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

A scalable two-level IS-IS design uses Level 1 areas for internal campus routing and Level 2 for the interarea backbone. Each campus should have a unique IS-IS NET area value so the internal routers belong to that campus’s Level 1 area. Internal campus routers should be configured as Level 1 routers, which keeps detailed campus prefixes inside the local area and prevents unnecessary propagation of more-specific routes to other campuses. At the edge of each campus, routers that connect the campus to the backbone should operate as Level 1/Level 2 routers. These routers connect the local Level 1 area to the Level 2 backbone and can summarize or control routing information between levels. IS-IS does not use BDR routers; that is an OSPF term, so option A is invalid. Assigning the same NET value everywhere collapses the hierarchy and defeats the segregation goal. MTU differences are not a hierarchy or route-segmentation tool. Therefore, the design should use unique NET values per campus with Level 1 internal routers and Level 1/Level 2 routers at the campus edge.

BGP outbound route filtering is the correct solution because the enterprise wants control over which routes it receives without waiting for a provider change cycle each time the prefix set changes. Cisco BGP prefix-based ORF allows a receiving BGP speaker to send an inbound prefix-list filter to the remote peer, which then applies that list as an outbound filter. This reduces unwanted routing updates before they cross the session and lets the enterprise update the desired prefixes locally when business requirements change. Requesting only fixed prefixes from the ISP would work initially but fails the flexibility requirement because the provider has a two-week change process. Requesting a full table and filtering inbound would provide flexibility but wastes memory and CPU on the customer router because all routes are still received before filtering. A maximum-prefix limit does not select the correct routes. ORF is therefore the scalable operational design for receiving a default route and selected prefixes with minimal provider involvement. Reference topics: BGP ORF, prefix-based filtering, inbound route control, ISP peering design, routing table scale.

Graceful restart and nonstop forwarding designs depend on preserving forwarding continuity during a control-plane processor switchover. Cisco Stateful Switchover provides the mechanism for a standby route processor or supervisor to maintain synchronized state so it can assume control when the active processor fails. When SSO is combined with NSF or graceful restart capabilities in routing protocols, neighboring devices can continue forwarding traffic while the restarting device preserves or rebuilds control-plane adjacencies. Cisco Express Forwarding is required for fast data-plane forwarding, but CEF alone does not provide route processor state synchronization after processor failure. Virtual Switch System is a chassis or switching architecture that can provide high availability in certain campus designs, but it is not the required feature named by graceful restart recovery. Bidirectional Forwarding Detection is a failure-detection protocol used to detect path or neighbor failures quickly; it does not recover a failed processor. Therefore, the feature that must be incorporated for graceful restart recovery from processor failure is Stateful Switchover. In a resilient enterprise design, SSO, NSF, and protocol graceful restart are coordinated to reduce outage impact.