Cisco 300-420 - Designing Cisco Enterprise Networks (ENSLD) v1.1

EIGRP variance is the correct design because the requirement is to use the best two paths while all links are available and maintain forwarding if one path fails. EIGRP supports unequal-cost load balancing when feasible successors exist and the variance multiplier allows additional loop-free paths to be installed in the routing table. Cisco EIGRP design uses the feasible distance and reported distance conditions to ensure that unequal-cost alternatives are loop free before they are used. The maximum-paths command controls how many equal or eligible paths can be installed, but it does not make an unequal path eligible. Add-paths is a BGP feature, not an EIGRP mechanism. Changing metric weights is rarely recommended because it affects metric calculation globally and can create inconsistent routing if not deployed everywhere. Variance 2 allows EIGRP to include backup paths whose metric is within two times the successor metric, assuming they satisfy the feasibility condition. Reference topics: EIGRP variance, unequal-cost load balancing, feasible successor, DUAL, path resiliency.

The RPF check verifies that multicast traffic arrives on the interface the router would use to reach the source or RP, and it prevents forwarding when traffic arrives from the wrong direction. Cisco multicast forwarding is intentionally different from unicast forwarding because a multicast packet may need to be replicated toward many receivers. Without Reverse Path Forwarding, multicast loops and duplicate packets could occur. Option A captures the forwarding condition: the packet is accepted when it arrives on the interface used to route back toward the source address. Option D is the closest available drop condition in the answer set, expressing that packets not aligned with the correct reverse path are discarded rather than forwarded. Option B incorrectly checks the route toward the destination group, because multicast groups are not reached through ordinary unicast destination routing. Option C would flood traffic and create loops. Option E reverses the correct logic by dropping packets that arrive on the proper reverse-path interface. Reference topics: multicast RPF, loop prevention, source-tree forwarding, shared-tree forwarding, PIM.

Route reflectors in different clusters should peer with one another as nonclients. BGP route reflection is used to reduce the full-mesh requirement inside an AS. Within a cluster, route-reflector clients peer with their route reflector and do not need to peer with every other client. However, route reflectors still need a way to exchange routes learned from their respective clusters. Cisco BGP route-reflection rules allow a route learned from a client to be reflected to other clients and to nonclient peers. A route learned from a nonclient peer is reflected only to clients. Therefore, route reflectors in different clusters should maintain iBGP peerings as nonclients so that routes can propagate between clusters and then be reflected down to each cluster’s clients. Making route reflectors clients of each other can create confusing policy and cluster behavior. Having no peering would isolate the clusters. Peering only through another nonclient router adds unnecessary indirection and does not improve scale. The correct scalable design is route-reflector-to-route-reflector nonclient peering. Reference topics: BGP route reflectors, clusters, client and nonclient peers, iBGP scaling, route reflection rules.