Fallback bridge Overview

The so-calledFallbackBridging refers to two or more virtual LANVLANs) And routable InterfacesRouted ports.FallbackThe traffic and non-routing protocols used by bridging to forward vswitches that cannot be routed are shown in figureDECnet) Traffic .
A virtual LAN bridge domain is a Virtual InterfaceSVIsA group of exchange virtual interfaces and routable interfaces can constitute a bridge group. An exchange virtual interface represents a virtual LAN consisting of an exchange port, which facilitates routing and bridging in the system. An exchange Virtual Interface corresponds to a virtual LAN, when you need to route traffic between virtual LAN or bridge the protocol between virtual LAN cannot be routed and provideIPWhen connecting a host to a vswitch, You need to configure the corresponding virtual interface for the corresponding virtual LAN; A routable interface is a physical port on the vswitch, it is like a physical port on a router, but it has nothing to do with a specific Virtual LAN and does not support virtual LAN sub-interfaces. Other aspects are similar to a normal routing interface.
FallbackThe Spanning Tree Protocol of the virtual LAN is not allowed for bridging.Spanning-tree) Is damaged. Each Virtual LAN has its own Spanning Tree process, which is called a virtual LAN bridging spanning tree to block the bridging ring ). When a vswitch creates a bridge group, the Spanning Tree process of the virtual LAN is automatically created, and the switch regards the switching Virtual Interface and the routable interface in the bridge group as the Spanning Tree Port.
The following isFallbackIn the example of bridging, we can see from the topology that two interfaces of the switch are configured as switching virtual interfaces.SVI1AndSVI2).IPAddress(SVI1: 172.20.128.1, SVI2: 172.20.129.1), Which represents two different virtual LAN, and the other interface is configured as a router interface with its ownIPAddress(172.20.130.1)If these three ports are allocated to the same bridge groupIPProtocol frames can be forwarded between the devices connected to the vswitch, even if they are in different CIDR blocks and different virtual LAN. For routable interfaces and switched virtual interfaces,IPAddress allocation is not required.
　Fallback BridgingExample