Configuring Transparent Bridging
Transparent and SRT Bridging Configuration Task List
BC-30
Cisco IOS Bridging and IBM Networking Configuration Guide
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’s destination address is known in the bridge table, it is forwarded on a single interface in the
bridge group. If the packet’s destination is unknown 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 process of bridging.
•To participate in the spanning-tree algorithm by receiving, and in some cases transmitting, 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. A bridge group establishes
a spanning tree based on the BPDUs it receives on only its member interfaces.
For SRT bridging, if the Token Ring and serial interfaces are in the same bridge group, changing the
serial encapsulation method causes the state of the corresponding Token Ring interface to be
reinitialized. Its state will change from “up” to “initializing” to “up” again within a few seconds.
After you assign a bridge group number and define a Spanning-Tree Protocol, assign each network
interface to a bridge group by using the following command in interface configuration mode:

Choosing the OUI for Ethernet Type II Frames

For SRT bridging networks, you must choose the organizational unique identifier (OUI) code that will
be used in the encapsulation of Ethernet Type II frames across Token Ring backbone networks. To
choose the OUI, use the following command in interface configuration mode:
Transparently Bridged VLANs for ISL
Traditionally, a bridge group is an independently bridged subnetwork. In this definition, bridge groups
cannot exchange traffic with other bridge groups, nor can they multiplex or de-multiplex different
streams of bridged traffic. The transparently bridged VLAN feature in Cisco IOS software permits a
bridge group to extend outside the router to identify traffic switched within the bridge group.
While bridge groups remain internal organizations of network interfaces functioning as distinct bridges
within a router, transparent bridging on subinterfaces permits bridge groups to be used to multiplex
different streams of bridged traffic on a LAN or HDLC serial interface. In this way, bridged traffic may
be switched out of one bridge group on one router, multiplexed across a subinterface, and demultiplexed
into a second bridge group on a second router. Together, the first bridge group and the second bridge
group form a transparently bridged VLAN. This approach can be extended to impose logical topologies
upon transparently bridged networks.
Command Purpose
bridge-group bridge-group Assigns a network interface to a bridge group.
Command Purpose
ethernet-transit-oui [90-compatible |
standard | cisco]
Selects the Ethernet Type II OUI encapsulation code.