Cabletron Systems 2.2 Manual De Usuario

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.

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