350-501 Networking — 25 Questions

Question 1

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?

Choose an answer to begin — your selection is scored in the full session.

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 2

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 3

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 4

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 5

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

Accepted Answer

Segment Routing via IGP. Segment Routing via IGP (A) is a valid label distribution method because SR encodes labels as MPLS label stack entries distributed through IGP extensions (IS-IS or OSPF), eliminating the need for a separate label distribution protocol. RSVP-TE (B) is valid because it distributes labels as part of the RSVP Path/Resv signaling used to establish traffic-engineered LSPs, binding labels to explicit routes. LDP (D) is valid because it is the standard protocol that maps FECs to labels and distributes those bindings between adjacent Label Switching Routers. BGP (C) is not a general label distribution method for MPLS LSPs; it distributes labels only in specific contexts such as BGP/MPLS IP VPNs (RFC 4364) or labeled-unicast, not as a standalone MPLS label distribution mechanism. TDP (E) is not a valid current method because it is the deprecated Cisco-proprietary predecessor to LDP and has been superseded by LDP.

Answer

BGP

Answer

TDP

Question 6

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 7

An engineer is troubleshooting a BGP peering issue between two routers. The peering is established, but routes are not being exchanged. On router R1, 'show bgp neighbors 192.0.2.2' shows the neighbor state as 'Established' but the 'Prefixes received' counter is zero. What is most likely the cause?

Accepted Answer

An outbound route-map on the neighbor is filtering all routes.. The neighbor state is 'Established', confirming that the TCP session and BGP open messages have been successfully exchanged. However, zero prefixes received indicates that R1 is not receiving any routes from the neighbor. An outbound route-map applied on the neighbor (the router sending routes to R1) can filter all prefixes before they are advertised, resulting in zero received prefixes while the session remains up.

Answer

The 'maximum-prefix' limit is exceeded, causing the session to reset.

Answer

The BGP session is in the Idle state due to a misconfigured update-source.

Answer

The 'next-hop-self' command is missing on R1.

Question 8

A service provider is deploying segment routing (SR) with MPLS data plane. The network uses OSPF as the IGP. Which configuration is required to enable SR-MPLS and ensure that routers advertise prefix-SIDs for their loopback interfaces?

Accepted Answer

Configure 'segment-routing mpls' globally and assign a prefix-SID to the loopback interface under OSPF.. To enable SR-MPLS with OSPF, you must globally enable segment routing with the 'segment-routing mpls' command, and then assign a prefix-SID to the loopback interface under the OSPF process using the 'prefix-sid' command. This ensures that routers advertise the prefix-SID for their loopback via OSPF extensions, which is the fundamental requirement for SR-MPLS operation without LDP.

Answer

Configure 'mpls ip' on all interfaces and 'router ospf 1 mpls ldp auto-config'.

Answer

Configure 'label mode per-prefix' under the OSPF process and assign a label to the loopback.

Answer

Configure 'segment-routing global-block 16000 23999' globally and enable 'mpls ip' on all interfaces.

Question 9

A network administrator is configuring a Cisco ASR 9000 router for MPLS L3VPN. The PE receives customer routes via eBGP from the CE. Which command is required to redistribute these routes into the VPNv4 BGP table on the PE?

Accepted Answer

Configure 'address-family ipv4 unicast' under the VRF configuration and then 'address-family vpnv4 unicast' under the BGP process.. In MPLS L3VPN, customer routes learned via eBGP from the CE must be placed into the VRF's IPv4 unicast table first, and then the 'address-family vpnv4 unicast' under BGP is required to redistribute those VRF routes into the VPNv4 BGP table, which carries the route with its route distinguisher (RD) and extended community. This two-step process ensures the PE can advertise the route as a VPNv4 prefix to other PEs via MP-BGP.

Answer

Configure 'address-family ipv4 vrf CUSTOMER' under the BGP process and activate the neighbor.

Answer

Enter 'vrf CUSTOMER' in the BGP process and then 'address-family ipv4 unicast' under the VRF.

Answer

Enter 'redistribute bgp 100' under the VRF's OSPF or static route configuration.

Question 10

A customer reports intermittent packet loss on a MPLS L3VPN connection. The PE router shows 'show mpls forwarding' entries for the CE prefix, but ping from the PE to the CE fails intermittently. Which action should be taken to isolate the issue?

Accepted Answer

Check 'show ip route vrf CUSTOMER' and 'show bgp vpnv4 unicast vrf CUSTOMER' to confirm the VRF routes.. The intermittent packet loss suggests a control-plane issue rather than a data-plane problem. By checking 'show ip route vrf CUSTOMER' and 'show bgp vpnv4 unicast vrf CUSTOMER', you can verify that the VRF route for the CE prefix is present in the routing table and that BGP is advertising the correct VPNv4 route with the proper next-hop and label. This isolates whether the failure is due to missing or incorrect route propagation, which is a common cause of intermittent reachability in MPLS L3VPN.

Answer

Use 'show mpls lsp' to verify the LSP to the CE's PE.

Answer

Examine the 'show mpls forwarding vrf CUSTOMER' output to see label operations.

Answer

Review the 'show bgp vpnv4 unicast all' output to verify route advertisement.

Question 11

A network engineer is troubleshooting a BGP route selection issue on a router that receives the same prefix from two different peers. Which THREE BGP attributes are considered in the route selection process before the 'prefer the route with the lowest metric' step?

Accepted Answer

Highest weight. The BGP best-path selection algorithm considers attributes in a specific order before using the metric (MED). The sequence is: highest Weight, highest Local Preference, prefer locally originated routes, shortest AS_PATH, lowest Origin type, and then lowest metric. Therefore, all five options A, B, C, D, and E are evaluated before the metric step.

Question 12

A service provider is deploying MPLS L3VPN to connect multiple customer sites. The PE router receives a route from a CE router via MP-BGP. Which attribute must the PE router add to the route before advertising it to the route reflector?

Accepted Answer

Route distinguisher (RD). When a PE router receives a customer route from a CE router via MP-BGP, it must first make the route unique within the MPLS VPN context by adding a Route Distinguisher (RD). The RD prepended to the IPv4 prefix creates a VPNv4 address (per RFC 4364), which is the format required for advertisement to a route reflector. Without the RD, the route would not be distinguishable from other customers' overlapping IP prefixes in the BGP table.

Answer

MPLS label

Answer

Route target (RT)

Answer

IGP metric

Question 13

A network engineer is troubleshooting MPLS LSP connectivity. The MPLS LDP session is up on both endpoints, but some MPLS labels are missing in the LFIB. Which configuration change would most likely resolve the issue?

Accepted Answer

Enable 'mpls ldp advertise-labels'. The 'mpls ldp advertise-labels' command ensures that LDP advertises label bindings to its peers. If this is disabled (default is enabled, but can be overridden), LDP sessions may be up but no labels are distributed, resulting in missing labels in the LFIB. Re-enabling it forces LDP to send label mappings for all FECs, resolving the connectivity issue.

Answer

Configure 'label protocol ldp' on all routers

Answer

Configure 'mpls ip' on all interfaces

Answer

Enable 'mpls ldp explicit-null' on the ingress

Question 14

A network administrator is designing an MPLS TE tunnel for a service provider backbone. The goal is to provide bandwidth protection for voice traffic. Which MPLS TE feature should be configured?

Accepted Answer

Fast Reroute (FRR). MPLS TE Fast Reroute (FRR) provides sub-50 ms protection against link or node failures by pre-computing and pre-signaling a backup tunnel that bypasses the protected element. This meets the requirement for bandwidth protection of voice traffic, which is sensitive to packet loss and delay during network convergence.

Answer

Autoroute announce

Answer

Path-option

Answer

Bandwidth protection

Question 15

Refer to the exhibit. An engineer is troubleshooting an MPLS LDP session between two routers. The output shows that the LDP session is operational. However, MPLS labels are not being exchanged. What is the most likely cause?

Accepted Answer

The LDP label advertisement mode is 'downstream on demand'.. The exhibit shows that the LDP session is operational (state = Operational), but no labels are being exchanged. In 'downstream on demand' mode, a router does not advertise labels unless explicitly requested by an upstream neighbor via a Label Request message. Since the session is up but no labels are exchanged, this mismatch in label advertisement mode is the most likely cause.

Answer

The peer's addresses are not bound to the LDP Ident.

Answer

The TCP connection is not using port 646.

Answer

The LDP session is not established.

Question 16

Which TWO MPLS VPN features are used to provide Layer 3 VPN services in a service provider network?

Accepted Answer

MPLS labels. MP-BGP (Option E) is used to exchange VPNv4 routes between Provider Edge (PE) routers, carrying both the route and its corresponding MPLS label. MPLS labels (Option C) are used to forward packets across the provider core via label-switched paths (LSPs), enabling the separation of customer traffic. Together, MP-BGP and MPLS labels form the foundation of Layer 3 MPLS VPN services.

Answer

LDP

Answer

VRF

Answer

RSVP-TE

Question 17

Match each QoS mechanism to its primary function.

Accepted Answer

Classification: Identifies traffic based on specific criteria.. In QoS, Classification identifies traffic, Marking sets priority, Policing drops/excess, Shaping buffers, and WRED drops probabilistically. Queuing handles packet scheduling, not rate enforcement.

Answer

Queuing: Drops packets that exceed a configured rate.

Question 18

Match each service provider architecture term to its description.

Accepted Answer

Provider Edge (PE) router: Connects to customer edge routers and provides MPLS VPN services.. The correct matches are: PE connects to customer edge, P is MPLS core, ASBR connects to other ASes. Common confusions: mistaking P for CE or confusing Route Reflector with ASBR.

Answer

Customer Edge (CE) router: Core router that forwards packets based on MPLS labels without customer awareness.

Answer

Route Reflector: Router that exchanges routing information with another autonomous system.

Question 19

A service provider is experiencing high CPU usage on a router running BGP. Which action should be taken first to mitigate the issue without disrupting traffic?

Accepted Answer

Apply inbound route filtering using prefix lists.. High CPU usage on a BGP-speaking router is often caused by processing a large number of BGP updates. Applying inbound route filtering using prefix lists (option C) reduces the number of routes the router must process and store, directly lowering CPU load without disrupting existing traffic flows. This is a non-disruptive, targeted mitigation that addresses the root cause of excessive route processing.

Answer

Disable BGP on the router.

Answer

Increase the BGP keepalive timer.

Answer

Implement BGP route dampening.

Question 20

An engineer is designing an MPLS L3VPN for a customer with multiple sites. The customer requires overlapping IP addresses between sites. Which method allows the provider to support overlapping customer addresses?

Accepted Answer

Use separate VRF per site with route distinguisher.. B is correct because a separate VRF per site with a unique route distinguisher (RD) allows the provider to maintain isolated routing tables for each customer site. This isolation enables overlapping IP addresses between sites, as each VRF treats its prefixes as unique within the MPLS L3VPN backbone, regardless of address duplication.

Answer

Implement VPLS instead of L3VPN.

Answer

Use BGP communities to control route distribution.

Answer

Use the same VRF for all sites with different route targets.