CISCO-BRIDGE-DOMAIN-MIB
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