Implementing Cisco Enterprise Advanced Routing and Services (ENARSI)

In IPv6 router EIGRP configuration, when a router is configured as a stub, it only advertises connected routes and summary routes by default. This means it does not advertise other routes such as the default route (::/0) learned from another router. Since R1 is learning the default route from R2 but not advertising it to the Branch router, it indicates that R1 is likely configured as a stub. By issuing the no eigrp stub command on R1, it will start advertising all EIGRP-learned routes, including the default route, thus allowing users in the branch network of 2001:db8:0:4::/64 to access the Internet.

The correct configuration is B. Here's the reasoning: The goal is to forward any traffic sourced from the 192.168.130.0/24 network to R2. The route-map should be applied to the interface closest to the traffic source, which is Gi0/1. For traffic destined to R2, the next-hop IP should be 172.20.40.2, which is the IP address of the interface on R2 that connects to R1. Therefore, the next-hop IP in the route map should be set to 172.20.40.2, making the configuration in option B correct.

When the route-map OUT command is applied toward an eBGP neighbor R1 (1.1.1.1) by using the neighbor 1.1.1.1 route-map OUT out command, R1 sees 192.168.130.0/24 as two AS hops away instead of one AS hop away. This is because the route-map matches the prefix 192.168.130.0/24 and sets the AS-path with an additional AS number (65000) using the prepend command. This makes the route appear to traverse two AS hops, making the route less preferred by other networks.

Policy-based routing (PBR) changes the forwarding decision that a router makes without first changing the routing table or influencing the IP data plane. PBR allows routers to make forwarding decisions based on policies set by network administrators rather than strictly following the routing table. This method enables more flexible and customized routing solutions without altering the fundamental routing information base (RIB) or impacting the forwarding information base (FIB).

The configuration needed to prevent the routing loop involves the proper tagging and filtering of routes between the two different routing protocols, EIGRP and OSPF. This is essential to ensure routes are not constantly reintroduced into the other protocol, creating a loop. The configuration must include tagging the routes when they are redistributed from EIGRP to OSPF and then filtering them out when they are redistributed back from OSPF to EIGRP. The provided configuration in Option A follows these steps correctly: R3 redistributes EIGRP into OSPF with a tag using a route-map, and R4 redistributes from OSPF to EIGRP while filtering out these tagged routes, thus preventing the loop. Therefore, the correct configuration to prevent the loop is shown in Option A.