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Q121. 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. 



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. 


Q122. What is the range of addresses that is used for IPv4-mapped IPv6 addresses? 

A. 2001. db9. . /32 

B. 2001. db8. . /32 

C. 2002. . /16 

D. . . ffff. /16 

E. . . ffff. 0. 0/96 



IPv4-Mapped Addresses FFFF:0:0/96 are the IPv4-mapped addresses [RFC4291]. Addresses within this block should not appear on the public Internet. 


Q123. Refer to the exhibit. 

Which two statements about the R1 configuration are true? (Choose two.) 

A. The IP TTL value is copied to the MPLS field during label imposition. 

B. The structure of the MLPS network is hidden in a traceroute. 

C. The LDP session interval and hold times are configured for directly connected neighbors. 

D. R1 protects the session for 86400 seconds. 

E. All locally assigned labels are discarded. 

Answer: B,D 

Q124. Refer to the exhibit. 

AS #1 and AS #2 have multiple EBGP connections with each other. AS #1 wants all return traffic that is destined to the prefix to enter through the router R1 from AS #2. 

In order to achieve this routing policy, the AS 1 advertises a lower MED from R1, compared to a higher MED from R3, to their respective BGP neighbor for the prefix Will this measure guarantee that the routing policy is always in effect? 

A. Yes, because MED plays a deterministic role in return traffic engineering in BGP. 

B. Yes, because a lower MED forces BGP best-path route selection in AS #2 to choose R1 as the best path for 

C. Yes, because a lower MED in AS #2 is the highest BGP attribute in BGP best-path route selection. 

D. No, AS #2 can choose to alter the weight attribute in R2 for BGP neighbor R1, and this weight value is cascaded across AS #2 for BGP best-path route selection. 

E. No, AS #2 can choose to alter the local preference attribute to overwrite the best-path route selection over the lower MED advertisement from AS #1. This local preference attribute is cascaded across AS #2 for the BGP best-path route selection. 



MED and AS path prepending can both be used to influence the way incoming traffic from other Autonomous Systems get sent to the local AS, but they provide no guarantee as the other AS ultimately has the final word in how they send traffic. Since local preference is preferred over MED in the BGP decision process, the other AS can configure local preference to override the MED settings you have configured. 

Q125. What is the most efficient way to confirm whether microbursts of traffic are occurring? 

A. Monitor the output traffic rate using the show interface command. 

B. Monitor the output traffic rate using the show controllers command. 

C. Check the CPU utilization of the router. 

D. Sniff the traffic and plot the packet rate over time. 



Micro-bursting is a phenomenon where rapid bursts of data packets are sent in quick succession, leading to periods of full line-rate transmission that can overflow packet buffers of the network stack, both in network endpoints and routers and switches inside the network. In order to troubleshoot microbursts, you need a packet sniffer that can capture traffic over a long period of time and allow you to analyze it in the form of a graph which displays the saturation points (packet rate during microbursts versus total available bandwidth). You can eventually trace it to the source causing the bursts (e.g. stock trading applications). 

Reference: Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 989-994). Kindle Edition. 

Q126. The no ip unreachables command is configured on interfaces to protect the control plane of a router. 

Which mechanism is impacted by using this command? 

A. ICMP redirects 

B. path MTU discovery 

C. source routing 

D. ICMP router discovery protocol 


Q127. Which two services are used to transport Layer 2 frames across a packet-switched network? (Choose two.) 

A. Frame Relay 


C. AToM 

D. L2TPv3 

Answer: C,D 


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 

Q128. A GRE tunnel is down with the error message %TUN-5-RECURDOWN: Tunnel0 temporarily disabled due to recursive routing error. 

Which two options describe possible causes of the error? (Choose two.) 

A. Incorrect destination IP addresses are configured on the tunnel. 

B. There is link flapping on the tunnel. 

C. There is instability in the network due to route flapping. 

D. The tunnel mode and tunnel IP address are misconfigured. 

E. The tunnel destination is being routed out of the tunnel interface. 

Answer: C,E 


The %TUN-5-RECURDOWN: Tunnel0 temporarily disabled due to recursive routing error message means that the generic routing encapsulation (GRE) tunnel router has discovered a recursive routing problem. This condition is usually due to one of these causes: 

. A misconfiguration that causes the router to try to route to the tunnel destination address using the tunnel interface itself (recursive routing) 

. A temporary instability caused by route flapping elsewhere in the network 


Q129. Which statement about the NHRP network ID is true? 

A. It is sent from the spoke to the hub to identify the spoke as a member of the same NHRP domain. 

B. It is sent from the hub to the spoke to identify the hub as a member of the same NHRP domain. 

C. It is sent between spokes to identify the spokes as members of the same NHRP domain. 

D. It is a locally significant ID used to define the NHRP domain for an interface. 



The NHRP network ID is used to define the NHRP domain for an NHRP interface and differentiate between multiple NHRP domains or networks, when two or more NHRP domains (GRE tunnel interfaces) are available on the same NHRP node (router). The NHRP network ID is used to help keep two NHRP networks (clouds) separate from each other when both are configured on the same router. The NHRP network ID is a local only parameter. It is significant only to the local router and it is not transmitted in NHRP packets to other NHRP nodes. For this reason the actual value of the NHRP network ID configured on a router need not match the same NHRP network ID on another router where both of these routers are in the same NHRP domain. As NHRP packets arrive on a GRE interface, they are assigned to the local NHRP domain in the NHRP network ID that is configured on that interface. 


Q130. Which statement about Cisco Discovery Protocol is true? 

A. The multicast address 0100.0cdd.dddd is used as the destination address for periodic advertisements. 

B. An inactive VLAN that is configured on an access port passes periodic Cisco Discovery Protocol advertisements. 

C. The multicast address 0100.0ccc.ccd is used as the destination address for periodic advertisements. 

D. A VLAN must be active on an access port before periodic Cisco Discovery Protocol advertisements are passed. 



All CDP packets include a VLAN ID. If you configure CDP on a Layer 2 access port, the CDP packets sent from that access port include the access port VLAN ID. If you configure CDP on a Layer 2 trunk port, the CDP packets sent from that trunk port include the lowest configured VLAN ID allowed on that trunk port. CDP messages on the active physical interfaces (Ethernet NIC) to a well-known multicast address (0100.0CCC.CCCC.