Cisco Updated 300-420 Exam Questions and Answers by pia

The design must influence how EIGRP-learned routes are injected into the OSPF domain, so R2 should redistribute EIGRP into OSPF as metric-type E1 and R3 as metric-type E2. Cisco OSPF documentation distinguishes external Type 1 and Type 2 routes. An E1 route includes the external cost plus the internal OSPF cost to reach the ASBR, while an E2 route uses only the external metric. Cisco path selection prefers O E1 over O E2 for the same external destination. Therefore, making R2 advertise the EIGRP routes as E1 causes OSPF routers to prefer R2 as the exit toward the EIGRP domain. Option C adjusts OSPF-to-EIGRP redistribution, which is the wrong direction for traffic passing toward EIGRP from OSPF. Removing redistribution on R3 would reduce redundancy. Option D reverses the preference and would favor R3. Reference topics: OSPF external route types, EIGRP-to-OSPF redistribution, ASBR preference, E1 versus E2 metrics.

The company must prevent itself from advertising routes learned from one ISP to the other ISP. If it receives full Internet tables from both providers and then re-advertises those learned prefixes, it can unintentionally become a transit autonomous system. The proper BGP design is to apply outbound route policy so only the company’s own originated prefixes are advertised to the ISPs, while routes learned from ISP1 are never advertised to ISP2 and routes learned from ISP2 are never advertised to ISP1. A route map or equivalent prefix filtering policy applied outbound to the ISP neighbors achieves that goal. Local preference influences outbound path selection inside the company AS; it does not prevent transit advertisements. MED influences how a neighboring AS may enter the company network, but it does not stop the company from advertising third-party routes. Tagging incoming routes with no-export is not as deterministic as simply filtering what is advertised to each provider and may still allow undesirable routing behavior within the directly connected provider. Therefore, the design should include outbound route filtering with a route map.

R3 must be made an L1/L2 router so that it can connect the Level 1 area to the Level 2 backbone. IS-IS uses a two-level hierarchy. Level 1 routers know the topology inside their local area, Level 2 routers form the backbone between areas, and Level 1/Level 2 routers provide the boundary between the two. A proper IS-IS backbone cannot have isolated Level 1-only routers positioned between L1/L2 routers that need to exchange interarea reachability. If the link failure exposed an area-continuity problem, converting R3 to L1/L2 gives the area an appropriate attachment point to the backbone and restores interarea forwarding behavior. Making an unrelated router Level 1 or Level 2 only would not necessarily create the required boundary function. Simply making Area 0 L2-only is also not the focused fix if the topology still lacks a correct L1/L2 transition point. The design principle is straightforward: every Level 1 area must have a dependable L1/L2 router through which traffic can reach destinations outside the area. Reference topics: IS-IS Level 1, Level 2 backbone, L1/L2 routers, interarea reachability, hierarchical IGP design.

An SD-Access intermediate node routes and transports IP traffic between fabric nodes. It is part of the underlay transport path, comparable to a routed distribution or core device that provides IP reachability between edge, border, and control-plane nodes. Intermediate nodes do not perform endpoint registration, VXLAN encapsulation for attached users, virtual-network mapping, or anycast gateway services. Those functions belong primarily to fabric edge nodes. They also do not act as LISP proxy tunnel routers, which is a border-node function used for communication between the fabric and external networks. The intermediate node’s role is deliberately simple: maintain underlay routing, provide resilient transport, and forward encapsulated traffic across the fabric infrastructure. This separation of roles is what lets Cisco SD-Access scale by keeping endpoint intelligence at the edge and mapping intelligence in the control plane while the intermediate layer remains a stable IP transport network. Reference topics: SD-Access intermediate node, routed underlay, IP transport, fabric roles, edge and border node separation.