350-501 Practice Test 50 — 15 Questions

Question 1

A network engineer needs to perform maintenance on a BGP router without causing traffic loss. They plan to use BGP Graceful Shutdown (GSHUT). What does GSHUT do?

Accepted Answer

It sets the local preference to a lower value to withdraw routes gracefully. BGP GSHUT adjusts the local preference of routes to make them less preferred, gracefully draining traffic before the session is shut down. This avoids packet loss.

Answer

It immediately terminates all BGP sessions

Answer

It uses BGP fast external failover to speed up convergence

Answer

It increases the MED to deprefer routes

Question 2

A service provider is deploying uRPF on customer-facing interfaces to prevent IP spoofing. The network has asymmetric routing due to multiple upstream connections. Which uRPF mode should be used?

Accepted Answer

Loose mode. Strict mode requires the source address to be reachable via the incoming interface, which fails with asymmetric routing. Loose mode only checks that the source address exists in the routing table, making it suitable for asymmetric paths.

Answer

Strict mode

Answer

uRPF with allow-default

Answer

uRPF is not recommended for asymmetric routing

Question 3

To prevent MPLS label spoofing in a Layer 3 VPN, which configuration should be applied on the PE-CE link?

Accepted Answer

Disable MPLS on the PE-CE link. Disabling MPLS on PE-CE links (by not enabling MPLS on the interface) prevents the customer from seeing or injecting MPLS labels, thus preventing label spoofing. TTL propagation is used for traceroute but not a primary anti-spoofing measure.

Answer

Enable MPLS TTL propagation

Answer

Configure uRPF on the PE-CE link

Answer

Use BGP FlowSpec to filter labels

Question 4

A service provider uses BGP to exchange routes with customers. To prevent the customer from announcing prefixes they do not own (BGP hijacking), which tool should the provider apply on the customer-facing BGP session?

Accepted Answer

Prefix-list and route-map to filter incoming updates. Prefix-lists and route-maps are used to filter incoming BGP advertisements based on prefix and attributes. RPKI validates origin but requires ROA; BGP GSHUT is for maintenance; communities are for tagging.

Answer

BGP Graceful Shutdown

Answer

RPKI Origin Validation only

Answer

Set BGP community to tag customer routes

Question 5

A service provider wants to protect its routers from CPU overload caused by excessive traffic to the control plane. Which mechanism should be configured on IOS XR routers to classify and rate-limit management traffic?

Accepted Answer

CoPP (Control Plane Protection). CoPP (Control Plane Protection) on IOS XR uses class maps and policy maps to classify and rate-limit traffic destined to the control plane, protecting the router from CPU overload.

Answer

uRPF strict mode

Answer

BGP Flowspec

Answer

MPLS TTL propagation

Question 6

An SP is deploying BGP FlowSpec (RFC 8955) to distribute traffic filtering rules. Which component is responsible for disseminating FlowSpec rules to routers in the network?

Accepted Answer

MP-BGP. BGP FlowSpec uses a separate BGP address family (AFI 1, SAFI 133) to carry flow specifications. The rules are encoded as BGP NLRIs and distributed via BGP sessions.

Answer

OSPF

Answer

PIM

Answer

LDP

Question 7

An SP is implementing RPKI to validate BGP origin AS. After configuring RPKI-to-Router (RTR) and setting BGP origin validation, a route is marked as 'invalid'. What action does BGP default take for invalid routes?

Accepted Answer

The route is not installed in the routing table unless it is the only path. By default, BGP does not drop invalid routes; they are considered but with a lower preference (typically not installed in the routing table if a valid route exists). To drop invalid routes, explicit policy is needed.

Answer

The route is installed with a lower local preference

Answer

The route is treated as valid but with a lower preference

Answer

The route is dropped and not considered for best path selection

Question 8

A service provider's network is experiencing suboptimal routing for BGP prefixes received from a customer. The customer is multihomed to two different PE routers in the same AS. Which BGP feature can ensure that traffic ingresses via the correct PE?

Accepted Answer

MED. MED (Multi-Exit Discriminator) is the correct BGP attribute for influencing inbound traffic from a multihomed customer. When the customer advertises the same prefix to two PE routers in the same AS, setting a lower MED value on one PE will cause the customer's upstream AS to prefer that path for returning traffic. This allows the service provider to control which PE receives the ingress traffic without altering any outbound routing policies.

Answer

Local Preference

Answer

AS-path prepend

Answer

Weight

Question 9

Which THREE components are required to deploy MPLS Layer 3 VPN?

Accepted Answer

VRFs on PE routers. VRFs (Virtual Routing and Forwarding) are required on PE routers to maintain separate, isolated routing tables for each customer VPN. This allows overlapping IP addresses between different customers and ensures that traffic from one VPN does not leak into another. Without VRFs, the PE router cannot distinguish between customer routes, making Layer 3 VPN operation impossible.

Answer

GRE tunneling

Answer

OSPF

Question 10

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 11

A service provider is configuring BGP community propagation in an MPLS VPN network. Which two actions are necessary to ensure that communities are passed from CE to remote PE?

Accepted Answer

Configure send-community on the CE-to-PE BGP session. B is correct because the `send-community` command must be configured on the CE-to-PE BGP session to allow standard BGP communities (e.g., NO_EXPORT) to be propagated from the CE router into the MPLS VPN network. Without this, the PE router will strip communities from incoming BGP updates, preventing them from reaching the remote PE.

Answer

Configure no bgp default community-mode

Answer

Configure vrf forwarding with route-target both

Answer

Configure route-map to filter communities

Question 12

Which congestion avoidance technique drops packets probabilistically before the queue becomes full?

Accepted Answer

WRED. WRED (Weighted Random Early Detection) is a congestion avoidance mechanism that monitors the average queue depth and, when it exceeds a configured threshold, begins dropping packets probabilistically before the queue becomes completely full. This proactive dropping signals TCP senders to reduce their transmission rates, thereby preventing tail drop and global synchronization.

Answer

FIFO

Answer

Priority Queuing

Answer

Custom Queuing

Question 13

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 14

When deploying EVPN with SR-MPLS or SRv6, which THREE statements about EVPN multi-homing and related concepts are correct? (Choose three.)

Accepted Answer

EVPN IRB (Integrated Routing and Bridging) enables both Layer 2 and Layer 3 forwarding over the same EVPN instance using an anycast gateway.. The three correct statements about EVPN multi-homing are A, B, and E. EVPN IRB does use an anycast gateway for Layer 3 forwarding. Aliasing allows forwarding based on ESI knowledge even without learning the specific MAC address. Designated Forwarder (DF) election is performed per (ESI, VLAN) to avoid duplicate traffic. Option C is incorrect because the Ethernet Segment Identifier (ESI) is used for identifying multi-homed sites and DF election, but split-horizon filtering is achieved through the ESI label, not the ESI itself. Option D is incorrect because EVPN VPWS is a point-to-point service only.

Answer

The Ethernet Segment Identifier (ESI) is a unique identifier for a multi-homed site, used to detect multi-homing and for split-horizon filtering.

Answer

EVPN VPWS (Virtual Private Wire Service) uses a single EVPN instance for both point-to-point and multipoint services.

Question 15

Which RSVP-TE message is used to reserve resources along the path and includes the Explicit Route Object (ERO)?

Accepted Answer

PATH. The PATH message is sent from the source to the destination to request resource reservation and includes the Explicit Route Object (ERO), which specifies the path to be taken. The RESV message, sent upstream in response, includes objects like LABEL, STYLE, and RECORD_ROUTE, but not the ERO.

Answer

RESV_CONFIRM

Answer

PATH_TEAR

Answer

RESV