Scenario practice questions

An engineer is troubleshooting a BGP route propagation issue. The customer is receiving a default route from upstream provider, but the route is not being installed in the routing table. The BGP table shows the route is valid but not best. What is the most likely cause?

Trap 1: The next-hop is unreachable

That would make the route invalid, not just not best.

Trap 2: The route is filtered by an inbound route-map

If filtered, it would not appear in BGP table at all.

Trap 3: The BGP session is not established

If session down, route would not be in BGP table.

A
The next-hop is unreachable
Why wrong: That would make the route invalid, not just not best.
B
The route is filtered by an inbound route-map
Why wrong: If filtered, it would not appear in BGP table at all.
C
The BGP session is not established
Why wrong: If session down, route would not be in BGP table.
D
A static default route with lower administrative distance exists
Static route AD 1 < BGP AD 20, so BGP route is not best.

Read the full MPLS explanation →

Question 2mediummultiple choice

A service provider's network uses MPLS TE with Fast Reroute (FRR) for link protection. After a fiber cut on a core link, the traffic is rerouted via a backup tunnel, but the backup tunnel's path suffers from high latency. The customer complains about poor voice quality. What can be done to avoid this scenario in future deployments?

Trap 1: Increase the bandwidth reservations on the backup tunnel

Bandwidth reservation does not affect latency.

Trap 2: Implement BGP PIC for edge routers

BGP PIC improves convergence at the network edge, not in core MPLS TE.

Trap 3: Use node protection instead of link protection

Node protection protects against node failures, but does not guarantee low latency for the backup path.

A
Configure backup tunnels with explicit paths that avoid high-latency links
Explicit path constraint ensures backup tunnel follows desired low-latency links.
B
Increase the bandwidth reservations on the backup tunnel
Why wrong: Bandwidth reservation does not affect latency.
C
Implement BGP PIC for edge routers
Why wrong: BGP PIC improves convergence at the network edge, not in core MPLS TE.
D
Use node protection instead of link protection
Why wrong: Node protection protects against node failures, but does not guarantee low latency for the backup path.

Review the full OSPF breakdown →

Question 3mediummultiple choice

A service provider is deploying MPLS L3VPN over an OSPF backbone. The PE routers are configured with OSPF as the IGP. The CE router of customer A is connected to two PEs for redundancy. Which configuration is required on the PE routers to ensure that the CE router can load-balance traffic across both PEs without loops?

Trap 1: Use OSPF sham-links between the two PEs.

Sham-links are used to connect backdoor links across an MPLS VPN, not for CE-PE loop prevention.

Trap 2: Use the BGP cost community to adjust the path selection on the CE.

BGP cost community influences BGP path selection, but does not prevent routing loops in OSPF.

Trap 3: Disable the DN-bit on the PE-CE OSPF interface.

Disabling the DN-bit would allow loopback routes to be propagated, potentially causing loops.

A
Use OSPF sham-links between the two PEs.
Why wrong: Sham-links are used to connect backdoor links across an MPLS VPN, not for CE-PE loop prevention.
B
Use the BGP cost community to adjust the path selection on the CE.
Why wrong: BGP cost community influences BGP path selection, but does not prevent routing loops in OSPF.
C
Configure OSPF with the capability vrf-lite and enable the down-bit on the PE-CE link.
The down-bit prevents the CE from re-advertising routes learned from one PE to the other PE, avoiding loops.
D
Disable the DN-bit on the PE-CE OSPF interface.
Why wrong: Disabling the DN-bit would allow loopback routes to be propagated, potentially causing loops.

Read the full VPN explanation →

Question 4hardmultiple choice

A service provider is deploying EVPN-VPWS to replace legacy pseudowire connections for point-to-point Layer 2 services. They notice that MAC addresses learned from a CE device are not properly advertised to the remote PE. Which EVPN route type is responsible for advertising MAC address reachability information in a VPWS scenario?

Trap 1: Route Type 2 (MAC/IP Advertisement)

Route Type 2 is for MAC/IP advertisement but not mandatory for VPWS.

Trap 2: Route Type 5 (IP Prefix)

Route Type 5 is for IP prefix advertisement, not MAC.

Trap 3: Route Type 3 (Inclusive Multicast)

Route Type 3 is used for multicast, not unicast MAC.

A
Route Type 1 (Ethernet Auto-Discovery)
Ethernet AD routes include MAC address information for split-horizon.
B
Route Type 2 (MAC/IP Advertisement)
Why wrong: Route Type 2 is for MAC/IP advertisement but not mandatory for VPWS.
C
Route Type 5 (IP Prefix)
Why wrong: Route Type 5 is for IP prefix advertisement, not MAC.
D
Route Type 3 (Inclusive Multicast)
Why wrong: Route Type 3 is used for multicast, not unicast MAC.

Study the full Python automation breakdown →

Question 5easymultiple choice

A network engineer wants to automate the backup of router configurations using Python and NETCONF. Which YANG model should the engineer use to access the running configuration?

Trap 1: urn:ietf:params:netconf:capability:rollback-on-error:1.0

This capability provides error rollback, not config retrieval.

Trap 2: urn:ietf:params:netconf:capability:candidate:1.0

Candidate capability allows using a candidate datastore, but for retrieval, any datastore works.

Trap 3: urn:ietf:params:netconf:capability:confirmed-commit:1.0

This capability enables confirmed commits, not retrieving config.

A
urn:ietf:params:netconf:capability:rollback-on-error:1.0
Why wrong: This capability provides error rollback, not config retrieval.
B
urn:ietf:params:netconf:capability:candidate:1.0
Why wrong: Candidate capability allows using a candidate datastore, but for retrieval, any datastore works.
C
urn:ietf:params:netconf:capability:confirmed-commit:1.0
Why wrong: This capability enables confirmed commits, not retrieving config.
D
urn:ietf:params:netconf:capability:xpath:1.0
Why wrong: XPath capability enables filtering, not required for full config retrieval.
E
urn:ietf:params:netconf:capability:writable-running:1.0
Why wrong: This capability allows writing to running, but retrieving is standard.

Question 6hardmultiple choice

An engineer is troubleshooting BGP convergence. The router has multiple paths for a prefix, but it selects a path with a lower local preference over a path with a higher local preference. The higher local preference path is from an EBGP peer. What could cause this?

Trap 1: The router is configured with 'bgp deterministic-med'.

Incorrect. This affects MED comparison order.

Trap 2: The path with lower local preference has a higher router ID.

Incorrect. Router ID is compared later if all else equal.

Trap 3: The path with lower local preference has a lower MED.

Incorrect. MED is compared after local preference.

A
The router is configured with 'bgp deterministic-med'.
Why wrong: Incorrect. This affects MED comparison order.
B
The path with lower local preference has a higher weight.
Correct. Weight is checked before local preference.
C
The path with lower local preference has a higher router ID.
Why wrong: Incorrect. Router ID is compared later if all else equal.
D
The path with lower local preference has a lower MED.
Why wrong: Incorrect. MED is compared after local preference.

Read the full MPLS explanation →

Question 7hardmultiple choice

A network operator is deploying segment routing in an MPLS network. They want to use a centralized controller to compute paths based on traffic demand and network constraints. Which architecture is being used?

Trap 1: LDP-based MPLS

LDP is distributed, not centralized.

Trap 2: Segment Routing Traffic Engineering (SR-TE)

SR-TE can use a controller but is not specifically an architecture name.

Trap 3: Segment Routing Best Effort (SR-BE)

SR-BE does not use a centralized controller.

A
LDP-based MPLS
Why wrong: LDP is distributed, not centralized.
B
Segment Routing Traffic Engineering (SR-TE)
Why wrong: SR-TE can use a controller but is not specifically an architecture name.
C
Segment Routing Path Computation Element (SR-PCE)
SR-PCE is a centralized controller for path computation.
D
Segment Routing Best Effort (SR-BE)
Why wrong: SR-BE does not use a centralized controller.

Read the full Scenario explanation →

Question 8easymultiple choice

Based on the exhibit, which label operation will occur when a packet destined to 10.2.2.1 enters PE1 with label 17?

Exhibit

Refer to the exhibit.

PE1# show mpls forwarding-table
Local      Outgoing   Prefix           Bytes Label   Outgoing   Next Hop
Label      Label      or Tunnel Id     Switched      interface
16         Pop       10.1.1.0/24      0             Gi0/0/0/0  point2point
17         18         10.2.2.0/24      0             Gi0/0/0/1  192.168.1.2
18         Untagged  10.3.3.0/24      0             Gi0/0/0/2  192.168.2.2

Trap 1: Forward with no label (Untagged)

Untagged is for prefix 10.3.3.0/24.

Trap 2: Pop the label and forward as IP

Pop operation is for prefix 10.1.1.0/24, not 10.2.2.0/24.

Trap 3: Push label 17 onto the packet

Label 17 is the incoming label, not pushed.

A
Forward with no label (Untagged)
Why wrong: Untagged is for prefix 10.3.3.0/24.
B
Pop the label and forward as IP
Why wrong: Pop operation is for prefix 10.1.1.0/24, not 10.2.2.0/24.
C
Push label 17 onto the packet
Why wrong: Label 17 is the incoming label, not pushed.
D
Swap label 17 to label 18
The forwarding table shows outgoing label 18 for this prefix.

Question 9mediummultiple choice

An ISP is implementing BGP communities to influence routing behavior for their customers. They want to ensure that a customer's routes are not advertised to a specific transit provider. Which BGP community should be used?

Trap 1: LOCAL_AS

LOCAL_AS is not a BGP community; it is a feature for AS-path prepending.

Trap 2: NO_PEER

There is no standard BGP community named NO_PEER.

Trap 3: NO_ADVERTISE

This community prevents advertisement to any BGP peer, including internal peers.

A
LOCAL_AS
Why wrong: LOCAL_AS is not a BGP community; it is a feature for AS-path prepending.
B
NO_EXPORT
This community ensures the route stays within the local AS and is not sent to any external AS.
C
NO_PEER
Why wrong: There is no standard BGP community named NO_PEER.
D
NO_ADVERTISE
Why wrong: This community prevents advertisement to any BGP peer, including internal peers.

Question 10mediummultiple choice

A service provider is troubleshooting an MPLS L3VPN scenario where a customer in VRF Blue cannot reach a server in VRF Blue at a remote site. The PEs are running MPLS with LDP. The VRF on both PEs shows the remote prefix. The PE at the local site shows the label from the remote PE for the prefix in the BGP table. However, when pinging from the CE, the packets are dropped. A packet capture on the core shows MPLS packets with the correct VPN label, but the transport label is missing. Further investigation shows that the LDP session between the two PEs is up, but the LDP label binding for the remote PE's loopback is not present. What is the most likely cause and correct action?

Trap 1: Increase the TTL on the MPLS packets

TTL issues would not cause missing label binding.

Trap 2: Change the transport address in LDP to the loopback IP

Transport address is already the loopback by default; this is not a missing binding issue.

Trap 3: Configure the remote PE's loopback to be advertised via BGP

The loopback is already in the IGP; LDP uses IGP to distribute labels.

A
Increase the TTL on the MPLS packets
Why wrong: TTL issues would not cause missing label binding.
B
Change the transport address in LDP to the loopback IP
Why wrong: Transport address is already the loopback by default; this is not a missing binding issue.
C
Configure the remote PE's loopback to be advertised via BGP
Why wrong: The loopback is already in the IGP; LDP uses IGP to distribute labels.
D
Check MPLS LDP interface configuration and enable LDP on all core interfaces
LDP must be enabled on each core interface to exchange label bindings for loopbacks.

Read the full MPLS explanation →

Question 11easymultiple choice

A service provider is migrating from a legacy MPLS network using LDP to Segment Routing (SR-MPLS). They have enabled SR on all routers and are using IS-IS. The migration plan is to keep LDP running alongside SR during the transition. After enabling SR, some traffic is being forwarded using the SR path, but the network operator notices that some CEs in an L2VPN are experiencing packet loss during failover scenarios. Troubleshooting shows that the primary pseudowire is using SR labels, but the backup pseudowire is still using LDP labels. The backup path is not working correctly. What is the most likely cause?

Trap 1: The backup path uses a different IGP metric

Metric difference would affect routing but not label allocation.

Trap 2: The MTU on the backup path is smaller than on the primary

MTU would cause fragmentation but not specifically for backup.

Trap 3: The prefix-SID for the remote PE is missing in the SR-TE database

SR-TE is not used here; pseudowires rely on IGP label distribution.

A
The backup path uses a different IGP metric
Why wrong: Metric difference would affect routing but not label allocation.
B
The backup pseudowire is still using LDP labels and the LDP session for the backup path is down
If LDP sessions are down for the backup path, the backup pseudowire cannot use SR labels, leading to packet loss.
C
The MTU on the backup path is smaller than on the primary
Why wrong: MTU would cause fragmentation but not specifically for backup.
D
The prefix-SID for the remote PE is missing in the SR-TE database
Why wrong: SR-TE is not used here; pseudowires rely on IGP label distribution.