A bridge domain is one of the means by which it is possible
to define a broadcast domain on a bridging device. It is an 
    alternative to 802.1D bridge-groups and to 802.1Q VLAN 
bridging.

Bridge domain is the service specification, and specifies the 
broadcast domain number on which this frame of this particular
service instance must be made available on. The physical and 
virtual interfaces that can comprise a bridge domain are 
heterogeneous in nature comprising Ethernet service instances,
WAN Virtual Circuit for ATM or Frame Relay and VFIs. However,
the frame encapsulations for all interface types are
essentially Ethernet. 

Without bridge-domains, VLANs would have to be globally unique
per device and one would only be restricted to the theoretical  
     maximum of 4095 VLANs for single tagged traffic. However
with        the introduction of bridge-domains, one can
associate a service        instance with a bridge-domain and all
service instances in the        same bridge-domain form a
broadcast domain. Bridge-domain ID        determines the
broadcast domain and the VLAN id is merely used        to match
and map traffic. With bridge domain feature configured       
VLAN IDs would be unique per interface only and not globally.   
    Thus bridge domains make VLAN ids have only local
significance        per port


Differences between Bridge Domains and 802.1AD Bridges:
=======================================================
1. Scope of the VLAN technology which uses 802.1 AD is global to
the box.
But in case of Bridge domain, the scope of vlan is local to
interface

2. Switchport 802.1AD restricts the number of broadcast domain  
    on a box to 4095.
However, with Bridge domains, we can have up to 16k broadcast   
       domain.

3. Under a single Bridge domain service instance, there can be  
        flexible service mapping criterion.(i.e match based on
outer vlan, outer cos, inner vlan, inner cos and payload
ethertype).
Whereas in case of switch port 802.1AD/dot1q this is not
supported.

Similarities between Bridge Domains and 802.1AD Bridges:
=======================================================

1. Both use the same MAC address lookup for forwarding.

2. Both work with protocols like STP, DTP etc.

3. Both of them classify 'ports' in a system into Bridges/Bridge
          Domains.

Ethernet service instance is the instantiation of an Ethernet 
virtual circuit on a given port on a given router. In other 
words, an Ethernet service instance is an object that holds 
information about the layer 2 service that is being offered
on a given port of a given router as part of a given Ethernet
virtual circuit. Bridge domains feature is currently supported
on ethernet service instances only and can be later extented
to other interfaces like ATM and Frame Relay.

This MIB helps the network management personnel to find out the 
details of various broadcast domains configured in the network.

Definition of terms and acronyms:

ATM: Asynchronous Transfer mode

BD: Bridge Domain

C-mac: Customer MAC

EVC: Ethernet Virtual Circuit

FR: Frame Relay

SH: Split Horizon

VFI: Virtual Forwarding Instance 

VLAN: Virtual Local Area Network

WAN: Wide Area Network