350-501 Sample Questions & Answers

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.

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.

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.

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.

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.

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.

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.

6VPE leverages the existing L3VPN architecture. The customer's IPv6 prefixes are placed into a VRF. The ingress PE uses MP-BGP with the VPNv6 address family (AFI 2, SAFI 128) to advertise these prefixes to other PE routers. The BGP update includes the IPv6 prefix, a route distinguisher to make it unique, a route target to control VPN membership, and an MPLS label. Critically, the BGP next-hop attribute for this VPNv6 route is set to the IPv4 address of the advertising PE router. This allows the remote PEs to resolve the next-hop via the IPv4/MPLS core and establish the correct transport LSP.

Bidirectional Forwarding Detection (BFD) is a lightweight, low-overhead protocol specifically designed for fast failure detection of the forwarding path between routers. It can operate with timers in the tens of milliseconds, far more aggressively than protocol-native keepalives like BGP's. When BFD is enabled for a BGP neighbor, BGP registers with the BFD process. If BFD detects a failure in the forwarding path, it immediately notifies the BGP process, which then tears down the peering session, allowing for sub-second failover. This is the industry-standard method for achieving fast convergence for BGP and other routing protocols.