Multi-Chassis Trunking Overview

Multi-Chassis Trunking (MCT) is an alternative to spanning tree protocols. Spanning tree is a technology that protects the network against loops by blocking necessary ports, and having the network span to relearn topologies when one link fails in a network. MCT is a technology that allows two MCT-supporting switches to cluster together and appear as a single logical device. Trunking is a technology that allows multiple links of a device to appear as one logical link. The combination of MCT and trunking allows for creating a resilient network topology that utilizes all links in the network, creating an ideal network topology for latency-sensitive applications.

Standard static or dynamic LACP trunks provide link-level redundancy and increased capacity. However, trunks do not provide device-level redundancy. If the device to which the trunk is attached fails, the entire trunk loses network connectivity. Two devices are needed for network resiliency with trunked links to both devices. With spanning tree, one of these trunks would be blocked from use until the failure of the other trunk is detected, taking from 1 to 30 seconds and potentially adding latency and jitter, not only on the affected devices locally, but throughout the span topology. With MCT, member links of the trunk are split and connected to two clustered MCT-supporting switches. MCT has integrated loop detection, which allows all links to be active. If a failure is detected, traffic is dynamically allocated across the remaining links. The failure detection and allocation of traffic occur in sub-second time, without impact on the rest of the network.

MCT inherits all of the benefits of a trunk group and allows multiple physical links to act as a single logical link. The resulting available bandwidth is an aggregate of all the links in the group. Traffic is shared across the links in the group using dynamic flow-based load balancing, and traffic is moved to a remaining link group in sub-seconds if a failure occurs on one of the links. MCT eliminates the single point of failure that exists at the device level when all links of a trunk terminate on the same device without the overhead associated with spanning tree. MCT diverts a subset of the links to a second device to provide redundancy and sub-second fault detection at the device level.