350-501 Practice Test 33 — 15 Questions

Question 1

A network engineer is troubleshooting an OSPF adjacency issue between two routers connected via a serial link. The routers are configured with point-to-point network type. The adjacency stays in EXSTART state. What is the most likely cause?

Accepted Answer

Mismatched MTU.. On a point-to-point OSPF link, the EXSTART state indicates that the routers have formed a bidirectional communication (2WAY) and are now negotiating the master/slave relationship for database description (DBD) packet exchange. A mismatched MTU between the two interfaces causes the DBD packets to be dropped or rejected, preventing the adjacency from progressing beyond EXSTART. This is a common issue on serial links where one side may have a different MTU configured.

Answer

Mismatched OSPF area ID.

Answer

Mismatched hello/dead intervals.

Answer

Duplicate router IDs.

Question 2

Which TWO queuing mechanisms best meet the requirements for voice traffic (low latency and jitter) in a service provider network?

Accepted Answer

LLQ (Low Latency Queuing). LLQ is correct because it combines a strict priority queue with CBWFQ, ensuring voice traffic (which is delay-sensitive) is always serviced first, thereby minimizing latency and jitter. The priority queue is policed to prevent starvation of other traffic, making it ideal for real-time applications like VoIP in service provider networks.

Answer

WRED (Weighted Random Early Detection)

Answer

CBWFQ without LLQ

Answer

FIFO (First In First Out)

Question 3

A service provider router receives a route with a higher specific prefix length than the same prefix in the routing table. Which route will be preferred?

Accepted Answer

The more specific prefix.. C is correct because a router always prefers the most specific prefix (longest prefix length) when forwarding packets, regardless of administrative distance or metric. This is a fundamental rule of IP routing: the route with the longest subnet mask match is chosen first, as it represents the most precise path to the destination.

Answer

The route with higher administrative distance.

Answer

The route with lower metric.

Answer

The less specific prefix.

Question 4

A service provider is implementing MPLS in their core network. They want to ensure that labeled packets for BGP prefixes are forwarded correctly without requiring an IGP label for every BGP prefix. Which technique should be used to achieve this?

Accepted Answer

Use BGP-free core with LDP or Segment Routing. A BGP-free core with LDP or Segment Routing allows the service provider to forward labeled packets for BGP prefixes without requiring an IGP label for every BGP prefix. In this design, edge routers (PEs) impose a label stack where the outer label (IGP label) is distributed by LDP or Segment Routing for the BGP next-hop, and the inner label is the BGP label. Core routers (P routers) only need to swap the outer label based on the IGP label, not the BGP prefix, thus eliminating the need for IGP labels per BGP prefix.

Answer

Enable MPLS on all interfaces and rely on BGP label distribution

Answer

Implement LDP for BGP prefixes

Answer

Configure static labels for BGP prefixes

Question 5

A service provider is deploying segment routing in their MPLS core. They want to use an IGP as the control plane for label distribution without running LDP or RSVP-TE. Which IGP is best suited for this purpose?

Accepted Answer

IS-IS. IS-IS is the best-suited IGP for segment routing in an MPLS core because it natively supports the Segment Routing (SR) extensions defined in RFC 8667. These extensions allow IS-IS to advertise Prefix-SIDs and Adjacency-SIDs directly within the link-state database, enabling label distribution without requiring LDP or RSVP-TE. This makes IS-IS a natural fit for service providers deploying SR-MPLS.

Answer

BGP

Answer

EIGRP

Answer

RIP

Question 6

In a VXLAN EVPN deployment, a host sends a broadcast ARP request. Which component in the fabric is responsible for replying on behalf of the target host to reduce flooding?

Accepted Answer

The VTEP that has the target host's MAC address in its local table (ARP suppression). In VXLAN EVPN, ARP suppression is a feature implemented on the ingress VTEP (the VTEP that receives the broadcast ARP request). The ingress VTEP maintains a local ARP/ND cache populated via EVPN Type-2 routes (MAC/IP advertisement routes). When a broadcast ARP request arrives, the ingress VTEP checks its local cache for the target IP; if found, it replies directly on behalf of the target host, suppressing the broadcast and preventing unnecessary flooding across the fabric. Option C correctly identifies this VTEP as the component responsible for the reply.

Answer

The VTEP that receives the broadcast

Answer

The spine switch

Answer

The default gateway (anycast IP)

Question 7

Which TWO statements about BGP route reflectors are true?

Accepted Answer

The cluster ID is used to detect routing loops.. Option A is correct because BGP route reflectors use the cluster ID to detect and prevent routing loops. When a route reflector receives an update containing its own cluster ID in the cluster-list attribute, it discards the route, breaking the loop. This mechanism is defined in RFC 4456 and is essential for loop-free route propagation in non-full-mesh iBGP topologies.

Answer

Route reflectors modify the AS_PATH attribute.

Answer

Route reflectors must be fully meshed.

Answer

A route reflector passes routes from non-client to non-client.

Question 8

An engineer is configuring VPLS on a Cisco ASR 9000. After verifying the pseudowire status, they notice that the 'pw status' shows 'down' for one of the PWs. Which two conditions could cause this? (Choose two.)

Accepted Answer

Mismatched encapsulation type on the pseudowire (e.g., Ethernet vs. VLAN). Option A is correct because a mismatched encapsulation type (e.g., Ethernet vs. VLAN) on the pseudowire causes the PW to fail to come up. In VPLS, the encapsulation must match between the local and remote PE for the pseudowire to be operational; otherwise, the PW status will show 'down' due to a negotiation failure.

Answer

VPLS ID mismatch on the same bridge domain

Answer

SNMP MIB not loaded

Answer

LDP session is missing

Question 9

Refer to the exhibit. An engineer notices that R1 has an LDP neighbor but 'show mpls forwarding-table' on R1 shows no label bindings for prefixes learned from R2. What is the most likely cause?

Accepted Answer

R1 is configured for 'label distribution on-demand'. R1 shows an LDP neighbor (session is up) but no label bindings for prefixes from R2. This occurs when R1 is configured for 'label distribution on-demand' (RFC 5036), meaning it only requests label bindings for prefixes in its routing table, not all prefixes from the peer. Since R1 has not yet needed those specific prefixes, it has not requested labels, so the forwarding table remains empty.

Answer

The LDP discovery source is incorrect

Answer

The peer LDP identifier is not reachable

Answer

LDP session is not established

Question 10

A service provider uses MP-BGP with IPv6 address family. They notice that routes redistributed from OSPFv3 are not being advertised to iBGP peers. The OSPF routes are internal. What is a likely reason?

Accepted Answer

The next-hop is not resolved for IPv6.. In MP-BGP for IPv6, the next-hop address for iBGP peers must be reachable via an IPv6 route in the global routing table or the appropriate VRF. When OSPFv3 redistributes internal routes into BGP, the next-hop is often set to the OSPFv3 router's own IPv6 address; if that address is not reachable (e.g., because the interface is not in the IPv6 unicast routing table or the next-hop is link-local), iBGP peers will not install the routes. This is a common cause of routes being learned but not advertised to iBGP peers.

Answer

The network command is missing under IPv6 address family.

Answer

The bgp default ipv4-unicast command is disabled.

Answer

The routes are not in the IPv6 unicast table.

Question 11

A service provider is migrating from LDP to Segment Routing in an IS-IS network. After enabling IS-IS with segment-routing on all routers, they observe that some prefixes still receive labels from LDP instead of from SR. Which configuration is most likely missing on these routers?

Accepted Answer

Missing 'segment-routing mpls' command under IS-IS. When migrating from LDP to Segment Routing in an IS-IS network, the 'segment-routing mpls' command must be explicitly enabled under the IS-IS routing process. Without this command, IS-IS will not allocate MPLS labels for prefixes using the Segment Routing (SR) extension, causing the router to fall back to LDP for label distribution. This is the most common missing configuration when SR labels are not being assigned.

Answer

OSPF segment-routing is still configured

Answer

TI-LFA is not enabled

Answer

SRGB range conflicts with MPLS label range

Question 12

In MPLS forwarding, what label operation does a transit LSR perform on the top label of a labeled packet?

Accepted Answer

Swap. A transit Label Switch Router (LSR) in an MPLS network receives a labeled packet and must forward it toward the egress LSR. The core operation on the top label is a swap: the incoming label is replaced with an outgoing label learned from the downstream LSR via Label Distribution Protocol (LDP) or other label distribution protocols. This maintains the label-switched path (LSP) and ensures the packet continues along the correct path.

Answer

None

Answer

Pop

Answer

Push

Question 13

Which OSPF neighbor state indicates that a router has received a hello packet from a neighbor but the neighbor's Router ID is not yet seen in its own hello packet?

Accepted Answer

Init. The Init state is the first step in OSPF neighbor formation after the Down state. A router enters Init when it receives a Hello packet from a neighbor but does not yet see its own Router ID in the neighbor's Hello packet, indicating that bidirectional communication is not yet confirmed.

Answer

Down

Answer

Two-way

Answer

ExStart

Question 14

An MPLS core network uses LDP to distribute labels. An engineer wants to verify that the LDP session between two routers is up and exchanging labels. Which command should be used?

Accepted Answer

show mpls ldp neighbor. The command 'show mpls ldp neighbor' displays the status of LDP sessions, including the neighbor's IP address, session state (e.g., Operational), and label exchange activity. Since LDP is the protocol used to distribute labels in this MPLS core network, this command directly verifies that the session is up and exchanging labels between the two routers.

Answer

show mpls interfaces

Answer

show mpls l2transport binding

Answer

show mpls forwarding-table

Question 15

Which THREE are valid methods to provide fast convergence in an MPLS/Segment Routing network? (Select three.)

Accepted Answer

Topology-Independent LFA (TI-LFA). B is correct because Topology-Independent LFA (TI-LFA) provides fast convergence in Segment Routing networks by computing a backup path that is guaranteed to be loop-free regardless of the network topology. It leverages the Segment Routing data plane to pre-install a repair path using a segment list, enabling sub-50ms failover without relying on IGP convergence.

Answer

BGP Prefix Independent Convergence (PIC)

Answer

IP Fast Reroute (IPFRR)