Cisco Systems 3560 Manual De Usuario

Fallback bridging does not allow the spanning trees from the VLANs being bridged to collapse. Each 
VLAN has its own spanning-tree instance and a separate spanning tree, called the VLAN-bridge 
spanning tree, which runs on top of the bridge group to prevent loops.

The switch creates a VLAN-bridge spanning-tree instance when a bridge group is created. The switch 
runs the bridge group and treats the SVIs and routed ports in the bridge group as its spanning-tree ports.

These are the reasons for placing network interfaces into a bridge group:

To bridge all nonrouted traffic among the network interfaces making up the bridge group. If the 
packet destination address is in the bridge table, the packet is forwarded on a single interface in the 
bridge group. If the packet destination address is not in the bridge table, the packet is flooded on all 
forwarding interfaces in the bridge group. A source MAC address is learned on a bridge group only 
when the address is learned on a VLAN (the reverse is not true).

To participate in the spanning-tree algorithm by receiving, and in some cases sending, BPDUs on 
the LANs to which they are attached. A separate spanning-tree process runs for each configured 
bridge group. Each bridge group participates in a separate spanning-tree instance. A bridge group 
establishes a spanning-tree instance based on the BPDUs it receives on only its member interfaces. 
If the bridge STP BPDU is received on a port whose VLAN does not belong to a bridge group, the 
BPDU is flooded on all the forwarding ports of the VLAN.

 shows a fallback bridging network example. The switch has two ports configured as SVIs 

with different assigned IP addresses and attached to two different VLANs. Another port is configured as 
a routed port with its own IP address. If all three of these ports are assigned to the same bridge group, 
non-IP protocol frames can be forwarded among the end stations connected to the switch even though 
they are on different networks and in different VLANs. IP addresses do not need to be assigned to routed 
ports or SVIs for fallback bridging to work.

These sections contain this configuration information:

101240

Host A

Host C

SVI 1

172.20.128.1

172.20.129.1

Layer 3 switch

Routed port
172.20.130.1

SVI 2

VLAN 20

Host B

VLAN 30