300-420 Exam Dumps : Designing Cisco Enterprise Networks

VRRP object tracking allows the effective priority of a VRRP router to change when a tracked object changes state. Cisco documentation states that VRRP object tracking ensures the best VRRP router is master by altering VRRP priorities according to tracked objects such as interface or IP route states. This directly supports answer A. It also supports answer D, because VRRP can track interfaces and routes through the tracking process. The priority does not have to remain fixed; object tracking is specifically designed so a router can lower its priority when an uplink, route, or critical dependency fails. A VRRP group is not limited to one tracked object in modern Cisco implementations, so option C is too restrictive. VRRP does not support only interface tracking; route tracking is also a common use case, especially when the LAN interface is still up but upstream reachability has failed. In a resilient campus or branch gateway design, object tracking prevents a router from remaining master when it no longer has a valid upstream path, improving deterministic failover.

DMVPN Phase 3 hub-and-spoke is the best design for this requirement. The customer wants secure dynamic site-to-site VPN connectivity for voice, video, and FTP, and also wants branches to communicate directly instead of hairpinning all traffic through headquarters. DMVPN supports dynamic spoke-to-spoke tunnels using multipoint GRE, NHRP, and IPsec protection. Phase 3 is more scalable than Phase 2 because the hub can send NHRP redirect messages and spokes can install shortcut routes, reducing the need for every spoke to maintain detailed next-hop information for every other spoke. That is important because the branch routers have limited memory. Phase 1 is hub-and-spoke only and does not provide dynamic spoke-to-spoke forwarding. A hierarchical Phase 3 design can be useful at very large scale, but the question describes a headquarters and remote branch design rather than multiple tiers of hubs. Phase 3 hub-and-spoke satisfies direct branch communication, security, and scale. Reference topics: DMVPN Phase 3, NHRP redirect, spoke-to-spoke tunnels, IPsec encryption, branch WAN scalability.

BIDIR-PIM is the correct multicast solution for interactive many-to-many communication with many sources when source trees must not be used. Bidirectional PIM is specifically designed for applications where receivers can also be senders and where the number of sources can grow without creating per-source multicast state throughout the network. Instead of building shortest-path trees for each source, BIDIR-PIM uses a shared tree rooted at the rendezvous point and forwards traffic bidirectionally along that tree. This reduces multicast routing-table growth and control-plane churn in environments with many active sources. PIM dense mode is not appropriate because it floods and prunes traffic and is inefficient at scale. Any-source multicast with PIM-SM may create source trees, which violates the requirement. MSDP is used to exchange source information between PIM-SM domains and is not itself the forwarding mode for this campus or WAN multicast design. Reference topics: BIDIR-PIM, many-to-many multicast, shared tree, multicast source scaling, PIM design.