Multi-node Amazon ElastiCache (Memcached) clusters must be deployed across multiple Availability Zones to maintain availability and avoid complete cluster outage if a single Availability Zone fails.
The AZMode property on AWS::ElastiCache::CacheCluster must be set to cross-az for resources with Engine set to memcached and NumCacheNodes greater than 1. If AZMode is omitted, the cluster defaults to single-az, so resources missing AZMode or with AZMode not equal to cross-az will be flagged as a configuration risk.