350-501 MPLS and Segment Routing Practice Test 5 — 15 Questions

Question 1

A service provider is implementing Segment Routing over IPv6 (SRv6) in their core. They want to provide path isolation for different services using SRv6 SIDs. Which SID structure is used to encode both the locator and the function?

Accepted Answer

The SID is divided into locator and function (and optionally arguments).. An SRv6 SID is 128 bits; the locator is the prefix portion, and the function is the remaining bits. Option A is incorrect because the prefix is the locator. Option C is incorrect because the SRv6 SID includes both. Option D is incorrect because the argument is optional and part of the function.

Answer

The SID uses a separate label stack for function encoding.

Answer

The SID is an IPv6 address without any encoding.

Answer

The SID consists of a prefix only.

Question 2

An MPLS network is experiencing micro-loops during convergence after a link failure. The network uses LDP and IS-IS as IGP. Which of the following solutions can prevent micro-loops during IGP convergence without requiring additional protocols?

Accepted Answer

Configure LFA (Loop-Free Alternate) on the IGP.. Loop-free alternates (LFA) provide fast convergence and micro-loop avoidance. Option B is incorrect because BGP PIC is for BGP convergence. Option C is incorrect because RSVP-TE is separate. Option D is incorrect because MPLS-TE FRR is for RSVP-TE tunnels.

Answer

Implement MPLS-TE FRR using bypass tunnels.

Answer

Enable BGP PIC (Prefix Independent Convergence).

Answer

Deploy RSVP-TE with fast-reroute.

Question 3

Which two statements about MPLS label operations are true? (Choose two.)

Accepted Answer

Push operation adds a label to the packet.. A correct: push is adding a label. B correct: pop is removing exactly one label, and PHP uses implicit-null. C wrong: swap replaces label. D wrong: push adds one or more. E wrong: pop removes exactly one, not multiple at once (except in PHP).

Answer

Push operation can only add one label at a time.

Answer

Pop operation can remove multiple labels at once.

Answer

Swap operation removes and replaces two labels.

Question 4

Which three are required components for deploying Segment Routing in an MPLS network? (Choose three.)

Accepted Answer

MPLS forwarding capability on routers. A correct: IGP must be extended with Segment Routing extensions (OSPF or IS-IS). C correct: Node-SID and Adj-SID are the basic SID types. D correct: MPLS forwarding plane is needed to forward labeled packets. B is optional for traffic engineering. E is not an SR component; it's for RSVP-TE.

Answer

RSVP-TE signaling protocol

Answer

LDP for label distribution

Question 5

Which two are characteristics of the Anycast SID in Segment Routing? (Choose two.)

Accepted Answer

It provides redundancy and fast convergence by allowing traffic to be rerouted to another router.. B correct: multiple nodes can advertise the same Anycast SID, enabling load balancing. E correct: Anycast SID is used for redundancy and fast convergence. A wrong: Node-SID is unique. C wrong: Anycast SID is not for adjacency. D wrong: The Anycast SID is advertised with a prefix SID type, not an Adj-SID type.

Answer

It identifies an adjacency between two routers.

Answer

It is typically used as an Adj-SID for load balancing.

Answer

It is unique per node and identifies a specific router.

Question 6

Refer to the exhibit. The show command output displays the LDP neighbor state. What does the 'Downstream' label mode indicate about label distribution?

Accepted Answer

Labels are advertised to all neighbors without a request.. Downstream label mode means the router advertises labels to its neighbors without being asked. Option A is incorrect because downstream-on-demand sends labels only when requested. Option C is incorrect because unsolicited downstream is the same as downstream. Option D is incorrect because DoD is not shown.

Answer

Labels are sent only to the peer that advertises the prefix.

Answer

Labels are only distributed when explicitly requested.

Answer

The neighbor must request labels via a Label Request message.

Question 7

Refer to the exhibit. A network engineer is configuring a segment routing traffic engineering policy. The output shows two candidate paths. Why is path1 selected as the active path even though path2 has a bandwidth constraint?

Accepted Answer

path2 has a lower preference value.. The active path is determined by the highest preference. Path1 has preference 200, which is higher than path2's 100. Option A is incorrect because path1 is explicit, not dynamic. Option B is incorrect because both paths can be up. Option C is incorrect because color is same for both.

Answer

path1 has a shorter segment list.

Answer

path2 is down due to insufficient bandwidth.

Answer

path1 is explicit and always preferred over dynamic paths.

Question 8

Refer to the exhibit. The output shows an MPLS forwarding entry with FRR protection. What is the purpose of the backup path shown?

Accepted Answer

To forward traffic if the primary outgoing interface fails.. The backup path is used when the primary next-hop fails, providing fast reroute. Option A is incorrect because VPN label is still present. Option C is incorrect because FRR does not create a load-balancing group. Option D is incorrect because backup path is not for PHP; PHP is for penultimate hop.

Answer

To handle penultimate hop popping for the primary path.

Answer

To provide a load-balancing alternative for the primary path.

Answer

To carry the VPN label separately.

Question 9

A service provider is designing a new MPLS core network and wants to simplify traffic engineering without deploying a centralized controller. Which technology should be used to enable source-routed paths with explicit path control?

Accepted Answer

Segment Routing (SR-MPLS). Segment Routing (SR-MPLS) allows source routing by encoding paths as a list of segment IDs, enabling traffic engineering without a centralized controller. LDP only distributes labels for shortest-path forwarding. BGP-LU is for inter-domain label distribution. RSVP-TE requires a centralized controller.

Answer

BGP-LU

Answer

LDP

Answer

RSVP-TE

Question 10

A network engineer is troubleshooting MPLS L3VPN issues on a PE router. The CE router is receiving routes from the PE, but the CE cannot ping the remote CE. The PE shows the remote prefix in its routing table and in the VRF. What is the most likely cause?

Accepted Answer

The LSP between the PEs is not operational. If the PE has the route in the VRF but the label-switched path (LSP) to the remote PE is down or nonexistent, the packet cannot be forwarded. The most common issue is that the LSP (e.g., via LDP or Segment Routing) is broken. Other options like wrong RT/RD or MTU would cause different symptoms.

Answer

The VRF route-target import/export is misconfigured

Answer

The MTU on the core links is too small

Answer

The RD is different on the two PEs

Question 11

A service provider is migrating from LDP-based MPLS to Segment Routing (SR-MPLS) with IS-IS. After enabling SR on all routers, the adjacency segment IDs (ADJ-SIDs) are not being advertised. Which configuration is missing?

Accepted Answer

The `segment-routing mpls` command is not enabled under the IS-IS process. In IS-IS, adjacency segment IDs are programmed only when a specific prefix-SID (e.g., the loopback) is configured and SR is enabled globally. Without an SR-capable IGP process on the interface, ADJ-SIDs are not allocated. The most common missing piece is the `segment-routing mpls` command under the IS-IS process.

Answer

The router is running OSPF instead of IS-IS

Answer

The interface has the `isis adjacency-sid` command incorrectly configured

Answer

The loopback interface does not have an ip address configured

Question 12

A network architect is designing an MPLS network that must provide fast convergence in case of a link failure. The network uses Segment Routing. Which mechanism provides sub-50ms protection by using a precomputed backup path?

Accepted Answer

TI-LFA (Topology-Independent Loop-Free Alternate). TI-LFA (Topology-Independent Loop-Free Alternate) with Segment Routing provides fast reroute by computing a backup path using segment lists that avoid the failed link. LFA provides protection but not always topology-independent. LDP FRR is older. BFD is a detection mechanism, not protection.

Answer

BFD (Bidirectional Forwarding Detection)

Answer

LFA (Loop-Free Alternate)

Answer

LDP FRR

Question 13

A service provider is deploying MPLS L2VPN using Virtual Private Wire Service (VPWS). After configuration, the two CEs cannot ping each other. The PE routers show the xconnect interface is up, but no MAC addresses are learned. What is the most likely cause?

Accepted Answer

The VC ID mismatch between the two PEs. If the attachment circuit (AC) is up but no MAC learning occurs, the issue is likely that the MPLS core LSP is not available for the pseudowire. The pseudowire status may be down due to LSP issues. VC ID mismatch would cause the pseudowire to not come up. MTU or encapsulation issues would show errors.

Answer

The MTU on the core links is less than 1500

Answer

The control word is enabled on one PE but not the other

Answer

The encapsulation type on the AC is different on the two PEs

Question 14

A network engineer notices that BGP-LU prefixes learned from an eBGP neighbor are not being installed in the MPLS forwarding table (LFIB). The BGP table shows the prefixes with the correct label. The IGP route to the neighbor is reachable. What additional configuration is needed?

Accepted Answer

Set `label mode per-prefix` under the BGP neighbor or address-family. For BGP-LU to install labels into LFIB, the `label mode` must be set to `per-prefix` or the router must have `mpls bgp forwarding` enabled. Without enabling label allocation per prefix, BGP-LU prefixes are not programmed in LFIB. The other options are not directly related.

Answer

Enable `no bgp default ipv4-unicast`

Answer

Configure `mpls ldp autoconfig` on the interface

Answer

Add `allowas-in` to accept routes with AS-path containing own AS

Question 15

A company is connecting two sites using MPLS L3VPN. The PE routers are running both LDP and Segment Routing (SR-MPLS) in the core. The CE router at Site A cannot reach the CE at Site B. On the PEs, the VRF routes are present. Which command would you use to verify the MPLS forwarding path for the VPN traffic?

Accepted Answer

show mpls forwarding-table vrf VPN-A 10.1.1.0. To verify the MPLS forwarding path, including the label stack used for VPN traffic, the command `show mpls forwarding-table vrf ` shows the outgoing labels and next hop. `show ip route vrf` only shows the IP route without labels. `show mpls ldp bindings` shows LDP bindings but not per-prefix forwarding. `show bgp vpnv4 unicast labels` shows the VPN labels but not the transport labels.

Answer

show mpls ldp bindings

Answer

show ip route vrf VPN-A 10.1.1.0

Answer

show bgp vpnv4 unicast labels