C H A P T E R 36 Configuring Fallback Bridging - Cisco Catalyst 3550 series Software Configuration Manual

Understanding Fallback Bridging
data units (BPDUs) are not exchanged between different bridge groups on a switch. An interface can be
a member of only one bridge group. Use a bridge group for each separately bridged (topologically
distinct) network connected to the switch.
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. The switch places source addresses in the bridge table as
it learns them during the bridging process.
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.
Figure 36-1
SVIs with different assigned IP addresses and attached to two different VLANs. Another interface 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.
Figure 36-1 Fallback Bridging Network Example
172.20.128.1
Catalyst 3550 Multilayer Switch Software Configuration Guide
36-2
shows a fallback bridging network example. The switch has two interfaces configured as
Catalyst 3550 switch
with enhanced
multilayer
software image
SVI 1
Host A
VLAN 20
Routed port
172.20.130.1
Si
SVI 2
Host B
VLAN 30
Chapter 36 Configuring Fallback Bridging
Host C
172.20.129.1
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