Juniper jn0-664 practice test

Answer:

D

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.
To create a sham link, you need to configure the local and remote addresses of the PE
routers under the [edit protocols ospf area area-id] hierarchy level1
.
https://www.juniper.net/documentation/us/en/software/junos/ospf/topics/topic-map/configuring-ospfv2-sham-links.html

Answer:

AB

Explanation:
The hello interval is the time interval between two consecutive hello packets sent by an OSPF router
on an interface. The dead interval is the time interval after which a neighbor is declared down if no
hello packets are received from it. These parameters must match between two OSPF routers for
them to form an adjacency. In the exhibit, router R1 has a hello interval of 10 seconds and a dead
interval of 40 seconds, while router R2 has a hello interval of 30 seconds and a dead interval of 120
seconds. This causes a mismatch and prevents them from becoming neighbors
.

Answer:

BC

Explanation:
class-of-service (CoS) is a feature that allows you to prioritize and manage network traffic based on
various criteria, such as application type, user group, or packet loss priority. CoS uses different
components to classify, mark, queue, schedule, shape, and drop traffic according to the configured
policies.
One of the components of CoS is drop profiles, which define how packets are dropped when a queue
is congested. Drop profiles use random early detection (RED) algorithm to drop packets randomly
before the queue is full, which helps to avoid global synchronization and improve network
performance. Drop profiles can be discrete or interpolated. A discrete drop profile maps a specific fill
level of a queue to a specific drop probability. An interpolated drop profile maps a range of fill levels
of a queue to a range of drop probabilities and interpolates the values in between.
In the exhibit, we can see that the class-of-service configuration shows an interpolated drop profile
with two fill levels (50 and 75) and two drop probabilities (20 and 60). Based on this configuration,
we can infer the following statements:
The drop probability jumps immediately from 20% to 60% when the queue level reaches 75% full.
This is not correct because the drop profile is interpolated, not discrete. This means that the drop
probability gradually increases from 20% to 60% as the queue level increases from 50% full to 75%
full. The drop probability for any fill level between 50% and 75% can be calculated by using linear
interpolation formula.
The drop probability gradually increases from 20% to 60% as the queue level increases from 50% full
to 75% full. This is correct because the drop profile is interpolated and uses linear interpolation
formula to calculate the drop probability for any fill level between 50% and 75%. For example, if the
fill level is 60%, the drop probability is 28%, which is calculated by using the formula: (60 - 50) / (75 -
50) * (60 - 20) + 20 = 28.
To use this drop profile, you reference it in a scheduler. This is correct because a scheduler is a
component of CoS that determines how packets are dequeued from different queues and
transmitted on an interface. A scheduler can reference a drop profile by using the random-detect
statement under the [edit class-of-service schedulers] hierarchy level. For example: scheduler test {
transmit-rate percent 10; buffer-size percent 10; random-detect test-profile; }
To use this drop profile, you apply it directly to an interface. This is not correct because a drop profile
cannot be applied directly to an interface. A drop profile can only be referenced by a scheduler,
which can be applied to an interface by using the scheduler-map statement under the [edit class-of-
service interfaces] hierarchy level. For example: interfaces ge-0/0/0 { unit 0 { scheduler-map test-
map; } }

Answer:

BC

Explanation:
Intermediate System to Intermediate System (IS-IS) is a link-state routing protocol that supports
Level 1 (L1), Level 2 (L2), or both (L1/L2) operations. The way IS-IS sends Hello (IIH) packets depends
on whether the interface is point-to-point (P2P) or broadcast (LAN).
Evaluating the Answer Choices
✅
Option A: "If a point-to-point interface is in both L1 and L2, separate hello messages are sent for
each level."
Incorrect!
On point-to-point (P2P) interfaces, only one combined Hello message is sent for both L1 and L2.
IS-IS P2P Hellos include both Level 1 and Level 2 TLVs in the same message.
Reference: Juniper IS-IS documentation confirms that P2P links use a single Hello message with both
levels included.
❌
This statement is incorrect.
✅
Option B: "If a point-to-point interface is in both L1 and L2, one combined hello message is sent
for both levels."
Correct!
On point-to-point (P2P) links, IS-IS sends a single Hello message that includes TLVs for both L1 and L2.
This reduces overhead and simplifies adjacency formation.
✅
This statement is correct.
✅
Option C: "If a broadcast interface is in both L1 and L2, separate hello messages are sent for each
level."
Correct!
On broadcast (LAN) interfaces, IS-IS sends separate Hello messages for L1 and L2.
This is because L1 and L2 use separate Designated IS (DIS) elections and different multicast
addresses:
L1 Hellos: Sent to AllL1IS (01:80:C2:00:00:14)
L2 Hellos: Sent to AllL2IS (01:80:C2:00:00:15)
Reference: Juniper IS-IS Configuration Guide confirms that broadcast interfaces send separate L1 and
L2 Hello messages.
✅
This statement is correct.
✅
Option D: "If a broadcast interface is in both L1 and L2, one combined hello message is sent for
both levels."
Incorrect!
As stated above, IS-IS sends separate Hello messages for L1 and L2 on broadcast interfaces because
they have independent DIS elections.
❌
This statement is incorrect.
Final Answer:
✅
B. If a point-to-point interface is in both L1 and L2, one combined hello message is sent for both
levels.
✅
C. If a broadcast interface is in both L1 and L2, separate hello messages are sent for each level.
Verification from Juniper Documentation
Juniper IS-IS Configuration Guide confirms:
Point-to-Point (P2P) interfaces send one combined Hello for both levels.
Broadcast interfaces send separate L1 and L2 Hellos due to separate DIS elections.
RFC 1195 (IS-IS Extensions for IPv4) specifies that broadcast networks require distinct Hellos per
level.

Answer:

B

Explanation:
Understanding the Problem in MPLS CoS Classification
How EXP Bits Are Used for CoS in MPLS
Traffic is sent from VPN-A Site-1 → CE-1 → PE-1 → P-1 → PE-2 → CE-2.
The MPLS LSP (Label Switched Path) from PE-1 to PE-2 is expected to carry MPLS EXP bits, which are
used for Class of Service (CoS) classification.
PE-2 should classify traffic based on EXP bits received in the MPLS label.
What Happens with PHP (Penultimate Hop Popping)?
By default, the penultimate router (P-1) pops the top MPLS label before sending the packet to PE-2.
Since the EXP bits are in the top MPLS label, they get removed along with the label.
This means that PE-2 no longer sees the correct EXP bits, leading to incorrect traffic classification.
Solution: Configure Explicit-Null on PE-2
Explicit Null (explicit-null) must be configured on PE-2 to ensure that P-1 does NOT remove the MPLS
label.
Instead of removing the label, P-1 will send a label of 0 (for IPv4) or 2 (for IPv6) to PE-2.
This preserves the MPLS EXP bits, allowing PE-2 to classify the traffic correctly.
Evaluating the Answer Choices Again
✅
B. Configure the explicit-null statement on PE-2.
Correct, because:
PE-2 is the egress LSR, where Ultimate Hop Popping (UHP) must be enabled.
Configuring explicit-null ensures that P-1 does not remove the label, preserving the EXP bits for CoS
classification at PE-2.
Configuration on PE-2:
set protocols mpls explicit-null
Juniper Documentation Reference:
"Explicit-null must be configured on the egress LSR to prevent PHP from removing the top MPLS
label, thereby preserving the EXP bits."
❌
A. Configure the explicit-null statement on PE-1.
Incorrect, because:
Explicit-null must be configured on the egress LSR (PE-2), not the ingress LSR (PE-1).
PE-1 only labels the traffic but does not control PHP behavior on P-1.
❌
C. Configure VPN prefix mapping for the PE-1_to_PE-2 LSP.
Incorrect, because:
VPN prefix mapping is used for mapping VPN routes to LSPs but does not solve the EXP bit issue.
The problem here is label removal (PHP), not route mapping.
❌
D. Set a static CoS value for the PE-1_to-PE-2 LSP.
Incorrect, because:
This does not preserve the original EXP bits, it only applies a static CoS value.
It’s a workaround, not a fix.
✅
Final Answer:
B. Configure the explicit-null statement on PE-2.
Explanation:
Key Takeaways
Penultimate Hop Popping (PHP) removes the outer MPLS label at P-1, which also removes the EXP
bits used for CoS classification.
To keep EXP bits intact, configure explicit-null on the egress PE (PE-2).
This forces P-1 to send a label (0 for IPv4, 2 for IPv6) to PE-2, preserving the EXP bits for CoS
classification.
Official Juniper Documentation Reference

Juniper MPLS CoS and PHP Behavior Guide
"To retain CoS EXP bits at the egress LSR, configure explicit-null on the egress PE. This prevents PHP
from stripping the MPLS label before reaching the final PE router."

Answer:

B, C, E

Explanation:
https://www.juniper.net/documentation/us/en/software/junos/bgp/topics/ref/statement/multihop-edit-protocols-bgp.html

Answer:

AC

Explanation:
https://www.juniper.net/documentation/us/en/software/junos/ospf/topics/topic-map/configuring-ospfv2-sham-links.html

Answer:

ABC

Answer:

A

Explanation:
Prefer the route with the lower origin code. Routes learned from an IGP have a lower origin code
than those learned from an exterior gateway protocol (EGP), and both have lower origin codes than
incomplete routes (routes whose origin is unknown).
https://www.juniper.net/documentation/us/en/software/junos/vpn-l2/bgp/topics/concept/routing-protocols-address-representation.html

Answer:

B

Explanation:
When a Juniper device receives a packet on an interface with both a Behavior Aggregate (BA)
classifier and a Multifield (MF) classifier, Junos OS follows a specific processing order to apply Class of
Service (CoS).
Understanding the Classifiers in Junos CoS
️
⃣
Behavior Aggregate (BA) Classifier
Uses packet headers (DSCP, IP precedence, or MPLS EXP bits) to classify traffic into forwarding
classes.
Applied at the ingress interface.
Example: A packet with DSCP 46 (Expedited Forwarding) is mapped to a high-priority queue.
️
⃣
Multifield (MF) Classifier
Uses match conditions (like source/destination IP, port numbers, protocol types) to classify traffic.
Typically used for more granular classification beyond what BA can provide.
Junos Processing Order:
When both BA and MF classifiers are configured on an interface, Junos first applies the BA classifier,
then the MF classifier.
MF classifier can override the BA classification if necessary.
Evaluating the Answer Choices
✅
B. The packet will be processed by the BA classifier first, then the MF classifier.
Correct, because Junos first applies BA classification based on DSCP/MPLS EXP bits.
After BA classification, the MF classifier is applied, which can refine or override the BA classification.
❌
A. The packet will be discarded.
Incorrect, because classification does not drop packets unless explicitly configured with a filter or
policing action.
❌
C. The packet will be forwarded with no classification changes.
Incorrect, because both classifiers are applied in a specific order, meaning classification changes will
occur.
❌
D. The packet will be processed by the MF classifier first, then the BA classifier.
Incorrect, because BA classification is always applied first, followed by MF classification.
✅
Final Answer:
B. The packet will be processed by the BA classifier first, then the MF classifier.
Explanation:
Official Juniper Reference:
"When both BA and MF classifiers are applied on an interface, Junos OS first classifies packets using
the BA classifier before applying the MF classifier."