Free 350-501 sample questions

Correct answer: True

True. The Site of Origin (SoO) extended community is a BGP attribute used to identify the site from which a route originated. When a customer site is connected to two PEs for redundancy, SoO is applied to the routes learned from the CE at each PE. This SoO value is carried with the VPNv4 route across the MPLS core. A PE router will not advertise a VPNv4 route back to a CE if the route's SoO value matches the SoO configured on the PE-CE link. This effectively prevents routes learned from one link to a site from being advertised back to the same site via the other link, thus preventing routing loops.

Correct answer: show mpls mldp database vrf CUST_A

The `show mpls mldp database vrf ` command is the most direct way to verify the mapping between a customer multicast (C-multicast) flow and the provider's mLDP label switched path (LSP). This command displays the mLDP Forwarding Equivalence Class (FEC) information, including the multicast source/group, the upstream PE router, the opaque value identifying the C-flow, and the assigned upstream label. If the customer's PIM join has been successfully processed by the PE, an entry for that (S,G) or (*,G) should appear in this database, confirming the C-flow to P-tunnel mapping.

Correct answer: EVPN E-Tree

EVPN E-Tree is a service specifically designed for hub-and-spoke Layer 2 topologies. It categorizes attachment circuits as either 'root' (hub) or 'leaf' (spoke). The forwarding logic of E-Tree allows root-to-leaf, leaf-to-root, and root-to-root communication, but explicitly blocks leaf-to-leaf communication at Layer 2. This perfectly matches the requirement to allow spokes to talk to the hub but not to each other, effectively preventing loops and controlling BUM traffic between spoke sites.

Correct answer: Pipe Model (Uniform)

The Pipe Model (also known as the Uniform Model) in MPLS DiffServ provides a single, consistent QoS treatment from end-to-end. In this model, the IP Precedence or DSCP value is mapped to the MPLS EXP bits at the ingress PE. This EXP value is then used for queuing and scheduling decisions across the MPLS core. Crucially, at the egress PE, the EXP value is mapped back to the IP header's ToS byte. This ensures that any QoS remarking done in the core (if any) is reflected in the IP packet, maintaining a uniform QoS policy. The requirement to map IP Precedence to EXP and use it throughout the core is the definition of the Pipe/Uniform model.

Correct answer: Applying the FlowSpec service policy to the ingress interfaces where the attack traffic is expected.

In IOS-XR, BGP FlowSpec has a separate control plane and data plane component. While the rules are received and programmed into BGP via the address-family configuration, they are not enforced in the data plane until a specific `flowspec` service policy is attached to the physical or logical ingress interfaces. This action instructs the forwarding plane (FIB) to program the dynamic ACLs generated by the FlowSpec rules onto the interface hardware. Without this step, the router knows about the rule but does not actively filter traffic based on it.

Correct answer: The L1/L2 router will advertise the summary route into the Level 1 area, summarizing routes learned from other IS-IS levels or protocols.

In IS-IS, summarization is performed as routes are advertised *into* an area or level. The command `summary-address 10.10.0.0 255.255.0.0 level-1` on an L1/L2 router instructs it to advertise a single summary route (10.10.0.0/16) *down into the Level 1 area*. This summary represents routes that the L1/L2 router has learned from other sources, such as the Level 2 backbone or redistributed routes. This prevents the more specific prefixes from being flooded into the Level 1 area, thus reducing the LSDB size on L1-only routers.

Correct answer: Deploy Segment Routing with an MPLS data plane (SR-MPLS) and enable OSPF extensions for Segment Routing.

Segment Routing (SR-MPLS) is the ideal solution. 1) It provides powerful source-based traffic engineering by allowing the headend router to specify an explicit path as a stack of labels (SIDs), meeting the granular steering requirement without the state and complexity of RSVP-TE. 2) It simplifies the control plane by removing LDP and RSVP, relying only on IGP extensions (in this case, for OSPF), which significantly improves scalability. 3) SR is the foundational technology for network slicing and service function chaining, directly addressing the future-proofing requirement for 5G. 4) By centralizing path control at the source or a controller, it simplifies operations compared to hop-by-hop provisioning.

Correct answer: Event-driven (on-change)

The event-driven or on-change subscription model is designed for this exact purpose. In this model, the router (or telemetry agent) monitors the state of the subscribed data (e.g., interface counters defined by a YANG path). It only streams an update to the collector when the value of that data actually changes. This is highly efficient as it avoids sending redundant data, reducing both network bandwidth and collector processing load, which is ideal for high-frequency state changes like counters.

Correct answer: A mismatched IP MTU value on the interfaces.

When OSPF neighbors are stuck in the EXSTART/EXCHANGE state, it indicates that they have successfully exchanged Hello packets and agreed on a master/slave relationship but are failing to exchange Database Descriptor (DBD) packets. The most common reason for this failure is a mismatched IP MTU on the connecting interfaces. If the master router sends a DBD packet that is larger than the MTU of the slave router's interface, the slave will drop the packet and the exchange process will stall, continuously restarting.