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質問 # 22
Exhibit
A network is using IS-IS for routing.
In this scenario, why are there two TLVs shown in the exhibit?
- A. There are both narrow and wide metric devices in the topology
- B. Both IPv4 and IPv6 are being used in the topology
- C. Wide metrics have specifically been requested
- D. The interface specified a metric of 100 for L2.
正解：A
解説：
Explanation
TLVs are tuples of (Type, Length, Value) that can be advertised in IS-IS packets. TLVs can carry different kinds of information in the Link State Packets (LSPs). IS-IS supports both narrow and wide metrics for link costs. Narrow metrics use a single octet to encode the link cost, while wide metrics use three octets. Narrow metrics have a maximum value of 63, while wide metrics have a maximum value of 16777215. If there are both narrow and wide metric devices in the topology, IS-IS will advertise two TLVs for each link: one with the narrow metric and one with the wide metric. This allows backward compatibility with older devices that only support narrow metrics12.
質問 # 23
Exhibit
CE-1 must advertise ten subnets to PE-1 using BGP Once CE-1 starts advertising the subnets to PE-1, the BGP peering state changes to Active.
Referring to the CLI output shown in the exhibit, which statement is correct?
- A. CE-1 is configured with an incorrect peer AS
- B. The prefix limit has been reached on PE-1
- C. CE-1 is unreachable
- D. CE-1 is advertising its entire routing table.
正解：A
解説：
Explanation
The problem in this scenario is that CE-1 is configured with an incorrect peer AS number for its BGP session with PE-1. The CLI output shows that CE-1 is using AS 65531 as its local AS number and AS 65530 as its peer AS number. However, PE-1 is using AS 65530 as its local AS number and AS 65531 as its peer AS number. This causes a mismatch in the BGP OPEN messages and prevents the BGP session from being established. To solve this problem, CE-1 should configure its peer AS number as 65530 under [edit protocols bgp group external] hierarchy level.
質問 # 24
Exhibit
You have MAC addresses moving in your EVPN environment
Referring to the exhibit, which two statements are correct about the sequence number? (Choose two)
- A. It identifies MAC addresses that should be discarded.
- B. It helps the local PE to identify the latest advertisement.
- C. It is advertised using a Type 2 message
- D. It resolves conflicting MAC address ownership claims.
正解：B、D
解説：
Explanation
The sequence number is a field in the MAC mobility extended community that is used to resolve conflicting MAC address ownership claims and to help the local PE to identify the latest advertisement. The sequence number is incremented by one for every MAC address mobility event, such as when a host moves from one Ethernet segment to another segment in the EVPN network. The PE device that receives multiple MAC advertisements for the same MAC address chooses the one with the highest sequence number as the most recent and valid advertisement.
質問 # 25
Which three mechanisms are used by Junos platforms to evaluate incoming traffic for CoS purposes? (Choose three )
- A. fixed classifiers
- B. behavior aggregate classifiers
- C. multifield classifiers
- D. traffic shapers
- E. rewrite rules
正解：A、B、C
解説：
Explanation
Junos platforms use different mechanisms to evaluate incoming traffic for CoS purposes, such as:
* Behavior aggregate classifiers: These classifiers use a single field in a packet header to classify traffic into different forwarding classes and loss priorities based on predefined or user-defined values.
* Fixed classifiers: These classifiers use a fixed field in a packet header to classify traffic into different forwarding classes and loss priorities based on predefined values.
* Multifield classifiers: These classifiers use multiple fields in a packet header to classify traffic into different forwarding classes and loss priorities based on user-defined values and filters.
Rewrite rules and traffic shapers are not used to evaluate incoming traffic for CoS purposes, but rather to modify or shape outgoing traffic based on CoS policies.
質問 # 26
Which statement is correct about IS-IS when it performs the Dijkstra algorithm?
- A. The local router moves its own local tuples into the candidate database
- B. Tuples with the lowest cost are moved from the tree database to the LSDB.
- C. When a new neighbor ID in the tree database matches a router ID in the LSDB, the neighbor ID is moved to the candidate database
- D. The algorithm will stop processing once the tree database is empty.
正解：A
解説：
Explanation
IS-IS is a link-state routing protocol that uses the Dijkstra algorithm to compute the shortest paths between nodes in a network. The Dijkstra algorithm maintains three data structures: a tree database, a candidate database, and a link-state database (LSDB). The tree database contains the nodes that have been visited and their shortest distances from the source node. The candidate database contains the nodes that have not been visited yet and their tentative distances from the source node. The LSDB contains the topology information of the network, such as the links and their costs.
The Dijkstra algorithm works as follows:
* The local router moves its own local tuples into the tree database. A tuple consists of a node ID, a distance, and a parent node ID. The local router's tuple has a distance of zero and no parent node.
* The local router moves its neighbors' tuples into the candidate database. The neighbors' tuples have distances equal to the costs of the links to them and parent node IDs equal to the local router's node ID.
* The local router selects the tuple with the lowest distance from the candidate database and moves it to the tree database. This tuple becomes the current node.
* The local router updates the distances of the current node's neighbors in the candidate database by adding the current node's distance to the link costs. If a shorter distance is found, the parent node ID is also updated.
* The algorithm repeats steps 3 and 4 until either the destination node is reached or the candidate database is empty.
質問 # 27
Exhibit
The network shown in the exhibit is based on IS-IS
Which statement is correct in this scenario?
- A. The routers are using unnumbered interfaces
- B. The area address is two bytes.
- C. The NSEL byte for Area 0001 is 00.
- D. The system IDofR1_2 is 192.168.16.1
正解：C
解説：
Explanation
IS-IS is an interior gateway protocol that uses link-state routing to exchange routing information among routers within a single autonomous system. IS-IS uses two types of addresses to identify routers and areas:
system ID and area address. The system ID is a unique identifier for each router in an IS-IS domain. The system ID is 6 octets long and can be derived from the MAC address or manually configured. The area address is a variable-length identifier for each area in an IS-IS domain. The area address can be 1 to 13 octets long and is composed of high-order octets of the address. An IS-IS instance may be assigned multiple area addresses, which are considered synonymous. Multiple synonymous area addresses are useful when merging or splitting areas in the domain1. In this question, we have a network based on IS-IS with four routers (R1_1, R1_2, R2_1, and R2_2) belonging to area 0001. The area address for area 0001 is 49.0001. The NSEL byte for area 0001 is the last octet of the address, which is 01. The NSEL byte stands for Network Service Access Point Selector (NSAP Selector) and indicates the type of service requested from the network layer2. Therefore, the correct statement in this scenario is that the NSEL byte for area 0001 is 01.
References: 1:
https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/iproute_isis/configuration/xe-16/irs-xe-16-book/irs-ovrvw-cf.
2:
https://www.juniper.net/documentation/us/en/software/junos/is-is/topics/concept/is-is-routing-overview.html
質問 # 28
In which two ways does OSPF prevent routing loops in multi-area networks? (Choose two.)
- A. All areas are required to connect as a full mesh.
- B. All areas are required to connect to area 0.
- C. The SPF algorithm prunes looped paths within an area.
- D. The LFA algorithm prunes all looped paths within an area.
正解：B、C
解説：
Explanation
OSPF is an interior gateway protocol that uses link-state routing to exchange routing information among routers within a single autonomous system. OSPF prevents routing loops in multi-area networks by using two methods: area hierarchy and SPF algorithm. Area hierarchy is the concept of dividing a large OSPF network into smaller areas that are connected to a backbone area (area 0). This reduces the amount of routing information that each router has to store and process, and also limits the scope of link-state updates within each area. All areas are required to connect to area 0 either directly or through virtual links2. SPF algorithm is the method that OSPF uses to calculate the shortest path to each destination in the network based on link-state information. The SPF algorithm runs on each router and builds a shortest-path tree that represents the topology of the network from the router's perspective. The SPF algorithm prunes looped paths within an area by choosing only one best path for each destination3.
References: 2:
https://www.juniper.net/documentation/us/en/software/junos/ospf/topics/concept/ospf-area-overview.html 3:
https://www.juniper.net/documentation/us/en/software/junos/ospf/topics/concept/ospf-spf-algorithm-overview.ht
質問 # 29
Exhibit
Click the Exhibit button-Referring to the exhibit, which two statements are correct about BGP routes on R3 that are learned from the ISP-A neighbor? (Choose two.)
- A. The next-hop value for these routes is changed by ISP-A before being sent to R3.
- B. The BGP local-preference value that is used by ISP-A is not advertised to R3.
- C. All BGP attribute values must be removed before receiving the routes.
- D. By default, the next-hop value for these routes is not changed by ISP-A before being sent to R3.
正解：B、D
解説：
Explanation
BGP is an exterior gateway protocol that uses path vector routing to exchange routing information among autonomous systems. BGP uses various attributes to select the best path to each destination and to propagate routing policies. Some of the common BGP attributes are AS path, next hop, local preference, MED, origin, weight, and community. BGP attributes can be classified into four categories: well-known mandatory, well-known discretionary, optional transitive, and optional nontransitive. Well-known mandatory attributes are attributes that must be present in every BGP update message and must be recognized by every BGP speaker.
Well-known discretionary attributes are attributes that may or may not be present in a BGP update message but must be recognized by every BGP speaker. Optional transitive attributes are attributes that may or may not be present in a BGP update message and may or may not be recognized by a BGP speaker. If an optional transitive attribute is not recognized by a BGP speaker, it is passed along to the next BGP speaker. Optional nontransitive attributes are attributes that may or may not be present in a BGP update message and may or may not be recognized by a BGP speaker. If an optional nontransitive attribute is not recognized by a BGP speaker, it is not passed along to the next BGP speaker. In this question, we have four routers (R1, R2, R3, and R4) that are connected in a full mesh topology and running IBGP. R3 receives the 192.168.0.0/16 route from its EBGP neighbor and advertises it to R1 and R4 with different BGP attribute values. We are asked which statements are correct about the BGP routes on R3 that are learned from the ISP-A neighbor. Based on the information given, we can infer that the correct statements are:
* By default, the next-hop value for these routes is not changed by ISP-A before being sent to R3. This is because the default behavior of EBGP is to preserve the next-hop attribute of the routes received from another EBGP neighbor. The next-hop attribute indicates the IP address of the router that should be used as the next hop to reach the destination network.
* The BGP local-preference value that is used by ISP-A is not advertised to R3. This is because the local-preference attribute is a well-known discretionary attribute that is used to influence the outbound traffic from an autonomous system. The local-preference attribute is only propagated within an autonomous system and is not advertised to external neighbors.
References: : https://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/13753-25.html :
https://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/13762-40.html :
https://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/13759-37.html
質問 # 30
Which two EVPN route types are used to advertise a multihomed Ethernet segment? (Choose two )
- A. Type 2
- B. Type 1
- C. Type 4
- D. Type 3
正解：B、C
解説：
Explanation
EVPN is a solution that provides Ethernet multipoint services over MPLS networks. EVPN uses BGP to distribute endpoint provisioning information and set up pseudowires between PE devices. EVPN uses different route types to convey different information in the control plane. The following are the main EVPN route types:
* Type 1 - Ethernet Auto-Discovery Route: This route type is used for network-wide messaging and discovery of other PE devices that are part of the same EVPN instance. It also carries information about the redundancy mode and load balancing algorithm of the PE devices.
* Type 2 - MAC/IP Advertisement Route: This route type is used for MAC and IP address learning and advertisement between PE devices. It also carries information about the Ethernet segment identifier (ESI) and the label for forwarding traffic to the MAC or IP address.
* Type 3 - Inclusive Multicast Ethernet Tag Route: This route type is used for broadcast, unknown unicast, and multicast (BUM) traffic forwarding. It also carries information about the multicast group and the label for forwarding BUM traffic.
* Type 4 - Ethernet Segment Route: This route type is used for multihoming scenarios, where a CE device is connected to more than one PE device. It also carries information about the ESI and the designated forwarder (DF) election process.
質問 # 31
What is the correct order of packet flow through configurable components in the Junos OS CoS features?
- A. Behavior Aggregate Classifier -> Multifield Classifier -> Input Policer -> Forwarding Policy Options -> Fabric Scheduler -> Scheduler/Shaper/RED -> Output Policer -> Rewrite Marker
- B. Behavior Aggregate Classifier -> Input Policer -> Multifield Classifier -> Forwarding Policy Options -> Fabric Scheduler -> Output Policer -> Scheduler/Shaper/RED -> Rewrite Marker
- C. Behavior Aggregate Classifier -> Multifield Classifier -> Input Policer -> Forwarding Policy Options -> Fabric Scheduler -> Output Policer -> Scheduler/Shaper/RED -> Rewrite Marker
- D. Multifield Classifier -> Behavior Aggregate Classifier -> Input Policer -> Forwarding Policy Options -> Fabric Scheduler -> Output Policer -> Rewrite Marker -> Scheduler/Shaper/RED
正解：B
解説：
Explanation
The correct order of packet flow through configurable components in the Junos OS CoS features is as follows:
* Behavior Aggregate Classifier: This component uses a single field in a packet header to classify traffic into different forwarding classes and loss priorities based on predefined or user-defined values.
* Input Policer: This component applies rate-limiting and marking actions to incoming traffic based on the forwarding class and loss priority assigned by the classifier.
* Multifield Classifier: This component uses multiple fields in a packet header to classify traffic into different forwarding classes and loss priorities based on user-defined values and filters.
* Forwarding Policy Options: This component applies actions such as load balancing, filtering, or routing to traffic based on the forwarding class and loss priority assigned by the classifier.
* Fabric Scheduler: This component schedules traffic across the switch fabric based on the forwarding class and loss priority assigned by the classifier.
* Output Policer: This component applies rate-limiting and marking actions to outgoing traffic based on the forwarding class and loss priority assigned by the classifier.
* Scheduler/Shaper/RED: This component schedules, shapes, and drops traffic at the egress interface based on the forwarding class and loss priority assigned by the classifier.
* Rewrite Marker: This component rewrites the code-point bits of packets leaving an interface based on the forwarding class and loss priority assigned by the classifier.
質問 # 32
Which two statements are correct about reflecting inet-vpn unicast prefixes in BGP route reflection? (Choose two.)
- A. A BGP peer does not require any configuration changes to become a route reflector client.
- B. Route reflectors do not change any existing BGP attributes by default when advertising routes.
- C. Route reflectors add their cluster ID to the AS path when readvertising client routes.
- D. Clients add their originator ID when advertising routes to their route reflector
正解：A、B
解説：
Explanation
Route reflection is a BGP feature that allows a router to reflect routes learned from one IBGP peer to another IBGP peer, without requiring a full-mesh IBGP topology. Route reflectors do not change any existing BGP attributes by default when advertising routes, unless explicitly configured to do so. A BGP peer does not require any configuration changes to become a route reflector client, only the route reflector needs to be configured with the client parameter under [edit protocols bgp group group-name neighbor neighbor-address] hierarchy level.
質問 # 33
In IS-IS, which two statements are correct about the designated intermediate system (DIS) on a multi-access network segment? (Choose two)
- A. A router with a priority of 10 wins the DIS election over a router with a priority of 1.
- B. On the multi-access network, each router only forms an adjacency to the DIS.
- C. On the multi-access network, each router forms an adjacency to every other router on the segment
- D. A router with a priority of 1 wins the DIS election over a router with a priority of 10.
正解：A、B
解説：
Explanation
In IS-IS, a designated intermediate system (DIS) is a router that is elected on a multi-access network segment (such as Ethernet) to perform some functions on behalf of other routers on the same segment. A DIS is responsible for sending network link-state advertisements (LSPs), which describe all the routers attached to the network. These LSPs are flooded throughout a single area. A DIS also generates pseudonode LSPs, which represent the multi-access network as a single node in the link-state database. A DIS election is based on the priority value configured on each router's interface connected to the multi-access network. The priority value ranges from 0 to 127, with higher values indicating higher priority. The router with the highest priority becomes the DIS for the area (Level 1, Level 2, or both). If routers have the same priority, then the router with the highest MAC address is elected as the DIS. By default, routers have a priority value of 64. On a multi-access network, each router only forms an adjacency to the DIS, not to every other router on the segment. This reduces the amount of hello packets and LSP
質問 # 34
Which two statements are correct about a sham link? (Choose two.)
- A. It creates a BGP multihop neighborship between two PE routers.
- B. The PEs exchange Type 3 OSPF LSAs instead of Type 1 OSPF LSAs for the L3VPN routes.
- C. It creates an OSPF multihop neighborship between two PE routers.
- D. The PEs exchange Type 1 OSPF LSAs instead of Type 3 OSPF LSAs for the L3VPN routes
正解：C、D
解説：
Explanation
A sham link is a logical link between two PE routers that belong to the same OSPF area but are connected through an L3VPN. A sham link makes the PE routers appear as if they are directly connected, and prevents OSPF from preferring an intra-area back door link over the VPN backbone. A sham link creates an OSPF multihop neighborship between the PE routers using TCP port 646. The PEs exchange Type 1 OSPF LSAs instead of Type 3 OSPF LSAs for the L3VPN routes, which allows OSPF to use the correct metric for route selection1.
質問 # 35
An interface is configured with a behavior aggregate classifier and a multifield classifier How will the packet be processed when received on this interface?
- A. The packet will be processed by the MF classifier first, then the BA classifier.
- B. The packet will be discarded.
- C. The packet will be processed by the BA classifier first, then the MF classifier.
- D. The packet will be forwarded with no classification changes.
正解：D
解説：
Explanation
behavior aggregate (BA) classifiers and multifield (MF) classifiers are two types of classifiers that are used to assign packets to a forwarding class and a loss priority based on different criteria. The forwarding class determines the output queue for a packet. The loss priority is used by a scheduler to control packet discard during periods of congestion.
A BA classifier maps packets to a forwarding class and a loss priority based on a fixed-length field in the packet header, such as DSCP, IP precedence, MPLS EXP, or IEEE 802.1p CoS bits. A BA classifier is computationally efficient and suitable for core devices that handle high traffic volumes. A BA classifier is useful if the traffic comes from a trusted source and the CoS value in the packet header is trusted.
An MF classifier maps packets to a forwarding class and a loss priority based on multiple fields in the packet header, such as source address, destination address, protocol type, port number, or VLAN ID. An MF classifier is more flexible and granular than a BA classifier and can match packets based on complex filter rules. An MF classifier is suitable for edge devices that need to classify traffic from untrusted sources or rewrite packet headers.
You can configure both a BA classifier and an MF classifier on an interface. If you do this, the BA classification is performed first and then the MF classification. If the two classification results conflict, the MF classification result overrides the BA classification result.
Based on this information, we can infer the following statements:
* The packet will be discarded. This is not correct because the packet will not be discarded by the classifiers unless it matches a filter rule that specifies discard as an action. The classifiers only assign packets to a forwarding class and a loss priority based on their match criteria.
* The packet will be processed by the BA classifier first, then the MF classifier. This is correct because if both a BA classifier and an MF classifier are configured on an interface, the BA classification is performed first and then the MF classification. If they conflict, the MF classification result overrides the BA classification result.
* The packet will be forwarded with no classification changes. This is not correct because the packet will be classified by both the BA classifier and the MF classifier if they are configured on an interface. The final classification result will determine which output queue and which discard policy will be applied to the packet.
* The packet will be processed by the MF classifier first, then the BA classifier. This is not correct because if both a BA classifier and an MF classifier are configured on an interface, the BA classification is performed first and then the MF classification. If they conflict, the MF classification result overrides the BA classification result.
質問 # 36
Exhibit
R1 and R8 are not receiving each other's routes
Referring to the exhibit, what are three configuration commands that would solve this problem? (Choose three.)
- A. Configure remove-private on advertisements from AS 64497 toward AS 64498
- B. Configure loops on routers in AS 65412 and advertise-peer-as on routers in AS 64498.
- C. Configure remove-private on advertisements from AS 64500 toward AS 64499
- D. Configure as-override on advertisement from AS 64500 toward AS 64512.
- E. Configure loops and advertise-peer-as on routers in AS 64497 and AS 64450.
正解：A、B、C
解説：
Explanation
The problem in this scenario is that R1 and R8 are not receiving each other's routes because of private AS numbers in the AS path. Private AS numbers are not globally unique and are not advertised to external BGP peers. To solve this problem, you need to do the following:
* Configure loops on routers in AS 65412 and advertise-peer-as on routers in AS 64498. This allows R5 and R6 to advertise their own AS number (65412) instead of their peer's AS number (64498) when sending updates to R7 and R8. This prevents a loop detection issue that would cause R7 and R8 to reject the routes from R5 and R62.
* Configure remove-private on advertisements from AS 64497 toward AS 64498 and from AS 64500 toward AS 64499. This removes any private AS numbers from the AS path before sending updates to external BGP peers. This allows R2 and R3 to receive the routes from R1 and R4, respectively3.
質問 # 37
Exhibit
Referring to the exhibit, PIM-SM is configured on all routers, and Anycast-RP with Anycast-PIM is used for the discovery mechanism on RP1 and RP2. The interface metric values are shown for the OSPF area.
In this scenario, which two statements are correct about which RP is used? (Choose two.)
- A. Source2 will use RP1 and Receiver2 will use RP1 for group 224.2.2.2.
- B. Source1 will use RP1 and Receiver1 will use RP1 for group 224.1.1.1.
- C. Source1 will use RP1 and Receiver1 will use RP2 for group 224.1 1 1
- D. Source2 will use RP2 and Received will use RP2 for group 224.2.2.2.
正解：B、D
解説：
Explanation
A sham link is a logical link between two PE routers that belong to the same OSPF area but are connected through an L3VPN. A sham link makes the PE routers appear as if they are directly connected, and prevents OSPF from preferring an intra-area back door link over the VPN backbone. A sham link creates an OSPF multihop neighborship between the PE routers using TCP port 646. The PEs exchange Type 1 OSPF LSAs instead of Type 3 OSPF LSAs for the L3VPN routes, which allows OSPF to use the correct metric for route selection1.
質問 # 38
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