Cabletron Systems CSX400, CSX200 manual About Source Route-Transparent Bridges, Bridging Basics

Bridging

About Source Route-Transparent Bridges

Because network topologies have developed in which bridges must be able to handle network trafÞc from end stations which support source routing and others which do not, a hybrid type of bridgeÑSource Route-Transparent (SRT)Ñcombines elements of both bridging methods.

An end stationÕs network drivers can be conÞgured in software to use a bit setting in the source address portion of a data frame to indicate whether the station is to operate in a Source Route or Transparently bridged network environment. The Routing Information Indicator (RII) bit of the source address is set to 1 if the station is to use Source Routing; if the station is to operate in a Transparently bridged environment, the RII bit is left unchanged (i.e., at 0).

Not all end stations in a Token Ring environment have network drivers which support Source RoutingÑwhether the drivers are improperly conÞgured via management or they simply are not source-route capable.

In a network with a mix of Source Route and Transparent end stations, data frames from both station types must be bridged correctly. An SRT bridge inspects the RII bit setting of incoming frames to determine whether they should be Transparently bridged (if the RII bit was at 0) or Source Routed (if the RII bit was set to 1) to their destination and will use the appropriate bridge method to forward the frame.

NOTE

Cabletron has extended the functionality of Ethernet ports on translational bridges, so the ports can be set to Source Route mode.

When an Ethernet port is in Source Route mode, on receipt of an SR packet from a Token Ring port, it will save the Source Routing information and send out the packet transparently. When the response comes back, the source routing information will be restored and sent to the Token Ring port.

About Source Route-Translational Bridges

Because SmartSwitch 2000, 6000 and 9000 modules have the ability to combine mixed network topologies, yet another hybrid bridge methodÑcalled a Source Route Translational bridge (SR-TB)Ñis used by a number of these SmartSwitch modules.

An SR-TB bridge supports both Source Routing and Transparent bridging capabilities, with the added requirement of maintaining Source Route information across an FDDI interfaceÑeither the SmartSwitch 9000 FNB backplane, or an installed FDDI High Speed Interface Module (HSIM).

An SR-TB bridge does this by ÒtranslatingÓ the Token Ring physical frame format (by stripping out routing information, if necessary) so that the frameÕs source address can be recognized on an FDDI, Ethernet, or ATM segment; and then, when data is returned to the source, restoring the necessary route information to forward it along a bridged Token Ring environment.

4-4 Bridging Basics