300-420 Exam Dumps : Designing Cisco Enterprise Networks

The multicast Reverse Path Forwarding check is a loop-prevention mechanism. Cisco multicast forwarding does not forward traffic merely because the packet arrived on any multicast-enabled interface. Instead, the router checks whether the packet arrived on the interface that would be used to route traffic back toward the multicast source or, in shared-tree cases, back toward the rendezvous point. If the packet arrives on the expected reverse path, it passes the RPF check and can be replicated to the outgoing interface list. If it arrives on the wrong interface, it is dropped to prevent loops and duplicate packets. This supports a loop-free multicast distribution tree from sources to receivers. Auto-RP mapping agents, bootstrap messages, and RP-set discovery are separate rendezvous point distribution mechanisms; they are not the function of the RPF check. In enterprise designs with redundant links, asymmetric routing, or multiple equal paths, multicast RPF behavior must be reviewed carefully because unicast routing choices directly influence whether multicast packets are accepted or discarded. Reference topics: multicast RPF, PIM forwarding, loop prevention, outgoing interface list.

Option B is retained because the subnetting task depends on the exhibit options and the chosen plan must divide 10.1.8.0/21 into nonoverlapping partner VPN subnets without allocating more address space than required. The design principle is variable-length subnet masking: allocate the smallest subnet that satisfies each partner requirement, then place the subnets on valid prefix boundaries inside the reserved aggregate. Cisco addressing design emphasizes conserving address space, avoiding overlap, and preserving summarization where possible. A /21 provides 2048 total addresses, so the designer can carve several smaller partner ranges while keeping the entire allocation summarizable as 10.1.8.0/21 toward the data-center core. Incorrect options would either waste addresses, overlap existing ranges, place subnets on invalid boundaries, or allocate blocks larger than necessary. Option B is the mapping that satisfies the exhibit constraints while keeping the design clean for routing, firewall policy, and VPN onboarding. Reference topics: IPv4 VLSM, subnet boundary alignment, partner VPN addressing, summarization, nonoverlapping address design.

The correct BGP design is to attach the well-known No-Export community to the prefixes that should not be advertised beyond the service provider boundary. Cisco BGP communities are policy attributes that allow routes to carry instructions or classifications across BGP peers. The No-Export community tells a receiving BGP speaker not to advertise the route outside its confederation or autonomous system. That behavior is exactly what is needed when branch prefixes may be exchanged with the provider but must not be propagated to the public Internet. The No-Advertise community is stronger: it prevents the route from being advertised to any BGP peer, which may block reachability even where the provider still needs to know the route. NOPEER is used for route-server or peering policy control and is not the clean answer for excluding branch prefixes from Internet propagation. Setting a generic “Internet community” is not a standard solution by itself. The architect should also ensure that outbound prefix filters and provider-side community support are documented in the service agreement. Reference topics: BGP communities, No-Export, No-Advertise, provider edge filtering, Internet route policy.