There are two problems in using single-arm routing to achieve data forwarding: 1. The router and switch are connected through a link, which is easy to become a single point of network failure, resulting in communication interruption between VLANs. 2. The communication between each VLAN is carried by a “single” link. Le “single-arm” link may become a bottleneck in traffic transmission. The solution is to replace it with a three-layer switch to achieve inter-VLAN routing.