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.

OSPF with the hub in area 0 and branch routers in stub areas with ECMP is the best fit because the customer has multivendor branch routers with limited memory and CPU. EIGRP is not suitable in this case because the branch environment is multivendor and EIGRP is not the safest open-standard design choice for broad interoperability. IS-IS can be efficient, but it is less common for small DMVPN branch deployments and the option does not address the requirement for resource-constrained branch routing. eBGP route reflection is not technically correct, because route reflectors are an iBGP scaling mechanism. OSPF provides an open-standard dynamic routing solution, and placing branches in stub areas limits the amount of external routing information that low-resource branch routers must process. ECMP allows both equal 10-Mbps DMVPN paths to be used when equal-cost paths exist. This design reduces operational overhead compared with static routing while respecting the platform constraints at the spokes. Reference topics: OSPF branch design, stub areas, ECMP, DMVPN routing, multivendor interoperability, branch resource optimization.

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