Cisco Updated 300-420 Exam Questions and Answers by myah

Single-topology IS-IS with the transition feature is the correct design when IPv6 is being added to an existing IPv4 IS-IS network and both topologies must match exactly. In single-topology mode, IS-IS calculates one topology and uses it for both IPv4 and IPv6, which fits the requirement that the IPv4 and IPv6 paths and router levels remain the same on each interface. Cisco documentation notes that single-topology IS-IS IPv6 requires routers to run the same set of address families for adjacency consistency; during migration, transition support allows the network to operate while IPv6 is introduced gradually. Multi-topology IS-IS is used when IPv4 and IPv6 should be allowed to follow different topologies or when not all routers support the same address families. The question explicitly says the topologies will match exactly, so multi-topology is unnecessary. Single topology without the transition feature is risky during phased migration because routers that do not yet run both address families may fail adjacency checks or create blackholing. Reference topics: IS-IS for IPv6, single-topology IS-IS, transition mode, address-family consistency, IPv6 migration.

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.

If the RPF check is successful in PIM sparse mode, the multicast packet is forwarded according to the outgoing interface list. The Reverse Path Forwarding check verifies that the packet arrived on the interface the router would use to reach the source, or the RP in the case of shared-tree state. This prevents loops and duplicate multicast forwarding. After the check passes, the router consults the multicast forwarding entry and sends the packet only out interfaces in the OIL where downstream receivers or PIM neighbors require the traffic. It is not automatically forwarded out every PIM-enabled interface, and it is not forwarded back out the receiving interface. If the RPF check fails, the packet is dropped to protect the multicast distribution tree. The OIL therefore represents the correct controlled forwarding behavior after a successful RPF validation. Reference topics: PIM sparse mode, RPF check, outgoing interface list, multicast forwarding state, loop prevention.

The company must configure a summary route on R1. The branch routers must remain normal EIGRP routers, and auto-summary is not allowed, so the scalable design choice is manual summarization at the hub. Cisco EIGRP documentation states that manual summarization can be configured on an interface using the ip summary-address eigrp command and can summarize routes on almost any bit boundary. In a DMVPN hub-and-spoke topology, a hub summary reduces the number of individual routes advertised to branches, limits routing-table growth, and helps contain EIGRP queries. It also keeps branch configuration simple because the branches do not need to be converted to stub routers to meet this specific requirement. Disabling split horizon is a common DMVPN Phase 2 consideration when the hub must advertise spoke routes back out the same multipoint tunnel interface, but it does not summarize HQ routes. A multipoint interface is already implied by DMVPN, and disabling the SIA timer is not a design solution. Reference topics: EIGRP manual summarization, DMVPN hub design, no auto-summary, query containment.