350-501 Practice Test 23 — 15 Questions

Question 1

An SP network uses RSVP-TE for MPLS traffic engineering with FRR link protection. Which THREE statements are correct about RSVP-TE operation and FRR? (Choose three.)

Accepted Answer

In FRR link protection, the 1:1 detour method creates a separate backup LSP for each protected LSP, while the bypass tunnel method protects multiple LSPs over a single backup tunnel.. Option B is correct because FRR link protection supports two models: the 1:1 detour method, where each protected LSP gets its own dedicated backup LSP, and the bypass tunnel method, where a single backup tunnel protects multiple LSPs traversing the same protected link. Option D is correct because RSVP-TE signaling is receiver-initiated: the PATH message flows downstream from headend (ingress) to tailend (egress) to carry the sender's traffic specification and ERO, while the RESV message flows upstream from tailend to headend to install reservation state and labels hop-by-hop. Option E is correct because the ERO is a PATH message object that specifies the explicit path as a sequence of hops, each marked as strict (the node must be directly adjacent) or loose (intermediate nodes may be traversed), enabling constraint-based TE routing. Option A is incorrect because CSPF does not use IGP metrics only; it prunes the topology based on TE constraints such as bandwidth, affinity/administrative groups, and explicit path requirements before running shortest-path computation. Option C is incorrect because MPLS-TE does not rely solely on the fixed-filter (FF) reservation style; shared-explicit (SE) style is also used, and FF is not the only style permitting multiple senders.

Answer

CSPF (Constrained Shortest Path First) is used to compute the best path based on IGP metrics only, ignoring TE constraints.

Answer

The RSVP 'fixed-filter' (FF) reservation style is the only style used in MPLS-TE to allow multiple senders.

Question 2

Which OSPF area type is most suitable for a service provider's customer-facing network where external routes are blocked, but internal routes (including inter-area) are allowed, and the area should not accept Type 5 LSAs?

Accepted Answer

Stub area. A stub area blocks Type 5 LSAs (external routes) from entering the area, but still allows Type 3 LSAs (inter-area routes) and Type 1/2 LSAs. This matches the requirement of blocking external routes while permitting internal and inter-area routes.

Answer

NSSA

Answer

Backbone area

Answer

Totally stubby area

Question 3

Given the output, which configuration mismatch would prevent a remote PE in the same VPN from installing this route into its VRF?

Accepted Answer

The remote PE has an import RT that does not include RT:65000:200.. In BGP/MPLS L3VPN, a PE installs a VPN route into a VRF only if the route's route target (RT) matches one of the VRF's import RTs. If the remote PE's import RT does not include RT:65000:200, the route will not be imported into that VRF, even if everything else is correct. This is the classic RT mismatch that prevents route installation.

Answer

The remote PE uses a different route distinguisher for its VRF.

Answer

The remote PE filters routes based on BGP AS-path containing AS 65000.

Answer

The next-hop 10.1.1.2 is not reachable in the remote PE's global routing table.

Question 4

A service provider is designing a new MPLS core network and needs to choose a routing protocol that supports traffic engineering and fast convergence. Which protocol should be used?

Accepted Answer

IS-IS. IS-IS is the correct choice because it natively supports MPLS Traffic Engineering (TE) via IS-IS TE extensions (RFC 5305) and offers fast convergence through mechanisms like IS-IS Fast Flooding and LSP throttling. Unlike OSPF, IS-IS operates directly over Layer 2, making it more scalable and robust in large service provider core networks, and it avoids the need for a separate flooding domain for TE.

Answer

RIP

Answer

EIGRP

Answer

OSPF

Question 5

Which THREE protocols are used for label distribution in an MPLS network?

Accepted Answer

LDP. LDP (Label Distribution Protocol) is correct because it is the primary protocol designed specifically to distribute MPLS labels between Label Switch Routers by mapping FECs to labels. BGP is correct because it can distribute MPLS labels along with BGP routes, notably in BGP/MPLS IP VPNs (RFC 4364) using the labeled-unicast address family and extended communities for VPN labels. RSVP is correct because RSVP-TE (RSVP with Traffic Engineering extensions) distributes labels when establishing traffic-engineered LSPs via PATH and RESV messages. OSPF and IS-IS are not label distribution protocols; they are link-state interior gateway routing protocols that can carry MPLS-related extensions (e.g., for TE) but do not themselves distribute labels.

Answer

OSPF

Answer

IS-IS

Question 6

An SP network is using IS-IS and wants to achieve fast convergence using BFD. Which two statements about BFD in IS-IS are correct?

Accepted Answer

BFD can provide sub-second failure detection for IS-IS adjacencies.. Option B is correct because BFD (Bidirectional Forwarding Detection) is designed to detect failures in the forwarding path between two routers at sub-second intervals, typically by exchanging control packets at negotiated intervals as low as 3.3 ms (or 50 ms with multipliers), which allows IS-IS to tear down an adjacency much faster than IS-IS hello/hold timers alone. Option C is correct because BFD operates independently of the IS-IS level hierarchy and can be enabled per interface for both Level 1 and Level 2 adjacencies, so an SP network running L1, L2, or both can use BFD for each adjacency type. Option A is not required: wide metrics are used for TE and larger metric values, not for BFD operation. Option D is inaccurate because IS-IS BFD is typically enabled at the interface level (e.g., under the interface configuration with 'isis bfd' or 'bfd' commands), not solely under the IS-IS router process. Option E is false because BFD does not replace IS-IS hello packets; IS-IS still uses IIH PDUs to form and maintain adjacencies, while BFD provides faster liveness detection alongside them.

Answer

BFD requires IS-IS to be configured with wide metrics.

Answer

IS-IS BFD configuration is done under the IS-IS router process.

Answer

BFD replaces the need for IS-IS hello packets.

Question 7

A service provider runs an IOS XR core where a single route processor (RP) failure must not interrupt forwarding or routing protocol adjacencies on the line cards. The design team wants to confirm which IOS XR architecture component is responsible for continuing to forward traffic and maintaining protocol state when the active RP fails and the standby RP takes over. Which component provides this capability?

Accepted Answer

Non-stop forwarding (NSF) coordinated by the standby RP, which maintains synchronization with the active RP so line cards keep forwarding during switchover.. In IOS XR, RP redundancy works together with non-stop forwarding: the standby RP is kept synchronized, and line cards retain forwarding state during switchover, so packets keep flowing and routing protocol adjacencies survive. The mechanism is built into the RP/line-card architecture rather than residing on a line-card ASIC, SysDB, or RPME. That is what allows a single RP failure to be transparent to the forwarding plane in this design.

Answer

The System Database (SysDB) on the standby RP, which owns all configuration and protocol state and resumes control of the line cards after failover.

Answer

The distributed Route Processor Management Engine (RPME) on each line card, which independently runs BGP and OSPF and forwards traffic using locally built RIBs.

Answer

The fabric management ASIC on each line card, which caches routing protocol hellos and re-advertises them if the active RP stops responding.

Question 8

An SP engineer is configuring NTP authentication on IOS XR routers in the management plane. Which TWO statements about NTP authentication are correct? (Choose two.)

Accepted Answer

NTP authentication uses pre-shared keys to authenticate time sources. Option B is correct because NTP authentication on IOS XR relies on symmetric pre-shared keys (configured with the ntp authentication-key command) that the router and its time source use to prove authenticity of the time packets. Option E is correct because the client must not only have the matching key configured but also mark it as trusted (ntp trusted-key ) and associate it with the server (ntp server key ); otherwise the authentication check fails and the source is not used. Option A is wrong because NTP authentication is also supported on classic IOS (and IOS XE), not exclusively on IOS XR. Option C is wrong because multiple authentication keys can be configured, each with a unique key number, allowing different keys for different servers. Option D is wrong because NTP authentication provides integrity and origin authentication via a keyed hash (MD5/SHA), not confidentiality—it does not encrypt the NTP packets.

Answer

NTP authentication is only supported on IOS XR, not on classic IOS

Answer

Only one NTP authentication key can be configured on a router

Answer

NTP authentication encrypts the NTP packets to ensure confidentiality

Question 9

An engineer is configuring IS-IS in a service provider network. The network uses wide metrics for traffic engineering. Which TLV is mandatory for IS-IS to carry traffic engineering (TE) information in accordance with RFC 5305?

Accepted Answer

TLV 22 (Extended IS Reachability). TLV 22 (Extended IS Reachability) is the mandatory TLV for IS-IS TE, defined in RFC 5305.

Answer

TLV 135 (Extended IP Reachability)

Answer

TLV 229 (Multi-Topology Reachable IPv4 Prefixes)

Answer

TLV 128 (IP Internal Reachability)

Question 10

What is the role of Network Services Orchestrator (NSO) in service provider automation?

Accepted Answer

To orchestrate service provisioning across multiple devices. NSO provides service lifecycle management by modeling services with YANG and configuring devices via NETCONF.

Answer

To encrypt data plane traffic

Answer

To provide real-time traffic monitoring

Answer

To replace the routing protocol decision process

Question 11

Which three mechanisms are used within the DiffServ model to provide QoS in an SP network? (Choose three.)

Accepted Answer

Classification and marking at the edge using DSCP. In the DiffServ model, QoS is implemented by classifying and marking packets at the network edge using the DSCP field in the IP header. This marking determines how each packet is treated within the core network, enabling scalable, class-based service differentiation without per-flow state.

Answer

End-to-end path computation via PCEP

Answer

IntServ resource reservation via RSVP

Question 12

Which IS-IS TLV is used to carry Traffic Engineering (TE) information such as link bandwidth and TE metric in modern SP networks?

Accepted Answer

TLV 22 (extended IS reachability). In IS-IS Traffic Engineering (TE), TLV 22 (Extended IS Reachability) is used to carry TE information such as link bandwidth, TE metric, administrative groups, and other attributes. It extends the original IS Reachability TLV (TLV 2) to support larger link metrics and additional sub-TLVs for TE parameters. This TLV is advertised in Link State PDUs (LSPs) to distribute TE topology information.

Answer

TLV 128 (IP reachability)

Answer

TLV 135 (extended IP reachability)

Answer

TLV 1 (area addresses)

Question 13

In OSPF TE extensions for RSVP-TE, which LSA type is used to carry TE information (such as link attributes) in OSPF?

Accepted Answer

Type 10 Opaque LSA. Opaque LSA Type 10 (Area-local) is used for TE information in OSPF. Type 9 is link-local, Type 11 is AS-scoped.

Answer

Type 11 Opaque LSA

Answer

Type 5 AS-external LSA

Answer

Type 9 Opaque LSA

Question 14

In a hierarchical QoS policy, the parent policy shapes to 20 Mbps. The child policy has a class for voice with priority and police rate 2 Mbps, and a class for data with bandwidth 10 Mbps. What is the maximum bandwidth voice traffic can consume under congestion?

Accepted Answer

2 Mbps. In a hierarchical QoS policy, the child policy's police rate for voice is 2 Mbps. Even though the parent shaper allows up to 20 Mbps, the police rate acts as a hard cap on voice traffic. Under congestion, voice traffic cannot exceed the police rate of 2 Mbps, so the maximum bandwidth it can consume is 2 Mbps.

Answer

8 Mbps

Answer

20 Mbps

Answer

10 Mbps

Question 15

A network architect is designing a Segment Routing (SR) network for a service provider with a requirement for fast convergence upon node failure. The network uses IS-IS as the IGP with segment routing extensions. Which design choice BEST ensures that the network can achieve sub-50ms convergence without relying on any signaling protocol beyond the IGP?

Accepted Answer

Enable TI-LFA (Topology Independent Loop-Free Alternate) on all ISIS interfaces. TI-LFA (Topology Independent Loop-Free Alternate) is an SR IGP-based fast reroute mechanism that pre-computes backup paths for any topology change, providing sub-50ms convergence without additional signaling protocols like RSVP-TE. It leverages the IGP's link-state database and segment routing to create a loop-free alternate that is guaranteed to be loop-free for any failure, using the post-convergence path as the backup. This meets the requirement of relying only on the IGP.

Answer

Deploy BFD with static routes for fast detection

Answer

Configure RSVP-TE with Fast Reroute on all label-switched paths

Answer

Use LDP with LDP FRR and path protection