MNHA: Redundancy domain

Use four-node Multinode High Availability (MNHA) to strengthen continuity by deploying two MNHA pairs across domains, including separate data centers. Each pair uses an interchassis link (ICL), and the pairs interconnect with an interdomain link (IDL) for secure intra-domain communication and failover if one pair becomes unavailable. The design supports SRG0 services such as firewall and NAT but does not support SRG1+ services?for example, IPsec VPN. Four-node MNHA provides resilience against localized disruptions. Four-node MNHA support is available only for the routing mode (Layer 3) of MNHA.