300-510 Sample Questions & Answers

True. The peering between member-ASNs inside a BGP confederation is known as an 'internal' eBGP (ceBGP) session. While it behaves like eBGP in terms of AS_PATH, it is designed to act like iBGP for attribute propagation. This means attributes like MED, Local Preference, and communities are carried unmodified between the member-ASNs, preserving the original path information throughout the confederation.

BGP FlowSpec rules consist of match criteria and an action. To mitigate an attack targeting a server, the match criteria must identify the destination address (the server), the destination port, and the protocol. The correct action to drop the traffic is to set the traffic-rate to 0. Therefore, matching destination-address 198.51.100.50/32, destination-port 53, and protocol UDP with an action of traffic-rate 0 is the correct rule. Matching the source address would block traffic from the server, not to it. redirect-to-vrf is for traffic redirection, and traffic-marking changes DSCP values, neither of which drops the traffic.

BGP Route Dampening is the feature specifically designed to penalize and suppress unstable (flapping) routes. It assigns a penalty to a route each time it flaps. When the cumulative penalty exceeds a suppress-limit, the route is dampened (suppressed). The penalty value then decays over a configurable half-life time, and the route is un-suppressed once the penalty drops below a reuse-limit. This matches the requirement to suppress excessively flapping routes for a period of time.

MAP-T is a stateless IPv4/IPv6 translation mechanism that embeds IPv4 address and port information into an IPv6 address. This allows for direct translation between IPv4 and IPv6 packets without maintaining state on the translator, making it highly scalable for service provider environments. NAT64 is a stateful mechanism. Static tunnels are not a translation mechanism, and 6to4 is another tunneling technology that doesn't fit the stateless translation requirement described.

This regular expression is broken down as follows:

• ^: Matches the beginning of the string (start of the AS_PATH).
• 65501: Matches the literal AS number 65501.
• : Matches a space or a delimiter between AS numbers.
• .*: Matches any character (.) zero or more times (*), representing any sequence of AS numbers in the middle.
• : Matches a space or a delimiter.
• 65502: Matches the literal AS number 65502.
• $: Matches the end of the string (end of the AS_PATH, representing the origin AS).

In BIDIR-PIM, traffic can flow both up and down the shared tree. On a multi-access segment (like Ethernet), multiple routers might be connected. The Designated Forwarder (DF) election process is used to select a single router on that segment responsible for forwarding multicast traffic from downstream sources up toward the Rendezvous Point Link (RPL). This prevents duplicate multicast packets from being sent upstream.

The main goal of a BGP free core design is to improve scalability and simplify operations. In this model, only the Provider Edge (PE) routers run BGP to learn customer routes. The Provider (P) routers in the core only run an IGP and LDP (or use Segment Routing). They forward traffic based on MPLS labels without needing any knowledge of the BGP prefixes. This significantly reduces the memory and CPU load on core routers, as they do not need to hold the full internet routing table.

Bidirectional Forwarding Detection (BFD) is a lightweight protocol designed for very fast failure detection of the forwarding path between routers. It can detect failures in milliseconds, far faster than BGP's native keepalive mechanism. By associating a BFD session with the BGP peering, BGP can be notified of a neighbor failure almost instantly, allowing it to trigger convergence and switch to the backup path via PE2 well within the 50ms requirement. Tuning BGP timers cannot safely achieve sub-second convergence. BGP PIC speeds up FIB update after a failure is known, but it relies on another mechanism (like BFD) for the initial failure detection. NSF/NSR are for control plane redundancy, not link failure detection.

A Path Computation Element (PCE) provides a centralized, off-box computation engine that can make complex pathing decisions based on a variety of constraints, including latency, bandwidth, and SRLG avoidance. Using the Path Computation Element Communication Protocol (PCEP), routers (PCCs) can request paths from the PCE, and the PCE can proactively push down optimized paths. This architecture is ideal for creating multiple, constraint-based SR-TE policies or RSVP-TE tunnels between the same endpoints, each satisfying different optimization criteria.