Fallback Bridging - Cisco NM-8B-U - HW ROUTERS L-M User Manual

Feature Overview

Fallback Bridging

With fallback bridging, the switch bridges together two or more VLANs or routed ports, essentially
connecting multiple VLANs within one bridge domain. Fallback bridging forwards traffic that the
multilayer switch does not route and forwards traffic belonging to a nonroutable protocol such as
DECnet.
Fallback bridging does not allow the spanning trees from the VLANs being bridged to collapse; each
VLAN has its own Spanning Tree Protocol (STP) instance and a separate spanning tree, called the
VLAN-bridge spanning tree, which runs on top of the bridge group to prevent loops.
A VLAN bridge domain is represented using the switch virtual interface (SVI). A set of SVIs and routed
ports (which do not have any VLANs associated with them) can be configured to form a bridge group.
Recall that an SVI represents a VLAN of switch ports as one interface to the routing or bridging function
in the system. Only one SVI can be associated with a VLAN, and it is only necessary to configure an
SVI for a VLAN when you want to route between VLANs, to fallback-bridge nonroutable protocols
between VLANs, or to provide IP host connectivity to the switch. A routed port is a physical port that
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 subinterfaces, but behaves like a normal routed interface.
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:
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Figure 19
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.
Cisco IOS Release 12.2(2)XT, 12.2(8)T, and 12.2(15)ZJ
42
16- and 36-Port Ethernet Switch Module for Cisco 2600 Series, Cisco 3600 Series, and Cisco 3700 Series
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.
shows a fallback bridging network example. The multilayer switch has two interfaces