Source-Route Transparent Bridging (SRT)

The IEEE 802.1 committee identified the need for source-route bridges to interoperate with transparent bridges in the same internetwork. A source-route transparent bridge (SRT) standard has been defined to achieve this goal. The principle behind SRT bridges is very simple. An SRT bridge inspects all received frames and looks for the presence of the routing information indicator (RII) and the routing information field (RIF). If these fields are present, the SRT bridge uses them and acts as a source-route bridge. If not, the SRT bridge operates in transparent bridge mode and forwards frames based on their MAC sublayer destination address and its associated entry in the filtering database. The source-route transparent bridge does not allow source-route bridge devices to communicate with transparent bridge devices. SRT bridge is the capability for its interfaces to understand both source-route bridging and transparent bridging devices. But an SRT bridge will never translate source-route bridge frames into transparent bridge frames, and vice versa.

Source-Route - Translational Bridge (SR-TB)

Source-Route - Translational Bridge (SR-TB) is not an ISO standard definition. However, more and more bridges are implementing the SR-TB because of the need to interconnect source-route bridge domain with transparent bridge domain. The goal of the source-route - translational bridge is to translate source-route bridge frame into a transparent bridge frame, and vice versa. The SR-TB bridges have to change the MAC layer protocol from (or to) Ethernet protocol to (or from) token-ring protocol. Actually, regarding the ISO bridge definition, this translation does not belong to a bridge. But it is implemented in a lot of bridges, in order to be able to interconnect source-route bridge domain and transparent bridge domain regardless of the protocol of the upper layer.

Tunnel Bridge

The tunnel bridge allows source-route bridge domains or transparent bridge domains to communicate across an IP network. The tunnel bridge receives bridged frames from its source-route bridge or transparent bridge domain. The frames are encapsulated into IP datagrams that are sent to the destination IP address. These IP datagrams are routed in the IP network as are other IP datagrams, with the IP rules. The destination IP address is actually another bridge implementing the tunnel bridge feature. This target bridge removes the IP envelope from these IP datagrams making them source-route bridge or transparent bridge frames. Then the target bridge sends these frames to its source-route bridge domain or transparent bridge domain in the same way that other bridged frames are sent.

With tunnel bridging, as far as the source-route bridge is concerned, the IP network is seen as a single LAN segment, regardless of the complexity of the IP network. Then it adds only one hop to cross this IP network. 

The number of hops from the source device to the source IP tunnel bridge, plus one hop to cross the IP network, plus the number of hops from the destination IP tunnel bridge to the destination device, must not exceed the 7-hop count limitation of the source-route bridge implementation.