Bridging

2-4 Bridging Basics

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.
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; then, when
data is returned to the source, the bridge restores the necessary route information
to forward it along a bridged Token Ring environment.
For data that is restricted to the Token Ring networks available from the SR-TB
bridgeÕs front panel, the bridging method is user-conÞgurable via local
management to be Source Route-only (bridged packets must include RIF
information and will be source routed; no transparent bridging is enabled),
Source Route-Transparent (bridging method will be determined by whether the
RII bit is set), or Transparent only (no source routed packets will be bridged).
Remote management of these interfaces is based upon their current mode (as set
through local management).
For data that will ultimately be sent across an FDDI interface to an ATM, Ethernet,
FDDI, or another Token Ring segment, the Routing Information Field will be
stripped from the packet so the packet can be transparently bridged onto Ethernet
or FDDI media; however, the RIF information as well as the source address of the
packet is stored in a RIF cache of the SR-TB bridge. When data is returned to that
source address, the SR-TB bridge can look up the address information in its RIF
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
and receives 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 then sent to the Token Ring port.