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Catalyst 3550 Multilayer Switch Software Configuration Guide
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Chapter26 Configuring Fallback Bridging
Understanding Fallback Bridging
acts like a port on a router, but it is not connected to a ro ut er. A routed po rt is not asso ci at ed wi th a
particular VLAN, does not support VLAN subinterfaces, but behaves like a normal rou ted interface. For
more information about SVIs and routed ports, see Chapter 8, “Configuring Interface Characteristics.”
A bridge group is an internal organization of network interface s on a sw it ch. Brid ge group s c annot 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, b ridged traffic and bridge protocol
data units (BPDUs) cannot be exchanged between different bridge g roups on a switch. An interface can
be a member of only one bridge group. Use a bridge group f or eac h sepa ratel y bridg ed ( to polo gical ly
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 b ridg e ta ble , i t is fl ood ed on a ll fo rwar ding
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 span ning- tre e i nsta nce. A b rid ge gro up
establishes a spanning-tree instance based on the BPDUs it receives on only its member interfaces.
Figure 26-1 shows a fallback bridging network example. The multilayer switch has two interfaces
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 addr ess . If all three of these ports are assigned to
the same bridge group, non-IP protocol frames can be for war ded a mong t he e nd stati on s c onnec te d to
the switch.
Figure26-1 Fallback Bridging Network Example
Si
Host A
Host C
SVI 1172.20.128.1 172.20.129.1
Routed port
172.20.130.1
SVI 2
Catalyst 3550 switch
with enhanced
multilayer
software image
VLAN 20
Host B
VLAN 30
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