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Juniper JN0-661 Exam Practice Questions (P. 5)

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Question #21
A PE provides VLAN VPLS service to a CE attached with two links. You want to prevent Layer 2 loops and provide link redundancy.
Which two actions will accomplish this task? (Choose two.)
  1. A
    Place both interfaces in a link aggregation group.
  2. B
    Configure different VLANs on each interface.
  3. C
    Configure all VLANs on both interfaces, on the PE, and on the CE.
  4. D
    Configure Spanning Tree Protocol between the PE and the CE.

Correct Answer:
BD
D: To prevent the formation of Layer 2 loops between the CE devices and the multihomed PE routers, Juniper recommends that you employ the Spanning Tree
Protocol (STP) on your CE devices. Layer 2 loops can form due to incorrect configuration. Temporary Layer 2 loops can also form during convergence after a change in the network topology.
References:
http://www.juniper.net/documentation/en_US/junos16.1/topics/topic-map/vpls-bgp-multihoming.html

Question #22
Click the Exhibit button.
R1 is exporting 2000:FEFE:100::/64 to R2 over the EBGP peering session as shown in the exhibit.


What will R1 use for this route's protocol next hop when advertising it to R2?
  1. A
    2000:FEFE:89::1
  2. B
    172.16.1.1
  3. C
    10.1.1.1
  4. D
    ::FFFF:172.16.1.1

Correct Answer:
B
When a BGP router reports itself as the next hop, whether because of an explicit neighbor next-hop-self configuration or implicitly as a result of participating in an
EBGP session, BGP allocates a new in label and adds an entry to the MPLS forwarding table, creating a label-to-next-hop mapping.
Note: When a BGP router does not report itself as the next hop, whether because of an explicit neighbor next-hop-unchanged configuration or implicitly as a result of a participating in an IBGP session, BGP does not allocate a new in label. Instead, if the route is advertised as a labeled route, BGP uses the existing out label.
This feature is used mainly on route reflectors.
References:
https://www.juniper.net/techpubs/en_US/junose16.1/topics/concept/mbgp-bgp-next-hop-self-overview.html

Question #23
You work for a service provider and need to build EVPN service which provides an active/active multihoming topology using a single CE at each site.
In this scenario, which two statements are true? (Choose two.)
  1. A
    An Ethernet segment appears as a LAG to the CE device.
  2. B
    A backup designated forwarder is elected for forwarding BUM traffic to the CE device.
  3. C
    The Ethernet segment identifier must be an all zeros identifier.
  4. D
    A designated forwarder is elected for forwarding BUM traffic to the CE device.

Correct Answer:
A
A: When a CE device is multihomed to two or more PE routers, the set of Ethernet links constitutes an Ethernet segment. An Ethernet segment appears as a link aggregation group (LAG) to the CE device .
B: When a CE device is multihomed to two or more PE routers, either one or all of the multihomed PE routers are used to reach the customer site depending on the multihoming mode of operation. The PE router that assumes the primary role for forwarding BUM traffic to the CE device is called the designated forwarder
(DF).
Incorrect Answers:
C: An Ethernet segment must have a unique nonzero identifier, called the Ethernet segment identifier (ESI). The ESI is encoded as a 10 octet integer.
References:
http://www.juniper.net/documentation/en_US/junos15.1/topics/concept/evpn-bgp-multihoming-overview.html

Question #24
Which two statements are true regarding the CSPF algorithm? (Choose two.)
  1. A
    The selected path for a given LSP is passed to the TED in the form of an ERO.
  2. B
    LSPs with lower numerical setup priorities are computed before LSPs with higher setup priority values.
  3. C
    The selected path for a given LSP is passed to RSVP in the form of an ERO.
  4. D
    LSPs with higher numerical setup priorities are computed before LSPs with lower setup priority values.

Correct Answer:
BC
The CSPF algorithm first calculates the highest-priority LSP (the one with the lowest setup priority value).
Do I need to enable CSPF to control where to send my traffic? The answer is no. You can manually configure an Explicit Route Objects (ERO) list, and let RSVP doing the job. In the presence of the ERO object, the RSVP Path messages will follow the path specified, thus the Resv messages carrying the labels in the opposite direction.
References:
https://www.netflask.net/jnpr-constrained-shortest-path-first/

Question #25
You have a strict-high queue configured. You notice that under bursty traffic conditions, there are tail drops on the strict-high queue.
Which action would solve this problem?
  1. A
    Assign a policer on ingress to assign a low packet loss priority to the strict-high queue.
  2. B
    Decrease the buffer size of the strict-high queue.
  3. C
    Assign a policer on egress to assign a low packet loss priority to the strict-high queue.
  4. D
    Increase the buffer size of the strict-high queue.

Correct Answer:
D
A queue with strict-high priority is assured unlimited transmission bandwidth but is not actually assigned a large delay buffer. Not configuring a transmit-rate or an explicit buffer-size on a strict-high priority queue only ensures that the queue gets assigned a default minimum delay buffer, making it possible, under bursty conditions, to see tail-drops on strict-high priority queues. Assigning a small transmit-rate or an explicit temporal or percentage buffer-size to the queue ensures that the queue has a large enough buffer to hold bursts and protect against tail-drops.
References:
http://www.juniper.net/documentation/en_US/junos15.1/topics/usage-guidelines/cos-configuring-schedulers-for-priority-scheduling.html

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