Multichassis

Multichassis is a networking design approach in which two or more physical devices cooperate to present a single, logical network element to connected devices. The goal is to combine bandwidth and provide redundancy without requiring end devices to treat multiple devices as separate paths. In practice, multichassis deployments typically use technologies such as multi-chassis link aggregation (MLAG) or other vendor-specific equivalents to create a single forwarding domain across multiple chassis or switches.

A key feature of multichassis architectures is the use of a peer relationship between the participating devices. A dedicated inter-switch or peer link allows the devices to synchronize state information such as MAC address tables, forwarding decisions, and link aggregation group (LAG) membership. This enables active-active uplinks and rapid failover, while reducing the likelihood of loops and ensuring consistent forwarding across the connected endpoints. Although standard link aggregation protocols like LACP (IEEE 802.1AX) apply to individual links, multichassis implementations extend these concepts to coordinate across multiple devices.

Benefits of multichassis include higher aggregate bandwidth, improved redundancy, and simplified device uplinking, which can reduce single points of failure and enable non-stop forwarding. Challenges include increased architectural and operational complexity, the need for precise configuration of peer links and synchronization, and potential risk of split-brain scenarios if inter-device communication is disrupted.

Common use cases are data center networks, campus networks, and service provider environments that require high availability and scalable bandwidth. Examples of multichassis concepts include MLAG deployments and systems that combine two or more switches into a single logical switch, sometimes marketed under vendor-specific terms such as virtual switching systems.