300-420 Exam Dumps : Designing Cisco Enterprise Networks

The design must enable IPv6 multi-topology behavior at the appropriate edge routers and move metric handling toward transition support where the current IS-IS network still uses narrow metrics. Multi-topology IS-IS is important during IPv4-to-IPv6 migration because IPv4 and IPv6 reachability do not always exist on identical links at every stage of the migration. If IPv6 follows the IPv4 topology when some devices or paths are not IPv6-ready, traffic can be blackholed. Multi-topology allows independent IPv4 and IPv6 SPF calculations, which prevents IPv6 traffic from being forwarded through IPv4-only portions of the network. The metric-style transition setting is used when migrating from narrow to wide metrics, allowing interoperability while routers are upgraded and the design scales. Because the requirement says IPv6 areas will grow and the current network uses narrow metrics, the selected edge devices must support the IPv6 topology and metric transition behavior shown in the exhibit. Reference topics: IS-IS IPv6, multi-topology IS-IS, narrow metrics, wide metric transition, IPv6 migration, blackhole prevention.

Cisco SD-Access uses virtual networks as macro-segmentation boundaries, and those VNs are represented as VRF instances in the fabric. By design, separate VNs do not communicate directly inside the overlay unless the design introduces controlled inter-VN routing. Cisco SD-Access designs commonly use an external fusion router, firewall, or shared-services device to map fabric VNs to external VRFs and selectively exchange routes between them. That is the correct design because it preserves segmentation while allowing policy-controlled access to shared resources or selected networks. GRE tunnels between fabric edges are not the normal SD-Access inter-VN communication method and would bypass the intended fabric policy model. Security Group Tags provide microsegmentation within or across policy domains, but SGTs do not by themselves leak routes between separate virtual networks. Route leaking on fabric border nodes is not the recommended standalone answer here because inter-VN communication is normally provided through a fusion device or firewall outside the fabric. Therefore, the correct solution is external fusion routing with controlled VRF route exchange.

Dual-stack is the correct IPv6 migration technique because the DWDM devices provide Layer 2 transport between routers that already support both IPv4 and IPv6. Layer 2 transport is generally transparent to IPv6 because it forwards Ethernet frames rather than participating in Layer 3 routing. The routers at the ends of the DWDM circuit can peer directly over the Layer 2 service and run IPv6 in parallel with IPv4, while mail servers can also retain IPv4 reachability during the migration. Cisco dual-stack migration guidance uses simultaneous IPv4 and IPv6 operation to support phased application and host transition with minimal tunneling overhead. 6to4, ISATAP, and 6rd are tunneling or transition mechanisms used when IPv6 islands must cross IPv4-only routed infrastructure. In this case, the limiting devices are in the transport layer and do not need to route IPv6. Deploying dual-stack on the capable routers and servers is simpler, more transparent, and avoids unnecessary encapsulation. Reference topics: IPv6 dual-stack migration, Layer 2 transparency, IPv6 over Ethernet, tunneling avoidance, phased migration.