350-501 Architecture Practice Test 4 — 15 Questions

Question 1

An SP is deploying Inter-AS MPLS VPN option B. Which design characteristic is unique to Option B compared to Option A?

Accepted Answer

ASBRs exchange VPNv4 routes directly via MP-EBGP. In Option B, ASBRs exchange VPNv4 routes directly via MP-EBGP without requiring a full mesh of MP-IBGP sessions between PEs. This eliminates the need for per-VPN VRF configuration on ASBRs, as they only carry labeled VPNv4 routes and perform label allocation per next-hop, not per VPN.

Answer

Requires a full mesh of MP-IBGP sessions between all PEs

Answer

ASBRs maintain separate VRF for each VPN

Answer

Traffic is forwarded using IP, not MPLS

Question 2

A service provider is implementing EVPN with MPLS data plane. Which encapsulation type is used in MPLS EVPN to carry Ethernet frames across the MPLS network?

Accepted Answer

MPLS label stack with a control word for Ethernet frames. In MPLS EVPN, Ethernet frames are carried across the MPLS network using an MPLS label stack that includes a control word. The control word (4 bytes) is inserted between the MPLS label stack and the Ethernet frame to prevent misordering and to enable proper frame delineation. This encapsulation is defined in RFC 8214 (formerly RFC 7432) and is distinct from older EoMPLS or simple IP/MPLS tunneling.

Answer

Ethernet over MPLS (EoMPLS) with VLAN encapsulation

Answer

EVPN encapsulation defined by IETF RFC 7432

Answer

IP/MPLS tunnel with Ethernet frame inside

Question 3

Which protocol is used to discover MPLS peers for LDP session establishment?

Accepted Answer

UDP. LDP uses UDP multicast (224.0.0.2) on port 646 to discover directly connected MPLS peers, as specified in RFC 5036. This allows LDP to dynamically find neighbors without manual configuration, using UDP for the initial discovery phase before establishing a TCP session for label exchange.

Answer

TCP

Answer

ARP

Answer

ICMP

Question 4

A service provider is designing a core network with IS-IS as the IGP. To support MPLS traffic engineering, which IS-IS extensions are required?

Accepted Answer

IS-IS TE extensions (RFC 5305). IS-IS TE extensions (RFC 5305) are required to carry MPLS Traffic Engineering information, such as link bandwidth, administrative groups, and TE metric, in Type-Length-Value (TLV) fields within IS-IS Link State PDUs. These extensions enable the IGP to flood TE attributes that MPLS TE uses for constraint-based path computation, such as with RSVP-TE or Segment Routing.

Answer

IS-IS multi-topology (MT)

Answer

IS-IS wide metrics

Answer

IS-IS L1/L2 routing

Question 5

In Segment Routing over IPv6 (SRv6), what is the function of the SRH (Segment Routing Header)?

Accepted Answer

It contains the segment list (ordered list of SIDs). The Segment Routing Header (SRH) is a new IPv6 extension header defined in RFC 8754 that carries an ordered list of Segment Identifiers (SIDs). When a packet is forwarded along a segment-routed path, the SRH encodes the explicit path by listing each segment (node or adjacency SID) in sequence, and the active segment is updated as the packet traverses each hop. This allows the source node to steer packets through a specific path without intermediate nodes needing to maintain per-flow state.

Answer

It replaces the IPv6 header to reduce overhead

Answer

It provides OAM capabilities for path monitoring

Answer

It uses the IPv6 flow label to encode the path

Question 6

Which TWO statements about EVPN Route Type 2 (MAC/IP advertisement) are correct?

Accepted Answer

It is used for MAC learning and ARP suppression. EVPN Route Type 2 is used to advertise MAC and IP addresses of hosts, enabling MAC learning and ARP suppression in EVPN-VXLAN fabrics. By distributing host MAC and IP bindings via BGP, it allows VTEPs to learn remote MAC addresses without data-plane flooding and to suppress ARP requests by responding locally when the IP-to-MAC mapping is known.

Answer

It provides load balancing via aliasing

Answer

It includes the Ethernet Segment Identifier (ESI)

Answer

It is not used for host mobility

Question 7

Which THREE are requirements for deploying MPLS Traffic Engineering with RSVP-TE?

Accepted Answer

Links must have reservable bandwidth advertised. MPLS TE with RSVP-TE requires that links advertise reservable bandwidth so that the Constrained Shortest Path First (CSPF) algorithm can compute paths that satisfy bandwidth constraints. Without reservable bandwidth advertised via IGP extensions (e.g., OSPF TE or IS-IS TE), RSVP-TE cannot reserve resources along the path.

Answer

OSPFv3 must be the IGP

Answer

LDP must be enabled on all routers

Question 8

Which TWO are functions of the MPLS data plane?

Accepted Answer

Label push at ingress LSR. The MPLS data plane is responsible for the actual forwarding of packets based on labels. At the ingress LSR, the data plane performs a label push operation, which involves inserting a new MPLS label (or a stack of labels) onto the incoming IP packet before forwarding it into the MPLS domain. This action is a fundamental data-plane function, as it directly manipulates the packet header to enable label switching.

Answer

Label distribution via LDP

Answer

Metric calculation for routing

Answer

Route exchange via BGP

Question 9

Refer to the exhibit. The router has an outgoing label of 'Pop' for prefix ***********. What does this imply?

Accepted Answer

The router is the penultimate hop and will remove the label before forwarding. When a router has an outgoing label of 'Pop' for a prefix in MPLS, it indicates that the router is the penultimate hop (PHP) in the LSP. PHP means the router will remove (pop) the top label before forwarding the packet to the next hop, reducing the burden on the egress LSR. This is standard MPLS behavior as per RFC 3031, where the egress router signals an implicit null label (label 3) to its upstream neighbor.

Answer

The router has not learned a label for that prefix

Answer

The router will not forward traffic for that prefix

Answer

The router will push a new label for that prefix

Question 10

Refer to the exhibit. The router is configured as a route reflector for VPNv4. What is the requirement for the route reflector to propagate VPNv4 routes received from a client to other clients?

Accepted Answer

The route reflector must include the originator ID attribute to prevent loops. The route reflector must include the originator ID attribute to prevent routing loops when reflecting VPNv4 routes between clients. The originator ID is set to the router ID of the originating IBGP speaker, and the route reflector ignores any route with an originator ID equal to its own router ID, thereby preventing loops in a client-to-client reflection scenario.

Answer

The route reflector must filter routes based on route-target communities

Answer

The route reflector must use the same cluster ID for all clients

Answer

The route reflector must set the next-hop to itself for all reflected routes

Question 11

Refer to the exhibit. This JSON policy is used for Segment Routing Traffic Engineering. What is the purpose of the 'color' attribute?

Accepted Answer

It identifies a specific SR-TE policy to be used for traffic steering. In Segment Routing Traffic Engineering (SR-TE), the 'color' attribute is a 32-bit numerical value that, combined with the destination prefix, uniquely identifies an SR-TE policy. This allows traffic to be steered into a specific policy based on color, enabling advanced traffic engineering use cases such as multi-topology routing or service-level agreement (SLA) differentiation.

Answer

It sets the metric type for the path calculation

Answer

It defines the preference value for the candidate path

Answer

It specifies the link affinity constraint for the path

Question 12

A service provider wants to provide Layer 3 VPN services to customers using MPLS. Which technology is used to distribute VPNv4 routes between PE routers?

Accepted Answer

MP-BGP. MP-BGP is used to carry VPNv4 routes between PE routers. OSPF, EIGRP, and IS-IS are IGPs and do not carry VPNv4 routes.

Answer

IS-IS

Answer

OSPF

Answer

EIGRP

Question 13

Which MPLS label is used to identify the egress PE in a Layer 3 MPLS VPN?

Accepted Answer

VPN label. In a Layer 3 MPLS VPN, the VPN label (also known as the service label) is assigned by the egress PE router and advertised via MP-BGP (RFC 4364). It uniquely identifies the VPN forwarding instance (VRF) on the egress PE, allowing the correct per-VPN forwarding decision to be made after the transport label has been removed. Without the VPN label, the egress PE would not know which VRF or customer-facing interface should receive the packet.

Answer

Transport label

Answer

LDP label

Answer

IGP label

Question 14

What is the purpose of the 'mpls ldp autoconfig' command on an interface?

Accepted Answer

Automatically configure LDP on the interface. The 'mpls ldp autoconfig' command is used on an interface to automatically enable LDP on that interface for all IPv4 address families configured under the OSPF process. This eliminates the need to manually configure 'mpls ldp' on each interface, simplifying deployment in large-scale networks where LDP should be enabled on all OSPF-enabled interfaces.

Answer

Enable MPLS forwarding

Answer

Enable LDP on all interfaces

Answer

Disable LDP on the interface

Question 15

A customer is experiencing intermittent packet loss in an MPLS L3VPN network. The PE routers show no errors on interfaces. Which tool can help isolate the issue by showing the complete path a packet takes through the MPLS network?

Accepted Answer

MPLS traceroute. MPLS traceroute (often implemented via LSP ping with the 'traceroute' option or the 'mpls traceroute' command) is the correct tool because it uses MPLS echo request/reply messages (RFC 8029) to traverse the label-switched path (LSP) hop by hop, revealing the exact label stack and transit LSRs. This allows identification of a failing or misbehaving node in the MPLS core that standard IP traceroute would not see, since IP traceroute relies on TTL expiry in the IP header, which may not occur in an MPLS network where labels are swapped and IP TTL is often copied from the label TTL.

Answer

Traceroute

Answer

Ping

Answer

MPLS ping