350-501 Practice Test 41 — 15 Questions

Question 1

An SP is deploying 6VPE (IPv6 L3VPN). Which BGP extensions are used to carry IPv6 VPN prefixes?

Accepted Answer

AFI=2 (IPv6), SAFI=128 with IPv4 next-hop encoded as IPv6-mapped.. 6VPE uses MP-BGP with the IPv6 VPN AFI (AFI=2) and SAFI=128 (MPLS-labeled VPN). The next-hop is an IPv4 address (the PE's loopback) encoded as a mapped IPv6 address, but the VPNv6 address family carries the prefixes.

Answer

AFI=1 (IPv4), SAFI=128 with IPv6 next-hop.

Answer

AFI=2, SAFI=1 (unicast) with MPLS labels.

Answer

AFI=1, SAFI=128 with IPv6 VPN prefixes.

Question 2

Refer to the exhibit. A PE router is showing a VRF route. What does the [200/0] indicate?

Accepted Answer

Administrative distance 200 and metric 0. In the VRF route output, the [200/0] notation represents the administrative distance (200) and the metric (0) for the route. The administrative distance is a trustworthiness value used by the router to select the best path when multiple routing protocols provide routes to the same destination; a lower value is preferred. The metric is a protocol-specific cost, and here it is 0, indicating a directly connected or very low-cost path within the VRF.

Answer

Local preference 200 and MED 0

Answer

Label value 200 and metric 0

Answer

AS path length 200 and weight 0

Question 3

Which DSCP value is recommended for Expedited Forwarding (EF) per-hop behavior, commonly used for voice traffic?

Accepted Answer

DSCP 46. Expedited Forwarding (EF) per-hop behavior is defined in RFC 3246 and is designed for low-loss, low-latency, and low-jitter services such as voice traffic. The recommended DSCP value for EF is 46 (binary 101110), which corresponds to the PHB that ensures a dedicated forwarding treatment with a minimal queueing delay. This value is widely adopted in Cisco QoS deployments for voice payloads to guarantee strict priority queuing.

Answer

DSCP 10

Answer

DSCP 0

Answer

DSCP 34

Question 4

In BGP confederation, how does the AS_PATH attribute handle the confederation sub-AS numbers to prevent loops?

Accepted Answer

Confederation sub-AS numbers are carried in a separate confederation AS_PATH segment and are stripped when routes leave the confederation. In BGP confederation, confederation sub-AS numbers are carried in a separate confederation AS_PATH segment and are not visible to external BGP peers. The confederation AS_PATH is used for loop prevention within the confederation, while the regular AS_PATH remains unchanged when routes leave the confederation.

Answer

Confederation sub-AS numbers are added to the regular AS_PATH

Answer

Confederation sub-AS numbers are replaced by the confederation identifier

Answer

Confederation sub-AS numbers are ignored for loop detection

Question 5

Which TWO QoS mechanisms are used to provide congestion avoidance? (Choose two.)

Accepted Answer

RED. RED (Random Early Detection) and WRED (Weighted Random Early Detection) are congestion avoidance mechanisms that proactively drop packets before a queue becomes full, signaling TCP senders to reduce their transmission rate. Unlike congestion management tools (like CBWFQ or LLQ) that queue packets during congestion, RED/WRED monitor average queue depth and drop packets probabilistically to prevent tail drops and global TCP synchronization.

Answer

Policing

Answer

CBWFQ

Answer

LLQ

Question 6

An SRv6 SID is composed of three parts: Locator, Function, and Arguments. Which part identifies the SRv6-capable node in the network?

Accepted Answer

Locator. The Locator portion of an SRv6 SID identifies the node and is routable in the IGP.

Answer

Argument

Answer

Function

Answer

SRH

Question 7

An engineer is configuring MPLS LDP on a Cisco router. The router is using downstream unsolicited mode and liberal label retention. Which statement correctly describes the label advertisement behavior?

Accepted Answer

Labels are advertised to all LDP peers for all known prefixes without a request.. In downstream unsolicited mode, the router advertises labels for all known prefixes to all neighbors, even if the neighbor did not request them. Liberal label retention keeps all received labels, even if not currently used.

Answer

Labels are retained only for directly connected neighbors.

Answer

Label advertisements are sent only for prefixes learned via BGP.

Answer

Labels are advertised only when explicitly requested by a neighbor.

Question 8

In BGP route selection, which attribute is considered before LOCAL_PREF?

Accepted Answer

Weight. BGP route selection starts with the highest weight (Cisco proprietary), then highest LOCAL_PREF, then locally originated routes.

Answer

AS_PATH length

Answer

MED

Answer

LOCAL_PREF

Question 9

An engineer is deploying MPLS in the core and wants to ensure that all core routers use the same label for a specific prefix, regardless of which router originated it. Which MPLS label allocation mode should be used?

Accepted Answer

Per-prefix label mode. Per-prefix label mode (option C) is correct because it assigns a single label for a specific prefix across all core routers, regardless of which router originated the route. This ensures label consistency, which is critical for proper MPLS forwarding and troubleshooting. In contrast, per-next-hop or per-interface modes would create different labels for the same prefix based on the next hop or interface, breaking the requirement for uniform label allocation.

Answer

Per-interface label mode

Answer

Per-next-hop label mode

Answer

Per-VRF label mode

Question 10

A service provider recently deployed MPLS L3VPN for a customer with four sites (Site1, Site2, Site3, Site4) connected to PE1, PE2, PE3, and PE4 respectively. All sites are in VRF CUST-A with route targets 100:1 import and 100:1 export on all PEs. The customer reports that Site4 cannot ping the loopback interface (10.1.1.1/32) of Site1, but Site2 and Site3 can reach it. The provider verifies that BGP sessions between all PEs and the route reflector are up and that VPNv4 routes are advertised. The VRF on PE4 shows the route 10.1.1.1/32 with next-hop 192.0.2.1 (PE1's loopback) but when Site4 initiates a ping, it fails. What should the provider check next?

Accepted Answer

Check the VRF route target import on PE1 to ensure it includes the route target exported by PE4 for Site4's subnet.. The issue is that return traffic from Site1 to Site4 is being dropped. Although PE4 has the route to Site1's loopback, PE1 needs to have a route back to Site4's subnet in its VRF CUST-A. This is achieved by ensuring that PE1 imports the correct route target (exported by PE4 for Site4's prefix). If the import RT on PE1 does not include the RT that PE4 exports for Site4, the return path is broken. Option A is incorrect because the IGP propagation of loopback interfaces is not directly related to VPNv4 route reachability; BGP sessions are already up. Option B is incorrect because the BGP session between PE4 and the route reflector is already up, and update source issues would affect route advertisement, not return path. Option C is incorrect because CE configuration is likely fine since Site2 and Site3 work.

Answer

Ensure that the IGP operating in the core has propagated the loopback interface address of PE1 to all P routers.

Answer

Verify that the BGP session between PE4 and the route reflector is using the correct update source.

Answer

Verify that the CE router at Site4 is configured with the correct VRF name and default gateway.

Question 11

In the DiffServ model, which DSCP value is used for Expedited Forwarding (EF) to support voice traffic?

Accepted Answer

DSCP 46. EF is defined by DSCP 46 (binary 101110) to provide low-loss, low-latency service for voice.

Answer

DSCP 0

Answer

DSCP 40

Answer

DSCP 34

Question 12

Which technology should be used to provide per-flow load balancing across multiple equal-cost paths in an MPLS network while preserving packet order?

Accepted Answer

ECMP with a hash algorithm based on layer 3 and layer 4 headers. ECMP with a hash algorithm based on Layer 3 and Layer 4 headers is the correct choice because it provides per-flow load balancing by computing a hash over source/destination IP addresses and TCP/UDP port numbers, ensuring that all packets belonging to the same flow are forwarded over the same equal-cost path. This preserves packet order while distributing different flows across multiple paths in an MPLS network.

Answer

LAG with default hashing

Answer

Policy-based routing with next-hop per destination

Answer

Per-packet round-robin load balancing

Question 13

A network operator is deploying Segment Routing with TI-LFA across an OSPF network. After configuring OSPF with 'segment-routing mpls' and 'fast-reroute per-prefix', they notice that backup paths are not being installed for some prefixes. 'Show ip ospf segment-routing protected' shows 'No FRR backup' for those prefixes. What is a possible reason?

Accepted Answer

Adjacency-SIDs are not configured on neighboring routers. TI-LFA (Topology Independent Loop-Free Alternate) for per-prefix fast-reroute in OSPF requires that adjacency-SIDs be configured on neighboring routers. Without adjacency-SIDs, OSPF cannot compute the necessary repair paths to provide backup protection, resulting in 'No FRR backup' for those prefixes.

Answer

LDP label distribution is still active

Answer

The prefix-SID index is not globally unique

Answer

OSPF does not support TI-LFA for per-prefix prefixes

Question 14

A service provider is deploying MPLS L3VPN and notices that BGP next-hop resolution for VPNv4 routes fails on the PE routers. The PE routers are running OSPF as the IGP and have loopback interfaces used for LDP and BGP peering. Which configuration change should the engineer implement to ensure that the BGP next-hop is reachable?

Accepted Answer

Configure 'neighbor x.x.x.x update-source loopback0' under the BGP router configuration.. The BGP next-hop for VPNv4 routes is typically the loopback interface of the remote PE router. For BGP to consider the next-hop reachable, the local PE must have an IGP route to that loopback address. The 'neighbor x.x.x.x update-source loopback0' command ensures that BGP uses the loopback interface as the source IP for the TCP session, which aligns the BGP peering address with the IGP-advertised loopback, making the next-hop reachable via OSPF.

Answer

Configure 'neighbor x.x.x.x allowas-in 1' under the BGP VRF configuration.

Answer

Apply the 'neighbor x.x.x.x next-hop-self' command under the BGP VRF configuration.

Answer

Increase the 'maximum-paths' value under the BGP address-family VPNv4.

Question 15

Which two protocols are commonly used for label exchange in an MPLS network? (Choose two.)

Accepted Answer

LDP. LDP (Label Distribution Protocol) is a standardized protocol (RFC 5036) specifically designed for distributing MPLS labels between routers to establish Label Switched Paths (LSPs) without requiring an IGP to carry label information. BGP (Border Gateway Protocol) can also exchange MPLS labels, particularly in MPLS VPN and Inter-AS scenarios, where it carries VPNv4 routes with MPLS labels using the BGP label-mapping capability (RFC 3107).

Answer

OSPF

Answer

PIM

Answer

IS-IS