on time-of-day, bandwidth usage, application usage, or quality of service. Enterprise customers might utilize the information for departmental chargeback or cost allocation for resource utilization. 

Reference: http://www.cisco.com/c/en/us/td/docs/ios-xml/ios/netflow/configuration/12-4t/nf-12-4t-book/ios-netflow-ov.html 


Q188. Which statement describes the native VLAN concept in an ISL trunk? 

A. It is the VLAN ID that is assigned to untagged packets. 

B. It is the VLAN with highest priority. 

C. It is the default VLAN for a trunk. 

D. There is no native VLAN concept in an ISL trunk. 

Answer: D 

Explanation: 

ISL has no native VLAN concept because it places the entire Ethernet frame in the payload of an ISL frame. Native VLANs is an 802.1Q specific concept 


Q189. What is a reason for 6PE to use two MPLS labels in the data plane instead of one? 

A. 6PE allows penultimate hop popping and has a requirement that all P routers do not have to be IPv6 aware. 

B. 6PE does not allow penultimate hop popping. 

C. It allows MPLS traffic engineering to work in a 6PE network. 

D. It allows 6PE to work in an MPLS network where 6VPE is also deployed. 

Answer: A 

Explanation: 

Q. Why does 6PE use two MPLS labels in the data plane? 

A. 6PE uses two labels: 

. The top label is the transport label, which is assigned hop-by-hop by the Label Distribution Protocol (LDP) or by MPLS traffic engineering (TE). 

. The bottom label is the label assigned by the Border Gateway Protocol (BGP) and advertised by the internal BGP (iBGP) between the Provider Edge (PE) routers. 

When the 6PE was released, a main requirement was that none of the MPLS core routers (the P routers) had to be IPv6-aware. That requirement drove the need for two labels in the data plane. There are two reasons why the 6PE needs both labels. 

PHP Functionality 

If only the transport label were used, and if penultimate hop popping (PHP) were used, the penultimate hop router (the P router) would need to understand IPv6. 

With PHP, this penultimate hop router would need to remove the MPLS label and forward the packet as an IPv6 packet. This P router would need to know that the packet is IPv6 because the P router would need to use the correct Layer 2 encapsulation type for IPv6. (The encapsulation type is different for IPv6 and IPv4; for example, for Ethernet, the encapsulation type is 0x86DD for IPv6, while it is 0x0800 for IPv4.) If the penultimate hop router is not IPv6-capable, it would likely put the Layer 2 encapsulation type for IPv4 for the IPv6 packet. The egress PE router would then believe that the packet was IPv4. There is time-to-live (TTL) processing in both the IPv4 and IPv6 headers. In IPv6, the field is called Hop Limit. The IPv4 and IPv6 fields are at different locations in the headers. Also, the Header Checksum in the IPv4 header would also need to be changed; there is no Header Checksum field in IPv6. If the penultimate hop router is not IPv6-capable, it would cause the IPv6 packet to be malformed since the router expects to find the TTL field and Header Checksum field in the header. Because of these differences, the penultimate hop router would need to know it is an IPv6 packet. How would this router know that the packet is an IPv6 packet, since it did not assign a label to the IPv6 Forwarding Equivalence Class (FEC), and there is no encapsulation field in the MPLS header? It could scan for the first nibble after the label stack and determine that the packet is IPv6 if the value is 6. However, that implies that the penultimate hop router needs to be IPv6-capable. This scenario could work if the explicit null label is used (hence no PHP). However, the decision was to require PHP. 

Load Balancing 

Typical load balancing on a P router follows this process. The P router goes to the end of the label stack and determines if it is an IPv4 packet by looking at the first nibble after the label stack. 

. If the nibble has a value of 4, the MPLS payload is an IPv4 packet, and the P router load balances by hashing the source and destination IPv4 addresses. 

. If the P router is IPv6-capable and the value of the nibble is 6, the P router load balances by hashing the source and destination IPv6 addresses. 

. If the P router is not IPv6-capable and the value of the nibble is not 4 (it could be 6 if the packet is an IPv6 packet), the P router determines it is not an IPv4 packet and makes the load balancing decision based on the bottom label. In the 6PE scenario, imagine there are two egress PE routers advertising one IPv6 prefix in BGP towards the ingress PE router. This IPv6 prefix would be advertised with two different labels in BGP. Hence, in the data plane, the bottom label would be either of the two labels. This would allow a P router to load balance on the bottom label on a per-flow basis. If 6PE used only the transport label to transport the 6PE packets through the MPLS core, the P routers would not be able to load balance these packets on a per-flow basis unless the P routers were IPv6-capable. If the P routers were IPv6-capable, they could use the source and destination IPv6 addresses in order to make a load balancing decision. 

Reference: http://www.cisco.com/c/en/us/support/docs/multiprotocol-label-switching-mpls/mpls/116061-qa-6pe-00.html 


Q190. In an STP domain, which two statements are true for a nonroot switch, when it receives a configuration BPDU from the root bridge with the TC bit set? (Choose two.) 

A. It sets the MAC table aging time to max_age + forward_delay time. 

B. It sets the MAC table aging time to forward_delay time. 

C. It recalculates the STP topology upon receiving topology change notification from the root switch. 

D. It receives the topology change BPDU on both forwarding and blocking ports. 

Answer: B,D 

Explanation: 

When the TC bit is received, every bridge is then notified and reduces the aging time to forward_delay (15 seconds by default) for a certain period of time (max_age + forward_delay). It is more beneficial to reduce the aging time instead of clearing the table because currently active hosts, that effectively transmit traffic, are not cleared from the table. Once the root is aware that there has been a topology change event in the network, it starts to send out its configuration BPDUs with the topology change (TC) bit set. These BPDUs are relayed by every bridge in the network with this bit set. As a result all bridges become aware of the topology change situation and it can reduce its aging time to forward_delay. Bridges receive topology change BPDUs on both forwarding and blocking ports. An important point to consider here is that a TCN does not start a STP recalculation. This fear comes from the fact that TCNs are often associated with unstable STP environments; TCNs are a consequence of this, not a cause. The TCN only has an impact on the aging time. It does not change the topology nor create a loop. 

Reference: http://www.cisco.com/c/en/us/support/docs/lan-switching/spanning-tree-protocol/12013-17.html#topic1 


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