300-420 Exam Dumps : Designing Cisco Enterprise Networks

Cisco SD-WAN uses Path MTU Discovery to avoid fragmentation problems across the overlay. SD-WAN traffic is encapsulated and encrypted, which adds overhead beyond the original packet size. If the effective path MTU is smaller than the encapsulated packet, fragmentation or packet loss can occur, especially when the DF bit is set or when intermediate devices block fragmentation-related ICMP messages. PMTUD allows devices to discover the largest packet size that can traverse the path without fragmentation and adjust forwarding behavior accordingly. Marking traffic with DF alone does not solve the issue; it may expose the problem by causing packets to be dropped when too large. Jumbo frames are not a universal WAN solution because service-provider, Internet, and broadband paths often do not support them consistently. Configuring every access circuit with a fixed 1600-byte MTU is not a reliable design assumption. The correct design is to rely on PMTUD and validate tunnel overhead, device MTU, TCP MSS adjustment where needed, and ICMP handling across transports. Reference topics: Cisco SD-WAN MTU, PMTUD, tunnel overhead, fragmentation avoidance, IPsec encapsulation.

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.

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.