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NEW QUESTION # 19
In an Ethernet frame carrying a VLAN tag, where does the VLAN tag appear?
- A. after the length field
- B. after the type field
- C. before the type field
- D. before the length field
Answer: C
Explanation:
Explanation
In an Ethernet frame carrying a VLAN tag, the VLAN tag appears before the type field. A VLAN tag is a
4-byte field that is inserted into an Ethernet frame to indicate the VLAN membership and priority of the frame.
The VLAN tag consists of two subfields: the tag protocol identifier (TPID) and the tag control information (TCI). The TPID subfield is a 16-bit field that identifies the frame as an IEEE 802.1Q-tagged frame, with a value of 0x8100. The TCI subfield is a 16-bit field that contains the priority code point (PCP), the drop eligible indicator (DEI), and the VLAN identifier (VID). The VLAN tag appears between the source MAC address and the type fields of the original frame, shifting the type field by four bytes. The type field indicates the type of the payload, such as IP or ARP .
References: [IEEE 802.1Q - Wikipedia], [VLAN Tagging Explained with DTP Protocol - GeeksforGeeks]
NEW QUESTION # 20
Which two statements are true about priority queuing (PQ)? (Choose two.)
- A. Traffic in the highest priority queue is only reserved for voice traffic.
- B. Traffic in all queues are always guaranteed a minimum bandwidth.
- C. Traffic in lower priority queues can be starved of bandwidth.
- D. Traffic in the highest priority queue will experience the least amount of jitter and delay compared to traffic in the other queues.
Answer: C,D
Explanation:
Explanation
Priority queuing (PQ) is a queuing method that establishes four interface output queues that serve different priority levels: high, medium, normal, and low. Traffic in the highest priority queue will experience the least amount of jitter and delay compared to traffic in the other queues, because PQ always services the higher-priority queues first. However, this can also cause traffic in lower priority queues to be starved of bandwidth, especially if the highest priority queue is oversubscribed. Traffic in the highest priority queue is not only reserved for voice traffic, but can also include network control and routing traffic. Traffic in all queues are not always guaranteed a minimum bandwidth, because PQ does not provide any bandwidth reservation mechanism. References: Quality of Service (QoS) Queues and Queuing Explained, Chapter:
Configuring Priority Queueing - Cisco
NEW QUESTION # 21
How many different priorities are available to use in a VLAN header?
- A. 0
- B. 1
- C. 2
- D. 3
Answer: A
Explanation:
Explanation
There are 8 different priorities available to use in a VLAN header. A VLAN header is a 4-byte field that is inserted into an Ethernet frame to indicate the VLAN membership and priority of the frame. The VLAN header consists of two subfields: the tag protocol identifier (TPID) and the tag control information (TCI). The TCI subfield contains three subfields: the priority code point (PCP), the drop eligible indicator (DEI), and the VLAN identifier (VID). The PCP subfield is a 3-bit field that specifies the priority level of the frame, ranging from 0 (lowest) to 7 (highest). The PCP values can be used to implement quality of service (QoS) mechanisms on bridges and switches12.
References: HTML rt Tag - W3Schools, VLAN 0 Priority Tagging Support - Cisco
NEW QUESTION # 22
Which two statements are correct for LDP? (Choose two.)
- A. LSRs send UDP hello messages for neighbor discovery.
- B. LSRs use TCP to negotiate LDP session parameters.
- C. LSRs use UDP for session establishment.
- D. LSRs send ICMP hello messages for neighbor discovery
Answer: A,B
Explanation:
Explanation
Two statements that are correct for LDP are:
LSRs send UDP hello messages for neighbor discovery. LDP is a protocol that distributes labels for MPLS forwarding along the shortest path calculated by an IGP. LDP uses two types of messages:
discovery messages and session messages. Discovery messages are used to discover and maintain adjacency with other LSRs that support LDP. Discovery messages are sent as UDP datagrams to the well-known multicast address 224.0.0.2 (all routers on this subnet) on port 64634.
LSRs use TCP to negotiate LDP session parameters. Session messages are used to establish, maintain, and terminate LDP sessions between LSRs. Session messages are also used to exchange label mappings, notification messages, and keepalive messages. Session messages are sent as TCP segments to port
64634.
References: Ldp | Microsoft Learn, Label Distribution Protocol - Wikipedia
NEW QUESTION # 23
What is the purpose of penultimate hop popping?
- A. to reduce the load on the LDP
- B. to reduce the load on the LSR
- C. to reduce the load on the LER
- D. to reduce the load on the LSP
Answer: C
Explanation:
Explanation
Penultimate hop popping (PHP) is a function performed by certain routers in an MPLS enabled network. It refers to the process whereby the outermost label of an MPLS tagged packet is removed by a label switch router (LSR) before the packet is passed to an adjacent label edge router (LER). The purpose of PHP is to reduce the load on the LER, as it saves one cycle of label lookup. The LER can forward the packet based on its IP header or another label in the stack, without having to process the outer label. PHPfunctionality is achieved by the LER advertising a label with a value of 3 to its neighbors. This label is defined as "implicit-null" and informs the neighboring LSRs to perform PHP12.
References: Penultimate hop popping - Wikipedia, MPLS Penultimate hop popping - NetworkLessons.com
NEW QUESTION # 24
Which action will influence BGP route selection within your AS?
- A. reducing number of hops in the network
- B. changing the default link metric
- C. changing the administrative distance for eBGP
- D. changing the default value of the local preference
Answer: D
Explanation:
Explanation
The action that will influence BGP route selection within your AS is changing the default value of the local preference attribute. The local preference attribute is used to indicate the preference of a path among multiple paths learned from different external BGP neighbors or autonomous systems (ASes). The higher the local preference value, the more preferred the path is within your AS, and vice versa. The default value of local preference is 100, but you can change it using route maps or other configuration methods on your BGP routers. References: Ericsson IP Networking - Routing Protocols, BGP Attributes and Path Selection, BGP Local Preference Attribute: Controlling Traffic Like a Pro
NEW QUESTION # 25
Which mismatched field would cause an IS-IS adjacency between two routers to fail?
- A. SEL
- B. MTU
- C. SysID
- D. Context
Answer: B
Explanation:
Explanation
MTU mismatch would cause an IS-IS adjacency between two routers to fail. MTU stands for maximum transmission unit, which is the maximum size of a packet that can be transmitted on a link. IS-IS hellos are padded to the full MTU size, partly to allow a router to implicitly communicate its MTU to its neighbors. If two routers have different MTUs on their interfaces, they will not be able to exchange hellos properly, and their adjacency will not form or will be dropped34. To solve this problem, the MTUs on both interfaces must be configured to match.
SEL, context, and SysID are not fields that would cause an IS-IS adjacency failure. SEL stands for selector, which is a one-octet field that identifies the network layer service to which a packet should be sent. For IS-IS routers, the SEL must always be 005. Context is not a field in IS-IS packets, but rather a term used to describe the scope of routing information in different levels or areas6. SysID stands for system identifier, which is a six-octet field that uniquely identifies a router within an area. SysID does not have to match between two routers for them to form an adjacency7.
References: MTU Mismatch Problem in IS-IS - Cisco, IS-IS adjacency is not formed due to MTU mismatch - Cisco Community, IS-IS NSAP address - Cisco Community, Understanding IS-IS Routing Contexts - Juniper Networks, IS-IS - Nokia
NEW QUESTION # 26
What is the correct ordering (outer to inner) of packet headers in an IP/MPLS packet?
- A. Ethernet, MPLS, IP
- B. IP, MPLS, Ethernet
- C. Ethernet, IP, MPLS
- D. MPLS, IP, Ethernet
Answer: A
Explanation:
Explanation
The correct ordering (outer to inner) of packet headers in an IP/MPLS packet is Ethernet, MPLS, IP. An IP/MPLS packet is a packet that is encapsulated with an MPLS label stack between the Ethernet header and the IP header. The Ethernet header contains the source and destination MAC addresses and the EtherType field that indicates the type of the payload. The MPLS label stack contains one or more labels that are used for forwarding decisions by the MPLS routers. Each label consists of four fields: label value, experimental bits, bottom of stack, and time to live. The IP header contains the source and destination IP addresses and other information for routing and fragmentation. The order of the headers is important because it determines how the packet is processed by different devices in the network12.
References: MPLS Label Stack - Cisco, MPLS Fundamentals: 3 - MPLS Packet Forwarding
NEW QUESTION # 27
What is the subnet address for 10.57.126.156/29?
- A. 10.57.126.0
- B. 10.57.126.144
- C. 10.57.126.156
- D. 10.57.126.152
Answer: D
Explanation:
Explanation
The subnet address for 10.57.126.156/29 is 10.57.126.152. To find the subnet address, we need to perform a bitwise AND operation between the IP address and the subnet mask. The subnet mask for /29 is
255.255.255.248, which in binary is 11111111.11111111.11111111.11111000. The IP address in binary is
00001010.00111001.01111110.10011100. The result of the AND operation is
00001010.00111001.01111110.10011000, which in decimal is10.57.126.152. References: Ericsson IP Networking - IP Addressing, Software Installation and Upgrade Overview (Junos OS)
NEW QUESTION # 28
How is a BGP session established between two routers?
- A. The BGP router sends a unicast OPEN message.
- B. The BGP router sends a multicast HELLO packet.
- C. The BGP router sends a multicast CONNECT message.
- D. The BGP router sends a unicast NOTIFICATION message.
Answer: A
Explanation:
Explanation
A BGP session is established between two routers by sending a unicast OPEN message. BGP uses TCP as its transport protocol, which means that before exchanging any BGP information, a TCP connection must be established between the routers. The TCP connection uses port 179 as both source and destination port. After establishing a TCP connection, each router sends an OPEN message to its neighbor, containing parameters such as BGP version number, AS number, hold time, BGP identifier, and optional capabilities. The OPEN message also serves as a keepalive message for BGP. If both routers agree on these parameters, they proceed to exchange UPDATE messages containing routing information and establish a BGP session.
A BGP router does not send a NOTIFICATION message to establish a session, but rather to terminate a session due to an error or a manual shutdown. A BGP router does not send a CONNECT message, as this is a TCP state, not a BGP message. A BGP router does not send a multicast HELLO packet, as this is an OSPF message, not a BGP message. References: BGP Neighbor States > BGP Fundamentals | Cisco Press, Demystifying BGP Session Establishments - Packet Pushers, The TCP/IP Guide - BGP Connection Establishment: Open Messages
NEW QUESTION # 29
In your network, video traffic is being marked with DSCP code AF31.
Based on industry standard practice, which code would you use to assign higher priority to VoIP traffic?
- A. EF
- B. AF21
- C. DF
- D. AF11
Answer: A
Explanation:
Explanation
The code that is commonly used to assign higher priority to VoIP traffic is EF, which stands for Expedited Forwarding. EF is a per-hop behavior (PHB) defined by the Differentiated Services (DiffServ) model for QoS.
EF provides low delay, low jitter, and low loss for real-time applications such as VoIP. EF is marked by setting the DSCP value to 101110, which corresponds to decimal 4678. AF31, which stands for Assured Forwarding class 3 low drop probability, is another PHB defined by DiffServ, but it provides lower priority than EF. AF31 is marked by setting the DSCP value to 011010, which corresponds to decimal 2678. DF, which stands for Default Forwarding, is a PHB defined by DiffServ that provides best-effort service for unmarked traffic. DF is marked by setting the DSCP value to 000000, which corresponds to decimal 078.
AF11, which stands for Assured Forwarding class 1 low drop probability, is another PHB defined by DiffServ, but it provides lower priority than EF and AF31. AF11 is marked by setting the DSCP value to 001010, which corresponds to decimal 1078.
References: Differentiated services - Wikipedia, Solved: Cisco VoIP Phone traffic marking - Cisco Community
NEW QUESTION # 30
Review the exhibit.
A service provider wants to provide L3VPN for two customers (indicated by red and green in the exhibit).
Referring to the exhibit, which statement is correct?
- A. The service provider's P routers are not aware of the customers' routes.
- B. LDP must be configured between PE and CE
- C. MP-BGP should be configured on all the service provider's routers (Ps and PEs).
- D. The two customers cannot use overlapping IP prefixes.
Answer: A
Explanation:
Explanation
Referring to the exhibit, the correct statement is that the service provider's P routers are not aware of the customers' routes. This is because P routers only need to forward MPLS packets based on their labels, without looking into their IP headers or VPN information. The P routers do not have any VRFs configured for the customers, nor do they participate in MP-BGP sessions with other PE routers. The PE routers are responsible for maintaining VRFs for each customer and for exchanging VPN routes with other PE routers using MP-BGP910.
References: Layer 3 VPNs (L3VPN) - Cisco, MPLS VPN - Wikipedia
NEW QUESTION # 31
Which two label actions are performed by a P router? (Choose two.)
- A. php
- B. swap
- C. drop
- D. push
Answer: A,B
Explanation:
Explanation
A P router is a provider router that is part of the service provider's core network in an MPLS environment. A P router does not have any customer routes or VPN information, but only has information about how to reach other P routers and PE routers in the same MPLS domain. A P router performs label switching, which means that it forwards labeled packets based on their top label in the label stack. A P router can perform two possible label actions:
Swap: The P router replaces the incoming label with a new label that corresponds to the next hop along the label-switched path (LSP). The new label is determined by looking up the label forwarding information base (LFIB) based on the incoming label and interface.
PHP: The P router removes the top label from the packet at the penultimate hop before reaching the egress PE router. This is done to avoid an extra lookup on the egress PE router, which can forward the packet based on its IP header or another label in the stack.
A P router does not perform push or drop actions on labels. A push action means adding one or more labels to the packet, which is done by an ingress PE router when initiating an LSP. A drop action means discarding a packet, which is done by any router when there is no matching entry in its LFIB or routing table. References: Provider (P) Router in IP MPLS Network - Cisco Community, MPLS Fundamentals: Forwarding Labeled Packets - Cisco Press, MPLS Label Switching | MPLS Operation | Push, Swap,Push IPCisco
NEW QUESTION # 32
Which two features does IPsec provide? (Choose two.)
- A. hiding the content of packets from observation
- B. controlling the path through the network
- C. protecting packets from modification
- D. protecting the length of packets from discovery
Answer: A,C
Explanation:
Explanation
IPsec provides two main features: protecting packets from modification and hiding the content of packets from observation. IPsec protects packets from modification by using cryptographic algorithms to generate and verify message authentication codes (MACs) or digital signatures for each packet. These mechanisms ensure that the packets have not been tampered with or altered in transit by unauthorized parties. IPsec hides the content of packets from observation by using encryption algorithms to scramble the payload of each packet. This prevents eavesdroppers or interceptors from reading or analyzing the data carried by the packets12.
References: What is IPsec? | How IPsec VPNs work | Cloudflare, IPsec - Wikipedia
NEW QUESTION # 33
In OSPFv2, which route characteristic is used to determine the best path?
- A. jitter
- B. packet loss
- C. cost
- D. delay
Answer: C
Explanation:
Explanation
In OSPFv2, the route characteristic that is used to determine the best path is the cost. The cost is a metric that represents the link bandwidth, delay, reliability, or other factors. The cost is inversely proportional to the bandwidth, meaning that a higher bandwidth link has a lower cost. The cost of a route is calculated by adding the costs of all links along the path. OSPFv2 uses the following formula to calculate the cost of an interface:
Cost = Reference bandwidth / Interface bandwidth in bps
The reference bandwidth is a constant value that can be configured by the network administrator. By default, it is 100 Mbps. The interface bandwidth is the actual bandwidth of the interface in bits per second. For example, if an interface has a bandwidth of 10 Mbps, its cost would be 100 Mbps / 10 Mbps = 101415.
OSPFv2 does not use jitter, packet loss, or delay as route characteristics to determine the best path. Jitter is the variation in latency or delay between packets. Packet loss is the percentage of packets that are dropped or corrupted during transmission. Delay is the time it takes for a packet to travel from source to destination. These characteristics are not part of the OSPFv2 protocol and are not advertised in OSPFv2 LSAs1617.
References: OSPF Metric cost Calculation Formula Explained - ComputerNetworkingNotes, OSPF Cost - OSPF Routing Protocol Metric Explained - Study-CCNA, Open Shortest Path First - Wikipedia, OSPF Metric
= Cost - Cisco
NEW QUESTION # 34
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