Cisco WS-C3550-12G Software Configuration Manual page 688
Multilayer switch
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Understanding Fallback Bridging
acts like a port on a router, but it is not connected to a router. A routed port is not associated with a
particular VLAN, does not support VLAN subinterfaces, but behaves like a normal routed interface. For
more information about SVIs and routed ports, see
A bridge group is an internal organization of network interfaces on a switch. Bridge groups cannot be
used to identify traffic switched within the bridge group outside the switch on which they are defined.
Bridge groups on the same switch function as distinct bridges; that is, bridged traffic and bridge protocol
data units (BPDUs) cannot be 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.
The purpose of placing network interfaces into a bridge group is twofold:
•
To bridge all nonrouted traffic among the network interfaces making up the bridge group. If the
packet destination address is in the bridge table, it is forwarded on a single interface in the bridge
group. If the packet destination address is not in the bridge table, it is flooded on all forwarding
interfaces in the bridge group. The bridge 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 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 26-1
configured as 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.
Figure 26-1 Fallback Bridging Network Example
172.20.128.1
Catalyst 3550 Multilayer Switch Software Configuration Guide
26-2
shows a fallback bridging network example. The multilayer switch has two interfaces
Catalyst 3550 switch
with enhanced
multilayer
software image
SVI 1
Host A
VLAN 20
Chapter 8, "Configuring Interface Characteristics."
Routed port
172.20.130.1
Si
SVI 2
Host B
VLAN 30
Chapter 26
Configuring Fallback Bridging
Host C
172.20.129.1
78-11194-03
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