350-501 Networking — 25 Questions

Question 1

Which TWO statements accurately describe MPLS LDP label distribution and autoconfiguration? (Choose two.)

Accepted Answer

LDP can be configured to allocate labels only for specific prefixes using label filtering.. LDP supports label filtering via the 'label advertise' command to allocate labels only for specific prefixes. Option B is correct because LDP uses hello messages sent to multicast address 224.0.0.2 for neighbor discovery, which is a key aspect of autoconfiguration. Option C is incorrect because LDP does not allocate labels for all prefixes in the IGP routing table by default; it only allocates labels for routes that are in the forwarding table, and there are exceptions such as BGP routes. Options D and E are incorrect as LDP sessions typically use loopback addresses for stability, and label allocation is per prefix, not per interface.

Answer

LDP allocates labels for all prefixes in the IGP routing table by default.

Answer

LDP sessions are always established using the physical interface IP address.

Answer

LDP label allocation is per interface by default.

Question 2

Which BGP attribute is NOT considered in the BGP best path selection process when all other attributes are equal and the routes are from different ASs?

Accepted Answer

MED. During BGP best path selection, MED is only compared when the routes come from the same AS (i.e., the neighboring AS is the same). When routes are from different ASs, the MED attribute is skipped in the decision process because it is not comparable across different ASs. In this scenario, since all other attributes are equal and the routes are from different ASs, MED is not considered.

Answer

LOCAL_PREF

Answer

AS_PATH length

Answer

Weight

Question 3

Which THREE BGP attributes are considered in the best path selection process before comparing IGP metric to the next-hop? (Choose three.)

Accepted Answer

MED. All five attributes are considered before the IGP metric to the next-hop in the BGP best path selection process. The order is: Weight, Local Preference, AS_PATH length, MED, and eBGP over iBGP. These steps occur before comparing IGP metrics.

Question 4

A service provider is designing an OSPF network with multiple areas. Which two area types prevent Type 5 LSAs from entering the area (choose two)?

Accepted Answer

Totally stubby area. Stub areas block Type 5 LSAs and rely on a default route. Totally stubby areas block both Type 4 and Type 5 LSAs, also using a default route. NSSAs block Type 5 LSAs but allow Type 7 LSAs, which are translated to Type 5 at the ABR. Thus, all three area types prevent Type 5 LSAs from entering the area.

Answer

Backbone area

Answer

Standard area

Question 5

Which BGP attribute is used to influence inbound traffic to a specific AS by indicating the preferred path?

Accepted Answer

MED. MED (Multi-Exit Discriminator) is a BGP attribute used to influence inbound traffic from a neighboring AS. A lower MED value is preferred, making it the correct choice for influencing inbound traffic to a specific AS.

Answer

LOCAL_PREF

Answer

AS_PATH

Answer

NEXT_HOP

Question 6

Which OSPF adjacency type requires DR/BDR election on a multi-access network?

Accepted Answer

Non-broadcast. Both broadcast and non-broadcast multi-access (NBMA) networks require DR/BDR election in OSPF to reduce the number of adjacencies. Point-to-point and point-to-multipoint networks do not use DR/BDR.

Answer

Point-to-multipoint

Answer

Point-to-point

Question 7

In IS-IS, which type of metric uses 24 bits (wide) to allow higher link bandwidth values and is required for TE support?

Accepted Answer

Wide metric. IS-IS originally used narrow metrics (6 bits). Wide metrics, using 24 bits, were introduced in TLV 135 (Extended IS reachability) and TLV 22 (Extended IP reachability) to support higher link bandwidth values and are required for Traffic Engineering (TE). The term 'Extended metric' is not the standard name; the correct term is 'Wide metric,' making option B correct and option A incorrect.

Answer

Extended metric

Answer

Narrow metric

Answer

TE metric

Question 8

An SP network uses OSPFv3 for IPv6 routing. Which three statements about OSPFv3 are true? (Choose three.)

Accepted Answer

OSPFv3 uses link-local addresses for next-hop. OSPFv3 runs on a per-link basis, uses link-local addresses for adjacencies, and supports multiple instances per link. OSPFv3 does not require IPv4; it is for IPv6. It uses the same LSA types but with modified formats. Option D is false because OSPFv3 uses Authentication Trailer for authentication, not IPsec.

Answer

OSPFv3 requires an IPv4 address on the interface to form adjacencies

Answer

OSPFv3 uses IPsec for authentication

Question 9

Which TWO of the following are characteristics of IS-IS Partial Route Computation (PRC)? (Choose two.)

Accepted Answer

PRC improves convergence by avoiding full SPF runs for prefix changes.. PRC (Partial Route Computation) is triggered when only leaf (IP prefix) information changes, such as when a prefix is added or withdrawn, without any change to the IS-IS topology (i.e., no change in router IDs, adjacencies, or link-state information). This allows the router to update only the Reachability TLV (IP prefixes) without recalculating the entire SPF tree, which significantly improves convergence time. Options C and D correctly describe these characteristics. Option A is incorrect because a change in system ID (router ID) is a topology change that requires a full SPF run. Option B is incorrect because forming a new adjacency is also a topology change requiring full SPF. Option E is incorrect because PRC does not require metric-style wide; it functions with either narrow or wide metrics.

Answer

PRC is used when a router's system ID changes.

Answer

PRC is triggered when a new adjacency is formed.

Answer

PRC requires metric-style wide to operate.

Question 10

Which TWO of the following are types of OSPF stub areas? (Choose two.)

Accepted Answer

Stub area. The two types of OSPF stub areas are Stub area (blocks Type 5 LSAs) and Totally stubby area (Cisco proprietary, blocks Type 3, 4, and 5 LSAs except default). NSSA is also a stub area type but is not one of the two asked for in this question.

Answer

Transit area

Answer

NSSA

Answer

Backbone area

Question 11

A service provider is implementing 6VPE to provide IPv6 L3VPN services. Which two statements about 6VPE are true?

Accepted Answer

6VPE supports overlapping IPv6 address spaces between different VPNs.. 6VPE uses MP-BGP extensions with AFI 2 (IPv6) and SAFI 128 (MPLS-labeled VPN address family) to exchange VPN-IPv6 routes. It supports overlapping IPv6 address spaces between different VPNs by using route distinguishers. PE routers do not require IPv6 connectivity to the MPLS core; the core can be IPv4-only with MPLS. 6VPE does not require a full IPv6 routing table on the PE, and it uses MPLS encapsulation, not GRE.

Answer

6VPE PE routers must have IPv6 connectivity to the MPLS core.

Answer

6VPE requires a full IPv6 routing table on the PE router.

Answer

6VPE encapsulates IPv6 packets using GRE tunnels.

Question 12

A service provider is deploying MPLS L3VPN over an OSPF backbone. The PE routers are configured with OSPF as the IGP. The CE router of customer A is connected to two PEs for redundancy. Which configuration is required on the PE routers to ensure that the CE router can load-balance traffic across both PEs without loops?

Accepted Answer

Configure OSPF with the capability vrf-lite and enable the down-bit on the PE-CE link.. Configuring OSPF with the capability vrf-lite and enabling the down-bit on the PE-CE link prevents routing loops in a multi-homed CE scenario. The down-bit is set by the PE when redistributing routes into OSPF, ensuring that the CE does not re-advertise those routes back to another PE, which would cause a loop. The vrf-lite capability allows the CE to understand the down-bit without requiring full MPLS/VPN functionality, enabling load-balancing across both PEs safely.

Answer

Use OSPF sham-links between the two PEs.

Answer

Use the BGP cost community to adjust the path selection on the CE.

Answer

Disable the DN-bit on the PE-CE OSPF interface.

Question 13

An ISP is designing an MPLS core network and needs to choose an IGP that supports fast convergence. Which IGP meets this requirement and is most commonly used in MPLS core networks?

Accepted Answer

IS-IS. IS-IS is the correct choice because it is a link-state IGP that inherently supports fast convergence through mechanisms like incremental SPF (iSPF) and prefix-independent convergence (PIC). It is widely deployed in MPLS core networks due to its scalability, extensibility via TLVs, and native support for MPLS Traffic Engineering (MPLS-TE) without requiring additional protocol extensions like OSPF's opaque LSA.

Answer

OSPFv3

Answer

EIGRP

Answer

RIPng

Question 14

A network engineer is troubleshooting an MPLS L3VPN where the CE router is receiving the correct VPN prefixes from the PE, but traffic from the CE to those prefixes is being dropped. The PE has a default route pointing to the CE. What is the most likely cause?

Accepted Answer

The PE does not have a specific route for the destination in its global routing table.. The CE is receiving the correct VPN prefixes from the PE, so the VRF import/export is working. However, when the CE sends traffic to those prefixes, the PE must forward the packets. The PE has a default route pointing to the CE, but if the PE's global routing table lacks a specific route for the destination prefix (which is normal for L3VPN, as VPN routes are in the VRF, not the global table), the PE will drop the traffic because it cannot find a valid next hop in the global table for the outer IP header. This is a classic issue where the PE's global table must have a route to the CE's loopback or the PE-CE link subnet to enable recursive forwarding.

Answer

The VRF on the PE is not configured with the correct route-target import.

Answer

The CE does not have a route back to the PE's loopback.

Answer

The PE-CE link MTU is smaller than the packet size.

Question 15

A service provider is deploying segment routing in its MPLS core. Which label allocation method is used by segment routing to distribute prefix SIDs?

Accepted Answer

IGP (IS-IS or OSPF). Segment routing uses the IGP (IS-IS or OSPF) to distribute prefix SIDs. The IGP extensions for segment routing (RFC 8665 for OSPF, RFC 8667 for IS-IS) carry the prefix SID sub-TLV within the prefix reachability information, allowing each router to allocate and advertise the SID associated with a prefix. This is the native label allocation method for segment routing, as it leverages the existing IGP database without requiring a separate label distribution protocol.

Answer

LDP

Answer

BGP

Answer

RSVP-TE

Question 16

An engineer is configuring an MPLS L3VPN and needs to ensure that the PE router installs VPNv4 routes from a remote PE into the VRF of a customer. The remote PE sends a VPNv4 route with route-target 100:1. Which configuration on the local PE causes the route to be imported into the VRF?

Accepted Answer

vrf definition CUSTOMER
 rd 100:1
 route-target import 100:1. The `route-target import 100:1` command under the VRF definition configures the local PE to accept VPNv4 routes that carry the specified route-target (100:1) from the remote PE. This import RT must match the export RT of the remote PE for the route to be installed into the VRF's routing table. The `rd 100:1` defines the route distinguisher, which is separate from the RT and ensures uniqueness of the VPNv4 prefix.

Answer

router bgp 100
 address-family ipv4 vrf CUSTOMER
 route-target import 100:1

Answer

vrf definition CUSTOMER
 rd 100:1
 route-target both 100:1 route-map IMPORT

Answer

vrf definition CUSTOMER
 rd 100:1
 route-target export 100:1

Question 17

Which TWO of the following are characteristics of MPLS-TE (Traffic Engineering)?

Accepted Answer

Uses explicit paths to route traffic away from shortest-path IGP.. MPLS-TE uses explicit paths (either strict or loose) to direct traffic away from the shortest path determined by the IGP (e.g., OSPF or IS-IS). This allows network operators to engineer traffic flows based on administrative policies, such as load balancing or avoiding congested links, rather than relying solely on the IGP's metric-based shortest path.

Answer

Uses LDP for label distribution along the TE tunnel.

Answer

Requires per-platform label space for TE tunnels.

Answer

Requires all routers in the TE tunnel to be in the same OSPF area.

Question 18

Which THREE are valid methods for label distribution in an MPLS network?

Accepted Answer

Segment Routing via IGP. Segment Routing via IGP (Option A) is a valid label distribution method because it uses the IGP (OSPF or IS-IS) to advertise prefix-SIDs, which are MPLS labels. The IGP distributes these labels as part of its link-state database, eliminating the need for a separate label distribution protocol like LDP. This is defined in RFC 8660 and RFC 8665 for OSPF and IS-IS respectively.

Answer

BGP

Answer

TDP

Question 19

Based on the exhibit, which prefix is NOT reachable via MPLS forwarding?

Accepted Answer

10.5.5.5/32. (10.5.5.5/32) is correct because the exhibit shows that the LDP label bindings are only present for prefixes 10.1.1.1/32, 10.2.2.2/32, and 10.3.3.3/32. The prefix 10.5.5.5/32 is not in the LDP label information base (LIB), meaning no MPLS label has been assigned to it, so it cannot be forwarded via MPLS and must be forwarded using standard IP routing instead.

Answer

10.1.1.1/32

Answer

10.3.3.3/32

Answer

10.2.2.2/32

Question 20

Based on the exhibit, what is the purpose of the 'mpls ldp neighbor ... password cisco' commands?

Accepted Answer

To authenticate the LDP session using MD5.. The 'mpls ldp neighbor ... password cisco' command configures a Message Digest 5 (MD5) authentication password for the LDP session with a specific neighbor. This ensures that the TCP connection used for LDP exchanges is authenticated, preventing spoofed or unauthorized LDP messages from being accepted, as defined in RFC 5036.

Answer

To synchronize LDP and IGP convergence.

Answer

To control the label allocation for prefixes from that neighbor.

Answer

To enable LDP session protection for the neighbor.