350-501 MPLS and Segment Routing Practice Test 6 — 15 Questions

Question 1

Which MPLS TE mechanism provides link protection by pre-establishing a backup LSP that bypasses a specific link, activated when the link fails?

Accepted Answer

Bypass tunnel. A bypass tunnel in MPLS TE FRR is a pre-established backup LSP that is created to protect a specific link or node, and it is signaled in advance so it can be activated immediately when the protected resource fails. It bypasses the failed link by taking an alternate path, providing sub-50ms recovery.

Answer

LDP FRR

Answer

Detour tunnel

Answer

TE backup path

Question 2

In Segment Routing (SR-MPLS), a label allocated from the Segment Routing Global Block (SRGB) for a specific router's loopback is known as what?

Accepted Answer

Node SID. In Segment Routing MPLS, a Node SID is a specific type of Prefix SID that is allocated from the SRGB and assigned to the loopback address of a specific router. While the Prefix SID is a broader term that can refer to any prefix, the Node SID uniquely identifies a particular node. Therefore, for a specific router's loopback, it is correctly known as a Node SID (option B). Option C (Prefix SID) is too general and not the specific term for this scenario.

Answer

Anycast SID

Answer

Prefix SID

Answer

Adjacency SID

Question 3

An engineer is configuring SR-MPLS in an IS-IS network. Which IS-IS TLV is used to advertise the Segment Routing Global Block (SRGB) and Node-SID assignments?

Accepted Answer

TLV 242. IS-IS TLV 242 is the Router Capability TLV, which carries the Segment Routing Capability sub-TLV used to advertise the SRGB and Node-SID assignments. It is defined in RFC 8667 for IS-IS SR extensions. TLV 135 is the extended IP reachability TLV, TLV 22 is the extended IS reachability TLV, and TLV 229 is the MPLS label binding TLV.

Answer

TLV 135

Answer

TLV 229

Answer

TLV 22

Question 4

In SRv6 micro-SID (uSID) compression, which part of the SID is used to carry a compressed instruction set, enabling efficient forwarding with a shorter SID list?

Accepted Answer

Function. In SRv6 micro-SID (uSID) compression, the Function part of the SID carries the compressed instruction set. The uSID format splits the 128-bit SID into a Block, a Node ID, and a Function, where the Function encodes the forwarding behavior (e.g., END, END.X) in a compressed form. This allows multiple uSIDs to be packed into a single 128-bit SID list.

Answer

Block

Answer

Locator

Answer

Argument

Question 5

In EVPN multi-homing with all-active mode, which mechanism is used to elect a designated forwarder (DF) for broadcast, unknown unicast, and multicast (BUM) traffic on a given Ethernet segment?

Accepted Answer

DF election via ESI label and route type 4. In EVPN all-active multi-homing, the Designated Forwarder (DF) for a given Ethernet Segment is elected per VLAN using the DF election procedure defined in RFC 7432, which relies on the Ethernet Segment Identifier (ESI) and the EVPN route type 4 (Ethernet Segment route). The PE routers attached to the same ES exchange type-4 routes, and the DF is chosen based on the highest IP address (or lowest, depending on configuration) among the candidates, with the ESI label used to identify the segment.

Answer

Load balancing via MLAG

Answer

ES-IS protocol

Answer

MSTP

Question 6

An SR-TE policy is configured with a candidate path containing multiple segment lists. Which selection criteria is used to choose the active segment list among the candidate paths?

Accepted Answer

Preference value. In SR-TE (Segment Routing Traffic Engineering), when a candidate path contains multiple segment lists, the active segment list is selected based on the preference value. The segment list with the highest preference among the valid, active candidate paths becomes the active forwarding path. This is defined in RFC 9256 (SR Policy Architecture).

Answer

Color

Answer

Metric type (IGP or TE)

Answer

Hop count

Question 7

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

Accepted Answer

Label swapping is performed at every transit LSR.. Option A is correct because every transit LSR along an LSP performs label swapping: it receives a labeled packet, looks up the incoming label in its LFIB, and replaces it with the outgoing label toward the next hop. Option B is correct because in Penultimate Hop Popping (PHP), the penultimate LSR pops the top (and typically only) label and forwards the packet unlabeled to the egress LSR, saving the egress a lookup. Option C is wrong because the ingress LSR performs a push (imposing a label), not a swap, since the packet arrives unlabeled. Option D is wrong because the egress LSR does not always pop — with PHP the penultimate LSR already removed the label, so the egress receives an unlabeled packet. Option E is wrong because popping is not always at the penultimate hop; it can occur at the egress LSR (e.g., when PHP is disabled) or at any LSR terminating an LSP.

Answer

The ingress LSR performs a swap operation.

Answer

The egress LSR always performs a pop operation.

Answer

Label popping always happens at the penultimate hop.

Question 8

Which two actions are performed by an MPLS label edge router (LER) when it receives an unlabeled IP packet destined for a prefix with an LSP? (Choose two.)

Accepted Answer

It forwards the packet based on the MPLS label.. An MPLS label edge router (LER), also called a provider edge (PE) router, is the ingress point into the MPLS domain, so when it receives an unlabeled IP packet whose destination prefix is reachable via an LSP, it must impose (push) an MPLS label onto the packet — this is exactly what option E describes, the label push operation that encapsulates the IP packet for transport across the MPLS core. Once the label is pushed, the LER forwards the now-labeled packet according to that MPLS label rather than re-examining the IP header, which is what option C states, since the LFIB entry associated with the pushed label dictates the outgoing interface and next-hop label operation. Option A is incorrect because the recursive lookup in the LFIB is a function of transit label switch routers (LSRs) resolving the next-hop for an already-labeled packet, not the ingress LER's initial imposition step. Option B is incorrect because label swap (swap the incoming label for an outgoing label) is performed by core LSRs on labeled packets, not by an LER receiving an unlabeled IP packet. Option D is incorrect because popping the top label and forwarding based on the inner header describes penultimate hop popping (PHP) or egress LER behavior for labeled packets leaving the MPLS domain, not ingress imposition.

Answer

It performs a recursive lookup in the LFIB.

Answer

It performs a label swap operation.

Answer

It pops the top label and forwards based on the inner header.

Question 9

A service provider is transitioning from LDP-based MPLS to Segment Routing (SR-MPLS). Which TWO statements correctly describe differences or interoperability considerations? (Choose two.)

Accepted Answer

The Segment Routing Global Block (SRGB) is a reserved label range that is used to allocate Prefix-SIDs, ensuring end-to-end label consistency.. Option C is correct because the Segment Routing Global Block (SRGB) is the reserved local label range (default 16000–23999 on many platforms) from which Prefix-SIDs/Node-SIDs are allocated, and when every node uses the same SRGB the resulting global SIDs remain consistent end-to-end across the SR-MPLS domain. Option D is correct because an Adjacency-SID is a local label bound to a specific adjacency/link (typically allocated from the SR Local Block, SRLB) and is used to explicitly steer traffic onto that link, for example in TI-LFA or explicit path forwarding. Option A is wrong because a Node-SID is allocated from the SRGB (global range), not the dynamic label range, and it is globally unique within the SR domain rather than only within an IGP area. Option B is wrong because LDP and SR-MPLS can coexist on the same router, with SR-MPLS mapping and preferring SR SIDs while LDP may still be used for interworking or legacy neighbors. Option E is wrong because SR-MPLS does not swap the top label like LDP; the ingress router pushes the label stack and each hop processes/forwards based on the top SID, with PHP or penultimate popping behavior rather than label swapping.

Answer

In SR-MPLS, the Node-SID is allocated from the dynamic label range and is unique only within the IGP area.

Answer

LDP and SR-MPLS cannot coexist on the same router; the router must be configured exclusively for one or the other.

Answer

In SR-MPLS, the label stack is built by the ingress router and nodes swap the top label without pushing additional labels.

Question 10

When deploying EVPN with SR-MPLS or SRv6, which THREE statements about EVPN multi-homing and related concepts are correct? (Choose three.)

Accepted Answer

EVPN IRB (Integrated Routing and Bridging) enables both Layer 2 and Layer 3 forwarding over the same EVPN instance using an anycast gateway.. Option A is correct because EVPN IRB integrates Layer 2 bridging and Layer 3 routing within the same EVPN instance, and the anycast gateway (a shared IP/MAC across PEs) lets hosts use a single default gateway while traffic is routed optimally at the ingress PE. Option B is correct because aliasing lets a PE that has not yet learned a specific MAC address still forward traffic toward a multi-homed Ethernet segment by using the ESI and the remote PE's EVPN Inclusive Multicast/IMET or MAC/IP advertisement, thereby avoiding drops during MAC learning. Option E is correct because Designated Forwarder election runs per (ESI, VLAN) — or per (ESI, VLAN bundle) — so that only one PE attached to the multi-homed segment forwards BUM traffic for a given VLAN to the CE, preventing duplicate frames. Option C is not marked correct: although the ESI does identify a multi-homed site and is used for split-horizon filtering, the statement as written is incomplete/less precise than the intended correct answers and omits the per-ESI DF and aliasing roles. Option D is not correct because EVPN VPWS is a point-to-point service that uses a pair of Ethernet Auto-Discovery routes and does not provide multipoint service within a single EVPN instance.

Answer

The Ethernet Segment Identifier (ESI) is a unique identifier for a multi-homed site, used to detect multi-homing and for split-horizon filtering.

Answer

EVPN VPWS (Virtual Private Wire Service) uses a single EVPN instance for both point-to-point and multipoint services.

Question 11

A service provider runs an MPLS L3VPN with a single BGP route reflector serving all PE routers. A new PE router is added but is not configured as a route reflector client. VPNv4 prefixes from the new PE are not reaching other PEs. Which BGP address family must be activated on the new PE and the route reflector to exchange VPNv4 routes?

Accepted Answer

address-family vpnv4 unicast. The VPNv4 unicast address family is specifically designed to carry MPLS L3VPN IPv4 prefixes between PE routers. When a new PE is added to a route reflector topology, both the PE and the route reflector must have this address family activated to exchange VPNv4 routes. Without it, the PE cannot advertise its VPN prefixes, and other PEs cannot reach its attached customer sites.

Answer

address-family ipv4 unicast

Answer

address-family ipv4 vrf

Answer

address-family vpnv6 unicast

Question 12

A network engineer is implementing Segment Routing over an MPLS data plane in a service provider core. The engineer must ensure that the IGP advertises the necessary information for SR-MPLS to function. Which TWO actions are required when configuring IS-IS for Segment Routing? (Choose two.)

Accepted Answer

Enable segment-routing mpls under the IS-IS address-family.. To enable SR-MPLS with IS-IS, the IGP must advertise Segment Routing extensions. This is achieved by enabling segment-routing mpls under the IS-IS address family. Additionally, a consistent Segment Routing Global Block (SRGB) must be defined on each router to ensure that Prefix-SIDs map to the correct MPLS labels. These two actions are fundamental for SR-MPLS to function correctly in an IS-IS network.

Answer

Enable MPLS traffic engineering under IS-IS.

Answer

Enable LDP under the IS-IS interface configuration.

Answer

Configure a unique Node SID for each router.

Question 13

A network engineer is deploying Segment Routing over an MPLS core using OSPFv2 as the IGP. The engineer must ensure that Prefix-SIDs are advertised and that the MPLS forwarding plane is properly configured. Which TWO statements are correct about the required configuration? (Choose two.)

Accepted Answer

OSPFv2 must be enabled with the segment-routing mpls command under the router ospf configuration.. To deploy SR-MPLS with OSPFv2, you must enable segment routing under the OSPF process so that Prefix-SIDs are advertised, and you must enable MPLS forwarding on the interfaces that will carry MPLS traffic. The SRGB can be different per router, and LDP does not need to be disabled.

Answer

LDP must be disabled on all interfaces to avoid conflicts with Segment Routing labels.

Answer

The Segment Routing Global Block (SRGB) must be identical on all routers to ensure consistent Prefix-SID to label mapping.

Answer

Each router must be configured with a unique Prefix-SID for each of its loopback interfaces.

Question 14

A service provider is deploying Segment Routing over an MPLS core. The network engineer must ensure that the IGP advertises Prefix-SIDs for all loopback interfaces. Which TWO statements about Prefix-SID advertisement in IS-IS are correct? (Choose two.)

Accepted Answer

The Prefix-SID is advertised in the IS-IS Prefix Reachability TLV using the SID/Label sub-TLV.. Prefix-SIDs in IS-IS are advertised in the Extended IP Reachability TLV (TLV 135) using the SID/Label sub-TLV, and the prefix attribute flags sub-TLV indicates the SID type. The Router Capability TLV is for SRGB and capabilities, not per-prefix SIDs. Hello PDUs are for adjacency, not prefix advertisement.

Answer

The Prefix-SID is advertised in the IS-IS Router Capability TLV using the SR Capability sub-TLV.

Answer

The Prefix-SID is advertised in the IS-IS Hello PDU to ensure neighbor adjacency.

Answer

The Prefix-SID is advertised only if the prefix is a loopback address.

Question 15

A network engineer is deploying Segment Routing over an MPLS network using IS-IS as the IGP. The engineer wants to ensure that the Prefix-SID for a loopback interface is advertised with the N-flag set, indicating that the Prefix-SID is from the Node-SID space. Which configuration is required on the router to achieve this?

Accepted Answer

Enable segment-routing mpls under the IS-IS process and configure the loopback interface with the prefix-sid index command using the node-sid keyword.. In SR-MPLS with IS-IS, the Prefix-SID is advertised in the IS-IS Prefix-SID sub-TLV. To set the N-flag, which indicates a Node-SID, the prefix-sid index command must include the node-sid keyword. Additionally, segment-routing mpls must be enabled under the IS-IS process. This combination ensures that the Prefix-SID is advertised as a Node-SID, allowing other routers to use it for node reachability in the SR domain.

Answer

Enable MPLS LDP on all interfaces and configure the loopback with a static label mapping.

Answer

Configure the loopback interface with the prefix-sid index command under the IS-IS address family.

Answer

Configure the router to use LDP with segment routing mapping server.