350-501 MPLS and Segment Routing — 40 Questions

Question 1

An engineer is configuring MPLS LDP on a Cisco router. The router uses downstream unsolicited mode with liberal label retention. Which behavior describes the label distribution and retention on this router?

Accepted Answer

The router sends label bindings to all neighbors without receiving requests and retains all received label bindings.. Downstream unsolicited means the downstream router sends label bindings to all neighbors without being asked. Liberal label retention means the router keeps all received label bindings, even if the next hop is not reachable.

Answer

The router requests labels from neighbors and retains only labels for the best path.

Answer

The router sends label bindings only for directly connected routes and discards labels from non-reachable next hops.

Answer

The router sends label bindings only in response to requests from upstream neighbors and retains only labels for directly connected next hops.

Question 2

In MPLS, which operation is performed when a packet arrives at a transit router and the top label matches the router's own label assigned for the FEC?

Accepted Answer

Pop the top label.. When the top label is the router's own label (often an implicit null or explicit null), the router pops the label and forwards the packet based on the next label or IP header.

Answer

Push a new label onto the stack.

Answer

Drop the packet.

Answer

Swap the top label with a new label.

Question 3

An engineer is deploying MPLS-TE with FRR using the 1:1 detour method. Which statement about link protection in this scenario is true?

Accepted Answer

The detour LSP merges back with the main LSP after the protected link.. 1:1 detour creates a one-to-one backup LSP for each protected LSP. The detour LSP is pre-signaled and protects against link failure by re-routing traffic around the failed link.

Answer

The detour LSP is created dynamically after a failure is detected.

Answer

Link protection requires node protection to be configured.

Answer

A single bypass tunnel protects multiple LSPs over the same link.

Question 4

In Segment Routing (SR-MPLS), which IGP TLV is used by IS-IS to advertise the Node SID for a prefix?

Accepted Answer

TLV 135 (Extended IP Reachability) with sub-TLV 3 for Prefix-SID.. IS-IS uses the IS-IS Router CAPABILITY TLV (135) with sub-TLV 19 to carry the SR-capable information and prefix-SID. Specifically, the Prefix-SID is advertised in the IS-IS Extended IP Reachability TLV (135) with sub-TLV 3.

Answer

TLV 22 (IS Reachability) with sub-TLV 22 for Adjacency SID.

Answer

TLV 130 (IP Address) with sub-TLV 1 for Node SID.

Answer

TLV 137 (Dynamic hostname) with sub-TLV 5 for SID.

Question 5

Which RSVP message is used to tear down an MPLS-TE LSP by being sent from the ingress toward the egress?

Accepted Answer

PATH Tear. RSVP-TE uses PATH messages to establish state and RESV to reserve. To tear down, a PathTear message is sent from the ingress to the egress, or a ResvTear from egress to ingress.

Answer

PATH Error

Answer

RESV Confirm

Answer

RESV Tear

Question 6

An engineer configures an SR-TE policy with a segment list containing adjacency SIDs. The required path includes a strict loose hop through a specific router. Which segment type should be used for the strict hop?

Accepted Answer

Adjacency SID for the desired link.. Adjacency SIDs force the packet to traverse a specific link. For a strict hop, the adjacency SID ensures the packet goes through that exact link. A Node SID would allow any path to the node.

Answer

Anycast SID for the group containing the router.

Answer

Binding SID (BSID) of another policy.

Answer

Node SID (Prefix SID) of the router.

Question 7

In SRv6, which Segment Identifier (SID) format is used to encode a function that performs IPv6 decapsulation and forwarding?

Accepted Answer

End.DT4. SRv6 SIDs are 128-bit and structured as Locator:Function:Arguments. End.DT4 is a function that decapsulates the outer IPv6 header and forwards the inner IPv4 packet based on destination.

Answer

End.X

Answer

End.B6

Answer

End

Question 8

What is the primary purpose of the SRGB (Segment Routing Global Block) when configuring SR-MPLS on a network?

Accepted Answer

To provide a common label space for global segments, ensuring consistent label to prefix mapping.. SRGB defines a range of labels reserved for global segments (e.g., Node SIDs). It ensures that a given Node SID maps to the same label across all routers in the SR domain, simplifying operations.

Answer

To set aside labels for MPLS-TE tunnels.

Answer

To allocate labels for adjacency SIDs only.

Answer

To define the range for LDP labels used in SR interworking.

Question 9

In EVPN-VPWS, which component is used to identify a multi-homed Ethernet segment and enable load balancing and fast convergence?

Accepted Answer

Ethernet Segment Identifier (ESI). The Ethernet Segment Identifier (ESI) is a unique identifier for an Ethernet segment that is multi-homed to two or more PEs. ESI is used for DF election and aliasing.

Answer

Ethernet Tag ID

Answer

MAC address

Answer

EVPN Route Target

Question 10

A service provider is migrating from LDP to SR-MPLS. Which mechanism allows a router to allocate labels for prefixes using both LDP and SR simultaneously during the migration?

Accepted Answer

Segment Routing Mapping Server (SRMS). Cisco's SR/LDP interworking allows coexistence. Routers can run both LDP and SR, and a mapping server advertises SR labels for LDP-learned prefixes. This enables seamless migration.

Answer

LDP over SR tunnel

Answer

RSVP-TE interworking

Answer

MPLS forwarding plane adjustment

Question 11

In MPLS-TE, what is the purpose of the Explicit Route Object (ERO) in the RSVP PATH message?

Accepted Answer

To specify the desired path as a list of nodes or links.. ERO specifies the strict or loose hops that the TE LSP must traverse. It allows the headend to enforce a specific path through the network.

Answer

To label the path with a tunnel ID.

Answer

To reserve bandwidth along the path.

Answer

To record the actual path taken by the LSP.

Question 12

An engineer configures an SRv6 SID with the behavior End.DT6. What is the primary function of this SID?

Accepted Answer

Perform IPv6 decapsulation and forward using a VPN IPv6 routing table.. End.DT6 (Endpoint with Decapsulation and IPv6 table lookup) is used for L3VPN over SRv6. It decapsulates the outer IPv6 header and forwards based on the inner IPv6 destination using a specific VRF table.

Answer

Perform IPv4 decapsulation and forward using a VPN IPv4 table.

Answer

Perform IPv6 encapsulation and forward.

Answer

Perform IPv6 decapsulation and forward using the global IPv6 routing table.

Question 13

An engineer is troubleshooting an MPLS-TE tunnel that is not coming up. The configuration includes an explicit path with strict hops. Which TWO conditions could prevent the tunnel from being established? (Choose two.)

Accepted Answer

RSVP is not enabled on an interface along the path.. An explicit path with strict hops requires each hop to be directly connected. If a hop is not directly connected, the path fails. Also, if there is no RSVP capability along the path, the tunnel cannot be signaled.

Answer

The tunnel interface has an IP address.

Answer

The destination loopback is not advertised in IGP.

Answer

The path includes a router that does not support MPLS.

Question 14

In SR-MPLS, which THREE statements correctly describe the properties of Adjacency SIDs? (Choose three.)

Accepted Answer

Adjacency SIDs can be used in segment lists for SR-TE policies.. Adjacency SIDs are local to a router and represent a specific link. They are not global and are used for strict path control. They are allocated from a local label range (not SRGB).

Answer

Adjacency SIDs are allocated from the SRGB.

Answer

Adjacency SIDs are global and must be unique across the SR domain.

Question 15

An engineer is configuring EVPN IRB (Integrated Routing and Bridging) to provide IP connectivity between tenants across a service provider network. Which THREE components are essential for EVPN IRB operation? (Choose three.)

Accepted Answer

IP-VRF for each tenant.. EVPN IRB requires an IP-VRF to represent the tenant, a bridge domain (EVI) for Layer 2, and IRB interfaces (Layer 3 gateway) that connect the bridge domain to the VRF. BGP EVPN control plane is also needed.

Answer

LDP or SR for label distribution.

Answer

MPLS-TE tunnels for transport.

Question 16

In MPLS, which label operation is performed by a Penultimate Hop Popping (PHP) router?

Accepted Answer

Pop the top label from the stack. In PHP, the penultimate router pops the label before forwarding to the egress, reducing the egress router's work.

Answer

Swap the top label with a new label

Answer

Push a new label onto the stack

Answer

Swap and push an additional label

Question 17

Which label distribution mode does LDP use by default in Cisco IOS?

Accepted Answer

Downstream unsolicited. LDP uses downstream unsolicited mode by default, where a router advertises labels to all neighbors without being asked.

Answer

Downstream on-demand

Answer

Upstream unsolicited

Answer

Upstream on-demand

Question 18

Which RSVP-TE message is used to reserve resources along the path and includes the Explicit Route Object (ERO)?

Accepted Answer

RESV. The RESV message is sent upstream from the destination to the source to reserve resources and confirm the path; the ERO is in the PATH message, not RESV.

Answer

RESV_CONFIRM

Answer

PATH

Answer

PATH_TEAR

Question 19

In Segment Routing for MPLS (SR-MPLS), which TLV is used in IS-IS to advertise the Node SID?

Accepted Answer

TLV 149. IS-IS uses the SR-Capabilities sub-TLV and the SR-Algorithm sub-TLV, but the Node SID is advertised in the SR-MPLS SID/Label Binding TLV (TLV 149) with a sub-TLV. The exact TLV is 149 for SR-MPLS.

Answer

TLV 232

Answer

TLV 135

Answer

TLV 22

Question 20

Which label is used for an Adjacency SID in SR-MPLS?

Accepted Answer

A label from the dynamic label range. The Adjacency SID is local to the router and represents a specific link; it is typically allocated from the dynamic label range, not the SRGB.

Answer

A label from RSVP-TE reserved range

Answer

A label from the static label range

Answer

A label from the SRGB (Segment Routing Global Block)