350-501 MPLS and Segment Routing Practice Test 14 — 15 Questions

Question 1

A network operator is implementing segment routing (SR-MPLS) using OSPF as the IGP. Which OSPF TLV is used to advertise the Node SID (Prefix SID) for a router's loopback address?

Accepted Answer

Extended Prefix TLV. In OSPFv2 Segment Routing extensions (RFC 8665), the Prefix SID (Node SID) for a router's loopback is advertised inside the OSPF Extended Prefix TLV, which is carried in an OSPF Extended Prefix Opaque LSA (type 7). The Extended Prefix TLV contains the prefix, the prefix length, and the SID/Label sub-TLV that encodes the Node SID index. This is the mechanism that lets other routers in the SR-MPLS domain build the MPLS label stack for the node.

Answer

Router Information TLV

Answer

Traffic Engineering TLV

Answer

Extended Link TLV

Question 2

An engineer is configuring RSVP-TE fast reroute for link protection. Which protection method uses a pre-configured bypass tunnel that protects multiple LSPs on the same link?

Accepted Answer

Facility backup. In RSVP-TE FRR, facility backup uses a bypass tunnel that can protect multiple LSPs on the same link. In contrast, 1:1 detour creates a separate detour LSP for each protected LSP.

Answer

Local Path Protection

Answer

Global Repair

Answer

1:1 detour

Question 3

In SRv6, which behavior is used for L3VPN decapsulation (removing the outer IPv6 header and SRH, and forwarding the inner IPv4 packet based on the VPN routing table)?

Accepted Answer

End.DT4. End.DT4 is the SRv6 endpoint behavior for decapsulation and table lookup for IPv4 L3VPN. End.DT6 is for IPv6 L3VPN, End.DT2U is for L2VPN.

Answer

End.DT2U

Answer

End.DT6

Answer

End

Question 4

What is the purpose of the SRGB (Segment Routing Global Block) in SR-MPLS?

Accepted Answer

It defines a contiguous block of labels used for global SIDs such as Node SIDs.. The Segment Routing Global Block (SRGB) is a reserved, contiguous range of MPLS labels (default 16000–23999 on Cisco) from which globally significant SIDs — most notably Node SIDs — are allocated. Because the SRGB is consistent across the SR domain, any node can interpret a global SID in that range to reach the same destination, enabling stateless source routing.

Answer

It defines the range of labels for Adjacency SIDs.

Answer

It is used to reserve labels for RSVP-TE tunnels.

Answer

It is the set of labels allowed for MPLS VPNs.

Question 5

In an EVPN-VPWS (Virtual Private Wire Service) deployment, which identifier is used to uniquely identify an Ethernet Segment for multi-homing?

Accepted Answer

Ethernet Segment Identifier (ESI). In EVPN-VPWS, the Ethernet Segment Identifier (ESI) uniquely identifies an Ethernet Segment across all multi-homed Provider Edge (PE) nodes. This 10-byte value is used in Designated Forwarder (DF) election and split-horizon filtering to prevent loops when a Customer Edge (CE) is multi-homed to multiple PEs. The ESI is carried in EVPN routes (Type 1 and Type 4) to signal multi-homing membership.

Answer

Ethernet Tag ID

Answer

MAC address of the CE

Answer

Route Distinguisher (RD)

Question 6

Which component of SRv6 SID format is used to identify the specific function (e.g., End, End.X) on the node?

Accepted Answer

Function. In the SRv6 SID format, the Function field identifies the specific behavior or function to be executed on the node, such as End, End.X, End.DX4, etc. The SID is structured as Locator:Function:Argument, where the Locator routes to the node and the Function selects the local behavior. Therefore, the Function component is what identifies the specific function.

Answer

Argument

Answer

Block

Answer

Locator

Question 7

Which label distribution protocol is replaced by segment routing in SR-MPLS deployments?

Accepted Answer

LDP. In SR-MPLS deployments, segment routing replaces LDP as the label distribution protocol. LDP traditionally distributes labels for each prefix in the IGP, while segment routing uses the IGP itself to advertise prefix-SIDs and adjacency-SIDs, eliminating the need for a separate label distribution protocol. This simplifies the control plane and removes the dependency on LDP.

Answer

RSVP-TE

Answer

IGP itself

Answer

BGP

Question 8

When configuring an MPLS-TE tunnel with affinity/constraint-based routing, what is the effect of assigning a link attribute of 'red' and setting the tunnel's affinity to include 'red' with the 'include-all' keyword?

Accepted Answer

The tunnel must use only links that have the 'red' attribute.. In MPLS-TE affinity/constraint-based routing, the 'include-all' keyword means the tunnel's affinity bits must all be present on a link for that link to be usable. Assigning the link attribute 'red' and setting the tunnel affinity to include 'red' with 'include-all' therefore forces the tunnel to use only links that carry the 'red' attribute. Links without 'red' are excluded from path computation.

Answer

The tunnel will prefer links with 'red' but may use others if necessary.

Answer

The tunnel will use a strict explicit path.

Answer

The tunnel will avoid links with 'red'.

Question 9

An engineer is configuring MPLS on a Cisco router. Which label operation is performed on the ingress LSR when a packet enters the MPLS domain?

Accepted Answer

Push a new label. On the ingress LSR (the first router a packet hits when entering an MPLS domain), the router performs a FIB lookup on the destination IP, determines the appropriate LSP, and imposes (pushes) a new label onto the packet before forwarding it into the MPLS core. This is the fundamental 'label imposition' function of an ingress LER.

Answer

Pop the top label

Answer

No label operation

Answer

Swap the top label

Question 10

An MPLS TE tunnel is configured with an explicit path. Which RSVP message is used by the headend to signal the path and carry the Explicit Route Object (ERO) listing the hops?

Accepted Answer

PATH. In RSVP-TE, the headend router sends a PATH message downstream to signal the establishment of an LSP. The PATH message carries the Explicit Route Object (ERO) that lists the explicit hops the LSP must traverse. The RESV message is sent upstream by the receiver to reserve resources and confirm the path. Therefore, PATH is the correct message.

Answer

RESV

Answer

PATH_TEAR

Answer

RESV_CONFIRM

Question 11

In RSVP-TE, which filter spec style allows multiple senders to share the same reserved resources on a link, as long as their flows do not exceed the reservation?

Accepted Answer

Shared Explicit (SE). The Shared Explicit (SE) filter spec style in RSVP-TE allows multiple senders to share the same reserved resources on a link, as long as their combined flows do not exceed the reservation. This is used for make-before-break and for merging flows from different senders into a single reservation. The SE style explicitly lists the senders that can share the reservation.

Answer

Distinct Explicit (DE)

Answer

Fixed Filter (FF)

Answer

Wildcard Filter (WF)

Question 12

In SRv6, which endpoint behavior is used for decapsulation and forwarding to a Layer 3 VPN IPv4 destination, removing the outer IPv6 header?

Accepted Answer

End.DT4. End.DT4 is the SRv6 endpoint behavior that decapsulates the outer IPv6 header and forwards the inner IPv4 packet into a specific IPv4 VPN routing table (the L3VPN IPv4 table). It is the SRv6 equivalent of MPLS VPN's per-VRF IPv4 lookup after label disposition, making it the correct choice for L3VPN IPv4 decapsulation.

Answer

End.X

Answer

End.DT6

Answer

End.DT2U

Question 13

Which component in EVPN is used to uniquely identify an Ethernet segment across multiple PE devices in a multi-homing scenario?

Accepted Answer

ESI. The Ethernet Segment Identifier (ESI) is a 10-byte value that identifies a specific multihomed site.

Answer

RT-2

Answer

MAC-VRF

Answer

EVI

Question 14

Which THREE are valid SRv6 endpoint behaviors for VPN services? (Choose three.)

Accepted Answer

End.DT2U. End.DT2U (A) is a valid SRv6 endpoint behavior that performs decapsulation and looks up the inner Ethernet frame in a specified VRF/bridge domain, enabling Layer 2 VPN (EVPN) services. End.DT4 (C) is valid because it decapsulates an IPv4 packet and performs an IPv4 table lookup in the associated VRF, which is the SRv6 behavior used for IPv4 L3VPN. End.DT6 (E) is likewise valid, decapsulating an IPv6 packet and doing an IPv6 table lookup in the VRF for IPv6 L3VPN services. End.B6 (B) is not a VPN endpoint behavior; it is used for SRv6 binding SID / steering (e.g., End.B6.Encaps) in traffic-engineering contexts, not VPN service termination. End.X (D) is a cross-connect behavior that forwards to a specific adjacency in a layer-3 adjacency set, used for TE and not for VPN service endpoint decapsulation.

Answer

End.B6

Answer

End.X

Question 15

Which THREE are mechanisms used in EVPN to support multi-homing and load balancing? (Choose three.)

Accepted Answer

Aliasing. Aliasing (A) is correct because EVPN uses the aliasing mechanism so that a PE can advertise reachability to a remote Ethernet Segment Identifier (ESI) even when it is not the Designated Forwarder, allowing all multi-homed PEs to receive and forward traffic toward the CE, which enables active-active load balancing. The Ethernet Segment Identifier (B) is correct because the ESI is the unique identifier assigned to a multi-homed Ethernet Segment, and it is the key construct that lets EVPN PEs recognize that they share the same CE attachment and coordinate multi-homing behavior. Designated Forwarder (DF) election (D) is correct because EVPN runs a DF election per ESI and per VLAN (or per Ethernet A-D route) to select which PE forwards BUM traffic, preventing loops and duplication while the other PEs remain available for unicast load balancing. Spanning Tree Protocol (C) is not an EVPN multi-homing mechanism; EVPN avoids relying on STP by using ESI, DF election, and aliasing, and STP is not used to coordinate EVPN multi-homed PEs. Virtual Router Redundancy Protocol (E) is a first-hop gateway redundancy protocol for IP hosts and is unrelated to EVPN multi-homing or load balancing across PEs attached to an Ethernet Segment.

Answer

Spanning Tree Protocol (STP)

Answer

Virtual Router Redundancy Protocol (VRRP)